The linux professional institute certification study guide

Linux Professional Institute LPIC-1

The world’s largest and most recognized Linux certification

LPIC-1 is the first certification in the multi-level Linux professional certification program of the Linux Professional Institute (LPI). The LPIC-1 will validate the candidate’s ability to perform maintenance tasks on the command line, install and configure a computer running Linux and configure basic networking.

The LPIC-1 is designed to reflect current research and validate a candidate’s proficiency in real world system administration. The objectives are tied to real-world job skills, which we determine through job task analysis surveying during exam development.

Current version: 5.0 (Exam codes 101-500 and 102-500)

Prerequisites: There are no prerequisites for this certification.

Requirements: Passing the 101 and 102 exams. Each 90-minute exam is 60 multiple-choice and fill-in-the-blank questions.

Validity period: 5 years unless retaken or higher level is achieved.

Cost: Click here for exam pricing in your country.

Languages for exam available in VUE test centers: English, German, Japanese, Portuguese (Brazilian), Chinese (Simplified), Chinese (Traditional), Spanish (Modern)

Languages for exam available online via OnVUE: English, Japanese, Portuguese (Brazilian), Spanish

To become LPIC-1 certified the candidate must be able to:

• understand the architecture of a Linux system;
• install and maintain a Linux workstation, including X11 and setup it up as a network client;
• work at the Linux command line, including common GNU and Unix commands;
• handle files and access permissions as well as system security; and
• perform easy maintenance tasks: help users, add users to a larger system, backup and restore, shutdown and reboot.

LPI is committed to providing the IT community with exams of the highest quality relevance and accuracy.

Источник

LPIC-1 Linux Professional Institute Certification Study Guide

Скачать книгу

О книге «LPIC-1 Linux Professional Institute Certification Study Guide»

Thorough LPIC-1 exam prep, with complete coverage and bonus study tools

LPIC-1Study Guide is your comprehensive source for the popular Linux Professional Institute Certification Level 1 exam, fully updated to reflect the changes to the latest version of the exam. With 100% coverage of objectives for both LPI 101 and LPI 102, this book provides clear and concise information on all Linux administration topics and practical examples drawn from real-world experience. Authoritative coverage of key exam topics includes GNU and UNIX commands, devices, file systems, file system hierarchy, user interfaces, and much more, providing complete exam prep for the LPIC-1 candidate. Get access to invaluable study tools, including bonus practice exams, electronic flashcards, and a searchable PDF of key terms featured on the exam.

Linux is viewed by many companies and organizations as an excellent, low-cost, secure alternative to expensive operating systems, such as Microsoft Windows. The LPIC-1 tests a candidate’s understanding and familiarity with the Linux Kernel. This book provides comprehensive preparation and review, helping readers face the exam with confidence.

Review the system architecture, Linux installation, and package management

Understand shells, scripting, and data management more completely

Practice administrative tasks and essential system services

Brush up on networking fundamentals and security issues

As the Linux server market share continue to grow, so too does the demand for qualified and certified Linux administrators. Certification holders must recertify every five years, but LPI recommends recertifying every two years to stay fully up to date with new technologies and best practices. As exam day approaches, LPIC-1Study Guide is the one source you will want by your side.

На нашем сайте можно скачать книгу «LPIC-1 Linux Professional Institute Certification Study Guide» в формате fb2, rtf, epub, pdf, txt или читать онлайн. Здесь так же можно перед прочтением обратиться к отзывам читателей, уже знакомых с книгой, и узнать их мнение. В интернет-магазине нашего партнера вы можете купить и прочитать книгу в бумажном варианте.

Источник

LPIC-1 Exam 101

Version: 5.0

LPIC-1 is the first certification in LPI’s multi-level Linux professional certification program. The LPIC-1 will validate the candidate’s ability to perform maintenance tasks on the command line, install and configure a computer running Linux and configure basic networking.

First Lesson

101.1 Determine and configure hardware settings (101.1 Lesson 1)

Linux Professional Insitute Inc. All rights reserved. Visit the Learning Materials website: https://learning.lpi.org
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

LPI is a non-profit organization.

© 2021 Linux Professional Institute (LPI) is the global certification standard and career support organization for open source professionals. With more than 200,000 certification holders, it’s the world’s first and largest vendor-neutral Linux and open source certification body. LPI has certified professionals in over 180 countries, delivers exams in multiple languages, and has hundreds of training partners.

Our purpose is to enable economic and creative opportunities for everybody by making open source knowledge and skills certification universally accessible.

Spot a mistake or want to help improve this page? Please let us know.

© 1999–2021 The Linux Professional Institute Inc. All rights reserved.

Источник

Start Here

Linux Professional Institute Certification (LPIC-1) Study Guide

LPIC certifications are reviewed by a group of Linux experts whom are not bound to any particular distro or company, meaning that if you certified as an LPIC, your knowledge (the whole point of getting certified, to prove your knowledge) can be exported to any other distro running a Linux kernel. that includes Red Hat. So, start small but solid, and continue relentlessly: let’s get LPIC-1 certified! 🙂 Modularity and Flexibility are features of the command line, where one tool does one task and does it well. For this exam you will need two applications: putty and WinSCP

This article contains my exam notes, the study guide I followed to structure the exam’s objectives for each of the LPIC-1 exams. Yes, there are 2 exams (level 1 known as 101 and level 2 known as 102) that you need to pass both in order to get your LPIC-1 certification

Those are the objective for exam 2, you can find a deep description of them here: http://www.lpi.org/our-certifications/exam-102-objectives The items with a weight of 4/5 need to be known by heart!

101.1 Determine and configure hardware settings (2)

The kernel is the mediator between the applications and hardware. To find out which kernel you’re running on your system, executes uname -r (or uname -a ), you can also search /etc/ for *.release files. Cat the file /proc/cpuinfo for the processor info, for memory the /proc/meminfo. Execute cat /etc/os-release on any distro to see where it comes from. On CentOS you can do cat etc/redhat-release to see version number. Issue yum install redhat-lsb-core to get the lsb_release -a command

The procfs (/proc folder) contains processes information (like ACPI power, CPU and devices information) so that all other programs can refer to. With time the /proc folder started accumulating system information which is not process information, therefore the /proc folder became messy and confusing

• From kernel 2.5; the folder sysfs (/sys folder) was introduced to contain information specifically about the system (devices, cpu, etc), which before was stored in /proc
• From kernel 2.6; the udev system was introduced, which populates the /dev folder.; udev provides persistent device names for all devices in the Linux system, it supports hotplugging; the dbus was also introduced which send signals to desktop application from the system /dev folder, so that devices that are plugged-in are shown and interact with the desktop. The correct order that udev will use to read rules of the system (rules that you may want to modify yourself) is:
  1. /etc/udev/rules.d
  2. /run/udev/rules.d
  3. usr/lib/udev/rules.d

In Linux, just like in Unix, everything is treated as a file

sudo lshw -C network ;will list your network adapter, to see if they have been loaded; lshw list hardware on the Debian (Ubuntu) systems

sudo rmmod uvcvideo ;remove the video module of the webcam, sudo modprobe uvcvideo will add it again, hopefully the webcam will work this time!

101.2 Boot the System (3)

After the POST (Power On Self Test) the system runs GRUB (Grand Unified Bootloader) from the MBR (Master Boot Record, the first 512KB of the drive). Back on the day, I remember very well, it used to be LiLo (Linux Loader) the default loader, which was really clunky (it uses the /etc/lilo.conf file, remember that if you make changes to this file, you need to run the lilo utility, else changes won’t be effective). Whichever bootloader you use, it always launches the initrd (initial ram disk) that loads a little kernel into ram to detect all hardware and load only the modules needed for the system to run efficiently; it is the initrd tiny kernel the one that mounts the hard drive. Once initrd loads, it then loads the beginning of the Linux kernel called » init » (which has a process number of 1, being the first process that runs), it is then init the one who initializes the Linux kernel. The file /proc/cmdline passes the parameters from the bootloader (grub) to the kernel.

Modern Linux distributions have these 3 different types of init programs (there is a fourth one called «BSD init» but not many distros use this one, because is not very flexible)

• SysV; around for a long time; it uses numerical order to start the systems one at a time; it can run at level 1 (single user mode), run level 3 (console mode) or run level 5 (GUI mode) etc; the first program SysVinit usually starts is /sbin/init. To disable the crtl-alt-delete configuration edit the file /etc/inittab. On systsemd systems, this setting is configured by usr/lib/systemd/system/ctrl-alt-del.target The correct order for processes running during SysVinit startup is: kernel loaded > init > rc.sysinit > runlevel scripts
  
• Upstart; developed by Ubuntu, start the system by working out dependencies, therefore it loads faster with this mode because different programs can start at the same time
• Systemd; the new guy, use by Ubuntu and RedHat, compatible with SysV but not compatible with other systems, and it may need «systemctl enable____» or «systemctl start _____» to load the systems; Systemd uses binary code, and not just scripts like «SysVinit» and «Upstart»

To disable a service, depending on the system you’re using, you can use the following:

«SuperServers» used to be files on the Linux distributions that used to control everything (it used to be the /etc/inetd.d or the /etc/inetd.conf file). What we still have is the /etc/xinetd.d/ where they are lots of files (daemons) all run by the /etc/xinetd.conf which basically says to run all files inside the xinetd.d directory

sudo service xinetd restart ;to restart the parent process once you enable any of its services, then run netstat -tuna to verity the services like FTP servers, etc are listening. Check the /etc/hosts.allow or / etc/hosts.deny to add or remove IP addresses (allow takes precedence) and then restart xinetd to prevent certain computers from accessing the relevant services; this method of allowing/denying IP address is called TCP Wrappers, though it is not in use anymore

initramfs ; At boot time, the boot loader loads the kernel and the initramfs image into memory and starts the kernel. The kernel checks for the presence of the initramfs and, if found, mounts it as / and runs /init. The init program is typically a shell script. Note that the boot process takes longer, possibly significantly longer, if an initramfs is used.

101.3 Change runlevels / boot targets and shutdown or reboot the system (3)

Here is a table comparison of the run levels usage between different systems; just to note that run level 4 doesn’t exist, and that Debian/Ubuntu have put all-the-run-levels on level 2

When the system is rebooting

systemctl isolate reboot.target

To change run level use » telinit » or «init» following by the level you need, though note that «init» is not the right command to use, stick to use «telinit» instead

The default running mode can be determined by examining the file /etc/inittab; (on a distro running systemd, that file probably doesn’t exist, and you need to examine the folder /etc/systemd/system instead). The init command read the /etc/inittab file to logon on to single-user mode; use the command cat to examine it or the command vi to edit it (on the screenshot below notice that we are using an operating system with systemd, therefore the command » systemctl » must be use to change the boot target):

To explore the different target that the system has, explore these locations:

The telinit q instruct the system to read the /etc/inittab file again. To mess-up your colleague, and enhance his/her troubleshooting techniques, you can set a system to «reboot» as the default running level. This table maps the different runlevels with their corresponding target strings:

Run Level (Boot target)	CentOS / Suse	Debian/ Ubuntu	Description
0 (power off)	Halt	Halt	The system is completed stop	systemclt isolate poweroff.target
1 (rescue)	Single User Mode	Single User Mode	Only one user can logon, to change the root passwd for example	systemctl isolate rescue.target
2	Mutli User, no network	Full, multiuser, GUI is available if installed	To troubleshoot the system ; for Debian and Ubuntu everything is at run level 2
3 (multi-user non-gui)	Multi User, with network	n/a	On CentOS, level 3 is console with net access	systemctl isolate multiuser.target
4	n/a	n/a	Not use at all
5 (GUI)	Multi user with GUI	n/a	With the fully GUI loaded	systemctl isolate graphical.target
6 (reboot)	Reboot	Reboot

Run level	Boot target	Command
0	poweroff	telinit 0
1	rescue, no network	telinit 1 OR telinit s
3	multi-user, with network
5	graphical
6	reboot	telinit 6

How to shutdown your Linux box? Oh well, just as a start, please don’t pull the plug! To shutdown your system, switch to one of these run levels:

If your system is frozen, you may try «reboot -f now» (not recommended, unless, of course, system is frozen); by using the -f flag you force the reboot, obviously all data and integrity is lost and compromise, and there is not warranty at all that the system may boot up normally again. You can also use these different variations to shutdown your system:

To display a message to all your buddies who are connected to the console, use the » wall » command with the pipe (|) operator

102.1 Design hard disk layout (2)

/var ;keep it if possible on a separate hard drive or partition, with fast access, as this folder is in constant use and it tends to be filled quickly with logs, etc

/swap ;this is a partition that acts as the page file in windows

LVM (Logical Volume Manager) adds all drives together on a pool, where you can expand it by adding drives as you go along, giving you in that way lots of flexibility. LVM provides no redundancy at all, therefore underneath the LVM some kind of RAID1 (mirror) or RAID5 (parity) must be established if you want to provide resilience to your system. LVM uses the following acronyms:

• PV ;represent Physical Volumes ( pvdisplay )
• VG ;represent Volume Groups, collectioManaging Linux Swapn of 1 or more PVs
• LV ;represent Logical Volumes, collection of 1 or move VGs ( lvdisplay )
• For example: sudo vgcreate VolumeG1 /dev/sda3 /dev/sda4 ;then create a new LV from the VG by doing sudo lvcreate -n Volume1 VolumeG1 ;and finally scan for changes by doing lvscan

Remember that, on a MBR drive, only 4 partitions are possible; if you need more than 4 partitions you need to start using extended partitions

Directories that may need their own partition during installation are:

• /usr ;because it contain all the commands, libraries, man pages, documentation and static files
• /tmp ;because every user can write to this directory

The FHS (File Hierarchy Standard) is organised as follows:

102.2 Install a boot manager (2)

Grub vs Grub2: They both stand for «GRand Unified Boot system», and both application work by inserting themselves on the MBR of the drive, then loading from there the boot sector of the Linux OS. The /etc/grub.conf gives the location of GRUB stage 2 to GRUB stage 1

To install it: grub-install

Located at: / boot/grub/menu.lst

To modify just edit the *.lst file

To install it: grub2-install

Located at: /etc/grub2/grub.cfg

To modify you need to use «grub2-mkconfig -o» (for output) to edit the *.cfg file; the grub.cfg file pics the configuration from the folders «etc/default/grub» and from «/etc/grub.d/»

Debian/Ubuntu has been running grub2 since 2009, and furthermore they are backward compatible

Enterprise 7+ and beyond uses grub2, and on these distros, notice that the «grub.cfg» file is located at /boot/grub2 (instead of the /etc/grub2 folder), making it no script backward compatible

In Debian/Ubuntu you need to type «grub-mkconfig -o» , while on Enterprise Linux you need to type «grub2-mkconfig -o«. It is very rare to find a Debian/Ubuntu OS running legacy grub, as since 2009 where grub2 was adopted by these distros, a long time has passed, but potentially you can find some Enterprise Linux 6 in production, and those guys will run legacy grub

To make changes to the grub2 configuration of a system, do as follows:

1. Edit the file /etc/default/grub , and make the adjustment that you need (for example, extend the default time out)
2. To activate the modified configuration, you need to re-generate the .cfg file by running: «grub-mkconfig -o /boot/grub/grub.cfg«

Adding rw init=/bin/bash to the boot parameters in GRUB will allow root access to the system upon booting, scary ah?

102.3 Manage shared libraries (1)

The command » ldconfig » read the file /etc/ld.so.conf and created the ld.so.cache, looks into the folder «ld.so.conf.d» and populate the cache with all the configuration files in that folder and the location of the share libraries. The «LD_LIBRARY_PATH» environment variable ensures that a certain set of directories are always searched when trying to use shared libraries.

See this diagram of the file system, of the Linux share libraries:

• etc (folder)
  • ld.so.conf ;this file tells the system where to look for library files. This file normally points to the folder underneath (ld.so.conf.d) where different program can use the share files
  • ld.so.cache ;this is a binary file (not editable at all) that is created by the ldconfig command, this file is used for programs to load the share files quickly
  • ld.so.conf.d (folder)
    • libc.conf
    • linux.conf
    • vmware.conf

The ldd application is used to determine which share libraries («so» for Share Object, meaning it is a dynamic, or «a» for static) a determine program needs in order to function, for example, typing «ldd /bin/ls» outputs how many share libraries the ‘ls’ program is using; ldd is mostly use when you’re troubleshooting a program, etc

Share Libraries work the same across different distros

If you add a modification to any of the conf files (or add a new conf file) under the «ld.so.conf.d» folder, you need to recreate the ld.so.cache file by running: sudo ldconfig, this is so that the different programs know and are aware of the share libraries available in your system

You can use the » export » command to add a specific share library to your local path

102.4 Use Debian package management (3)

Debian package build is determined by a «control» file, which makes the creation process relatively easy. In the other hand, the creation of a Debian (APT) repository is more difficult, it needs a web/ftp server and the use of certain tools such as Debian Archive Kit. There are different Linux Philosophy between Debian and Red Hat packages:

Grub	Grub2
Enterprise Linux 6 and before use only grub

RPM	Debian
Doesn’t autoconfigure	Autoconfigures
Doesn’t autostart services	Autostarts services
Doesn’t prompt the user	Prompt the users
Used on the majority of commercial Linux server installations	Used on the majority of Linux desktop installations
Packages complex to create	Packages easy to create
Repository easy to create and mirror	Repository complex to create and mirror

In Debian there are basically 3 ways of installing software:

• apt ;is the newest program and is the one to be use
• aptitude ;is older and still work, but should not be use as doesn’t work well with dependencies
• apt-get ;the oldest and superseed by apt
• dpkg ;Debian Package, should not be use because it does not check for dependencies at all

APT/YUM ;searches, downloads software from repository and manage dependencies

To find out what online repositories your system is using, visit the file /etc/apt/sources.list (some Ubuntu distros also includes the folder etc/apt/sources.list.d to hold information about the repos, when you add a file into the sources.list.d folder it will automatically populate to the the sources.list file. The different types of repos that can be activated on an Ubuntu server/desktop, can be classified as follows:

• main ;store the official supported software
• restricted ;officially supported but not completely free, in other words not truly open source
• universe ;community supported
• multiverse ;these repos contain restricted software by copyright or legality like for example video codecs

Each repo has a ‘deb http’ for the pre-compible binary and a ‘deb-src’ for the source code

aptitude is the package manager that provides a front-end for Advance Package Tool (apt) infrastructure in Debian/GNU Linux systems. Use the following commands to interact with apt:

The first thing to do is to run » sudo apt-get update «, which will sync your repo with the online repos to ensure you have the latest packages entries in your cache, basically updating your system. The dpkg configuration file is kept in /etc/dpkg/dpkg.cfg

• apt-cache search apache2 ;searches the cache for applications that contains the word apache2
• apt-cache show apache2 ;shows information about the apache2 program
• sudo apt-get install apache2 ;will install apache2 program and any dependencies that it may have, as well as suggested packages
• sudo aptitude remove apache2 ,it will remove the app, we could have used the apt-get tool if we wanted to
• sudo dpkg-reconfigure tzdata ;reconfigure existing installation of packages by running the start wizard if applicable
• sudo dpkg -s vim ;shows you details of the vim package

A nice utility to install is tree, issue sudo apt-get install tree ; if you don’t know the name of the package you’d like to install, use the GUI utility called Synaptic

dkpg ;package utility for Debian systems, -P to purge the application and delete configuration files, -p display package details including version number (OMG, -P is purge and -p is info, so close!), -R to install a package recursively, -r remove the package but not its configuration files,

102.5 Use RPM and YUM package management (3)

The distros that use RPM are Red Hat, Fedora, CentOS and openSUSE; Zipper is used by SuSe Linux and Mandriva has Urpmi. A «spec file» determines how to compile and build an RPM package, the process is complex but could be make easier by macros. In the order hand, creating a YUM repository package is very easy, all you need is a Web/FTP server and the use of the createrepo command. YUM is a smart application that check for dependencies whenever you install a RMP package, checking the Internet and your local cache before actually installing anything, always keeping it updated. RPM mostly manages and can query installed packages or files thanks to its database rpm -qa | sort will -query -all packages in the whole system.

To configure YUM edit these two files:

• /etc/yum.conf (set keepcache=1 if you want to keep the packages locally, not needed though), ensure gpgcheck=1 to ensure packages are md5 verified;
• /etc/yum.repos.d/

yum update —security ;only updates the security packages

yum update-minimal —security ;these commands in red hat works, but on CentOS it will download the buggy ones too

yum changelog updates ;shows all changes logs or yum changelog firefox shows only for firefox

yum install -y epel-release ;add the epl repository, once added install yum install -y yumex which gives you a GUI extender for yum

rpm ;also install packages, -ql query and list the all the files of a package, qi query and display name, version and description of a package

• rpm -I —nodeps package1.0.3.rpm ;notice the double dash (—) for the no dependencies option
• rpm -qc -dump firefox ;display information regarding the configuration files of firefox; -c display configuration files; -dump parameter must be use with either -l -c or -d (the -q must support the -dump too)
• rpm -qd yum ;query yum for the location of documentation files
• rpm -qa Group=»System Environment/Shells» ;query for packages that fall in this category
• rpm -qa —last ;query for packages that were installed recently
• rpm -q —changelog firefox ;will show you all the changes that have happened to the package
• rpm -qpi httpd-2.4.centos.x64.rpm ;to query a packages for info you must use the -p switch, otherwise it will query the rpm database instead
• To upgrade an installation use the -F string (freshen)
• To verify an installation use the -V specifying only the name of the package
• To perform an MD5 checksum use the -K switch
• Capital -F (Freshen) will only updates the packages that are already installed
• To find information about a package use rpm -qi bash
• rpm -qi centos-release ;shows you the exact version you’re running

rpm tags ;you can issue this command to query tags rpm —querytags and you’ll see all the tags that you can use on your commands, then you can be creative and constructs nice queries like, need to surrounder in curly braces and percents:

• rpm -qa —queryformat «% %\n» | sort ;notice that double dash for —queryformat and the curly braces
• rpm -q —queryformat «[% %\n»] firefox ;put the expression between square brackets and will list you the files
• rpm -q —queryformat «% %\n» ;tells you the installation date of firefox
• rpm -q —queryformat «[% %\n]» firefox ;show you the permission of each of the files

rpm2cpio ;extracts cpio archives from rpm packages

yumdownloader ;download rpm packages from yum repositories, use rpm -ivh httpd2.4xxxx.rpm to install a package

If a file get damaged and you need to extract the config file again, do as follows:

• rpm -qcp httpd xxx.rpm ;the -q for query, the -c for contents and the -p for package, this will list the .conf files of the package
• rpm2cpio httpdxxx.rmpm | cpio -id ;this will extract the contents of the package, run ls to verify

The package database contain the attributes of the package, which are SM5DLUGTC ;to verify a package use rpm -V firefox if nothing is shown it means the package has not been modified, use rpm -Vv firefox with the lower -v to verbose mode so it can verify all files of the package; the rpm -Va will verify your whole OS

• S ;file size
• M ;file permission mode
• 5 ;file digest or checksum
• D ;device major or minor numbers
• L ;link path
• U ;user ownership
• G ;group ownership
• T ;last modified time
• C ;and capabilities

rmp -K —nosignature httpd-2.4xxxx.rpm ;you should get an ok saying signature is okay

Sometimes the RPM database may become corrupt, to recover it first do an archive of it by visiting /var/lib and typing tar -czvpf rpmdb.tar rpm ;once the backup is created delete the current database using rm /var/lib/rpm/__db* and do after a rebuild by rpm —rebuilddb ;you can also use rpm to restore permissions to files by rpm —ssetperms firefox and will put the permissions as default

yum —showduplicates list samba-test-libs ;it will show you duplicates samba packages that you have

yum —showduplicates list available ;it will show you on colour code

yum list installed ;shows all the installed packages, showing you in the 3rd column the repository (anaconda is for the CD) the color code of the packages listed is as follows: bold (update in repository), bold and underlined (current kernel) red (package doesn’t exist in repository) and yellow (newer than package in repository)

• yum group install system-admin-tools —setopt=group_package_types=mandatory,default,optional

yum groups hidden ;shows you the hidden installed packages; yum groups list ids ;will show all groups

yum groups info «Development Tools» ;shows you info of the packages group; to properly remove a group (all of its packages) use yum autoremove development-tools

yum localinstall whoishxxx.rmp ;this is how to use yum to install already downloaded rpm packages, the yum reinstall whatever.rpm will reinstall the package

yum reinstall —skip-broken whateverxxx.rpm ;will skip the dependencies packages that are faulty or can’t be downloaded, use yum remove package.rmp to re-install a package

yum install -y yum-utils ;install utility for yum, from where you can issue package-cleanup —leaves ;so this will tell you which packages are no longer needed

Repositories ;all repos have to end in .repo otherwise yum will ignore them. To disable a repository add «enabled=0″ to the repo file; yum repolist shows you what you have yum repolist disable ; yum repoinfo ;shows you the size of the repos; visit here to add more repositories if needed: https://li.nux.ro/repos.html The command yum —disablerepos»*» —enablerepos=»centosplus» list available disables all repos but the centosplus

103.1 Work on the command line (4)

Here there are some tricks that you can use while operating a Linux machine, note that you can invoke the | by pressing Shift + backslash

apropos «string» ;use the apropos command to find out what other commands can do stuff in Linux

stat ;find out the statistics of your file, eg stat file.txt

Login Shell ;after you logon the system reads the following configuration files and path variables

The /etc/bashrc loads the /etc/profile (this is where the #path is located), where one of the lines will load any scripts files found in /etc/profile.d/* ; then it will go the user home folder and executes the relevant files on this ‘specific’ order:

• /home/user/.bash_profile
• /home/user/.bash_login
• /home/user/.profile
• /home/user/.bashrc

Interactive Shell ;in that shell you’re not prompted for username/password, and the only loaded on this mode are:

You can create your own scripts under /etc/profile.d so they don’t get overwritten when there is a system update. The two main files system-wide locations to look for are «/etc/profile » and «/etc/bash.bashrc «

If you want to do customization (aliases, etc) you will do it in the /home/user/.bashrc

export ;the export command makes the value of a varialbe available across varios subshells

unset ;destroy a previously exported variable

env ;see a list of current variables (bash shell only), NOT local variables values or functions

set ;display the values of shell variables, INCLUDING local variables and functions

• set +o history ;turn off history
• set -o history ;turn on history

setenv ;similar to env but for tsch sell

.bash_history ;this file keep your history, check it out by cat

/.bash_history The command !! recalls the entire last line which is quite useful if you have entered a command and then realised that needed to put «sudo» at the front, you can just do sudo !! , the commnad !-4 will execute the command numbered 4 beginning from the bottom

reverse-i-search ;you can enter this mode by typing crtl-r , and it will call from the history a matching of the commands you enter; if you keep pressing crtl-r the reverse-i-search will jump to the next line in the history file

HISTCONTROL ;you can use this variable to hide some commands from the history, first of all type export HISTCONTROL=ignorespace and then all the commands that you enter with a space at the front won’t be recorded, cool ah? If you type HISTCONTROL=ignoredup s the history won’t record duplicated entries, and if you type HISTCONTROL=erasedups it will delete from the history the duplications, to do both use HISTCONTROL=erasedups:ignorespace . To keep certain lines from being recorded you can keep it in the history ignore variable, e.g. the following will not record the history or ls commands itself export HISTIGNORE=»history*:ls» .If you would like to add date and time to your history command type export HISTTIMEFORMAT=»%h %d %H: %M: %S:> » Type man stftime to see other possibilities for customisation. Type export HISTSIZE=10000 to increase the number of lines in the history entries, if you set the HISTSIZE to zero it will not record anything and if you set it to a negative value it will be infinite

103.2 Process text streams using filters (3)

man ;the famous manual pages, use grep to find a particular item, use its switches (man 5 passwd) to find a particular category of the enquired command, as follows: 1 (user commands), 2 (system calls), 3 (subroutines), 4 (services), 5 (file formats), 6 (games), 7 (miscellaneous), 8 (system administration), l (local) and n (new). Use man 1 intro to start with Linux.The —help is also built-in on almost every command (for commands built-in into the shell itself, use instead help command, eg.help cd). You can also use the info command, e.g. info crontab. You can also use «BROWSER=firefox man -H crontab» to open html help files.The -k switch is a similar to apropos, so by using it the man will display all manual related to the entered string man -k passwd

cd ;takes you to the folder you wish. if you type only «cd» it takes you to your home folder ( cd

also takes you back home), once at home do less .bash_history

• cd — ;takes you to the previous folder you were working on
• cd ../../folder1 ;takes you to that folder1

cat ; short for concatenate, use to display the file on the screen and to merge more than 2 files

cut ;on this example use the delimiter as: to chop the file, thereafter only print the first field of every line cut -d: -f1 /etc/passwd (-d is use to change the delimiter from TAB designation to the DELIM designation, while -f specify the column, the -c specify the characters to be displayed, while -b specify the bytes)

less ;similar to cat, use the «f» and «b» keys to travel around, press «h» to see the command available

more ;display file page by page, but you can only move forward

fmt ;format files in different way, this will start a new line every 60 character fmt -w 60 file1

pr ;modifies the file before display, the -d adds a double space while -l x add maximum number of lines pr -d -l 10 /etc/passwd

sed ;allows you to edit a file on the fly, without actually interacting with it; sed most use string is «s» for substitute where you can replace a given string with a different one, remember to use the «g» for global string at the end, so that the replacement occurs recursively in the whole file

• The way it works is like this: sed ‘s/string1/string2/g’ file1.txt ;this will replace string1 with string2 inside file1.txt, don’t forget the ‘g’ for global
• You can also replace more than 1 string at the same time: sed ‘s/string1/string2/g’ file1.txt | sed ‘s/string3/string4/g’
• You can also use regular expressions, on this example it will look for a capital/lower case S on string1: sed ‘s/[Ss]tring1/string2/g’ file1.txt
• In this example ^ symbol means that the substitution will occur only if string1 is at the beginning of the line: sed ‘s/^String1/String2/g’ file1.txt

tr ;translate command, use to delete or replace characters in a file, eg echo ‘Hello World’ | tr -d aeiou will actually display Hll Wrld

head ;give you the first 10 lines only by default of a file, handy to see the first entries on a log file for example. To modify the number of lines that you want to see use the -n switch head -n 15 file1.txt

tail ;same as «head», but only gives you the last 10 lines tail file1.txt

split ;create several files from a single one, depending on how many lines (-) you want on each file split -l 5 file1.txt file_ ;this will create file_aa, file_ab, file_ac, etc, where each file contains only 5 lines in sequential order from the parent file

nl ;number of lines, very useful command that helps you create a file of a given file listing the number of lines this file had, very handy when a script it has an error in line number 273 and you had no idea what line is that

• nl -b a context.xml > context.lines ;this creates a file called ‘context.lines’ that contains all lines (a switch) of all the body (-b switch) of the context.xml file

paste and join ;they both work by merging data together from 2 x files, but they do it on different way paste/join file1.txt file2.tx > file3.txt

• paste ;copy the files line-by-line into a single file, separating them by tabs; with the -s will print them sequentially paste -s file1 file2
• join ;combine the lines depending on the fields identifier, so you can bind line 1 with line 3 as long as you identify these two lines with the same number (let’s say 2)

expand ;converts a tabbed-file into a spaced-file, so the lines won’t have any tabs but spaces instead; expand file1.txt > expanded.txt or you can use the other way around unexpand -a unexpanded.txt

sort ;sort lines of text files, input sort -k2n file to organise the data by column 2 (k2) and in numerical order

• sort -u file ;removes the duplicated lines

uniq ;only print lines that are unique on the file, with the -u will only display lines that are never repeated uniq -u /etc/passwd

od ;the od command displays the contents of non-ASCII files

wc ;provides lines, words and number of character (LWC), don’t worrry for the number of characters if they don’t match, wc always add one extra character to them, the \n line character.

103.3 Perform basic file management (4)

ls ;list of the files on current directory

• ls -lh * ;list of files and its properties, with the size ready for humans understanding. oh yeah!
• ls -s ;list the size of the files
• ls -a ;list hidden files and directories, in Linux a file/directory is hidden when it starts with a dot (.)
• ls -R ;recursive mode, list all contents of all stuff, a kind of similar to the tree command
• ls —color=always ;to see the colour of the files
• ls -1 | wc -l ;notice the number 1 after ls and the «l» after wc, that command will tell you the number of files you have in a directory

mkdir ;creates a directory, use it with the -p to also create the parent directory at the same time: mkdir -p folder3/folder2/folder1

rm ;deletes files, use * for all; if you type rm file? it will delete all files called file1, file2, file3, etc, the question mark stands for 1 character. The combination rm -rf is very dangerous as it deletes everything without asking; rm *.txt deletes all these extensions while rm * deletes all files

rmdir; removes folders only if they are empty

touch ;creates an empty file, touch try this one too touch file_

cp ;copy files, use the combination copy -R to copy all contents inside the folder

mv ;move files and folders, you can use this command to rename a file mv file1.txt file2.txt

dd ;copy files at the block level, it uses the structure input file > output file > block size (4k normally)

• dd if=/dev/sda out=/dev/sdb ;it copies all of the blocks from the drive sda (input file, if) to the drive sdb (output file, of), byte by byte

tar ;copy files at the file level, it stands for tape archive, always use the switches -cvf (c f or create, v for verbose and f for file, always put f last)

• tar -cvf file1.tar /home/user1/stuff ;that will includes all the contents of the «stuff» folder into the file1.tar
• tar -xvf file1.tar ;extract the contents of the file1.tar to the current directory you are working on, notice the -x switch that stand for extract

gzip ;older compression command, for better compression use bzip2, they work exactly the same

• gzip file1.txt ;compresses file1.txt by deleting that file and leaving it file1.txt.gz
• gunzip file1.txt.gz ;uncompressed the file

bzip2 ;the newer compression command

• bzip2 file1.txt ;it created the compressed file with the extension bz2
• bunzip2 file1.txt.bz2 ;uncompress the file

Both gzip and bzip2 can only compress files, that’s why you have to use tar to compress folders into a file, and then gzip/bzip2 that file

With gzip do as follows:

• tar -zcvf file1.tar.gz /home/user1 ;use the -c (compress) will create the specified tar file that will contain all stuff into the user1 folder
• tar -zxvf file1.tar.gz ;this will extract (use the -x for extraction) the contents of file1.tar.gz (could also have the extension tgz) into your current folder

With bzip2, do as follows, it uses the -j to say you are using the bzip2 command

• tar -jcvf file1.tar.bz2 /home/user1 ;to compress
• tar -jxvf file1.tar.bz2 ;to extract

xz ;similar compression program as gzip and bzip2 (.xz format), the *.lzma is a legacy format

cpio ;(copy in copy out) an older version of the tar command, rarely use nowadays, because is not as flexible as tar, you actually have to pipe the data into cpio in order to user it. To uncompress a cpio archive, you need to use the gunzip utility. The cpio program copy all the files and put them on a kind of zip file with the extension .cpio, then that .cipo file is package with a description and dependencies of the files to create the .rmp file. The way to extract the contents of a .rpm package is therefore as follows:

• ls | cpio -ov > file.cpio ; gathers the data from ls and pipe into the cpio program, who outputs it (the o switch) to a file
• cpio -idv
• rmp2cpio package.rpm ; this will extract all the files that are inside the rpm package, one of those files would be a .cpio file
• cpio -idv packagefile.cpio ;this will extract the original file structure
• cpio is useful mostly when you use the find command to search for files that you want, and then pipe them into a single .cpio file

find ;search for stuff in real-time, (#find / -name ping)

• find / -type f -perm u+rwx -ls ;find and display all the files whose owner has rwx permissions
• fin . ; return files on current directory, no filter being applied
• find /home/user1 -size +50M ;find files that are bigger than 50MB in size
• find . -mtiime +1 ;show files that have been modified more than one day ago (yesterday)
• find . -mtime -2 ;show files that have been modified less than 2 days ago
• find . -size +50M -mtime +1 ;show files bigger than 50MB that were modified yesterday

file ;determines the file type in use, file * in any directory to see

rev ;for reverse, prints the file on the screen the other way around

tac ;prints it out from the bottom up, it is the reverse of cat

103.4 Use streams, pipes and redirects (4)

/ ;use it to cancel a space/special character in a folder/file name, for example: ls «folder 2» or ls folder/ 2

Operators ; and $$

• ls ; ls ;the semicolon allows to enter 2 commands independently
• ls && ls ;the double ampersand executes the 2nd command only if the 1st one succeed

All Linux commands have 3 streams opened for them: STDIN (numerical value of 0), STDOUT (1) and STDERR (2)

read VAR ;this command will stop at the cursor and wait for you to type anything, and once you type it the string will be stored in the variable VAR

echo $VAR ;using the echo command, you will be able to recall the string you stored into the VAR variable

read ;this command uses the Redirects (and careful! > always override the file) to STDOUT, standard output, normally the screen or a file, for example:

• ls > test-out-pipe ;this redirects the output by creating a file called «test-out-pipe», that will contain the result of the command ls

2> Redirects to STDERR

• ls myfolderA 2> error ;this command will try to list the contents of «myfolderA», but because it doesn’t exist will give an error through the STDERR pipe, which in this case will be captured by the directected parameter «2>» and put into a file called «error»
• ls > out 2> error ;this example create the «out» file from the STDOUT pipe and the «error» file from the STDERR pipe
• tar cvf backup.tar /dev/sda1 2> errors.txt ;this will keep all the error of trying to backup a hard drive, handy ha?

>> Append all standard output to a file that already contain data

&> This operator creates a new file that contain both standard output and standard error

&1 ;this operator redirects the input into the output, like a look, for example:

• ls myfolderA > results.txt 2>&1 ;the error generated by the lack of existence of this folder will be take from the STDERR and put it in STDIN by &1

2>&1 This operator redirects standard error as standard output to the same place

1>&2 This operator redirects standard output to standard error

<> use the same file as both standard input and standard output

With the concept of standard input/output, we can manipulate data so that the output of an application becomes the input of another, for example

1. ls > file1.txt ;will create a file called file1.txt with the listing result from ls
2. grep is file1.txt ;will search for the string «is» on the file1.txt and display the result on the screen
3. You can do this command, with the pipe operator, to get the output of ls and piped into the input of grep, cool ah? ls | grep is

xargs ;is a program that listens in STDOUT and executes the data as an input to another command, for example

• ls | xargs rm -rf ;this get the STDOUT from ls and pipe it to xargs, which executes the rm command, just like if you typed: rm -rf
• sudo find. -maxdepth 4 -name «*.txt» | xargs tar cvf textfiles.tar ;this will find all *.txt files within 3 directory levels and compress them on a file

tee ;similar command to xgars, get the output of a command and pipe it into another, plus shows in the screen the output

• echo «print this» | tee file.txt ;the tee command will get the output from the each and put it in the screen, creating in addition the file.txt

103.5 Create, monitor and kill processes (4)

Every single process in Linux has a number (process ID — PID — that identifies itself; these are the different signal commands with their signaling number

Name	Number	Description
SIGHUP	1	Hang-up, instruct the program to reinitialize itself, including reading configuration files
SIGINT	2	Interrupt, this is done also by Crtl + C
SIGKILL	9	Kill the process immediately
SIGTERM	15	Terminates a program, it is doesn’t terminate you can kill it by using the «9» signal

& ;by putting the ampersand at the end of a command it sends the job to the background but they are only there in the terminal window that you are working on, the minute you close that terminal all background commands are stopped; to stop jobs just use the popular Crtl+Z

• sleep 60 & ;
• jobs ;allows you to see what jobs are running in the background
• fg %2 ;allows you to bring to the foreground (fg) the job id number 2; when you see the + next to a job number that indicates that this is the most recent command you sent to the foreground, meaning that if you just type «fg» it will bring back the one with the plus

nohup ;it sends jobs to the background too, but they will stay running in the system even if you close the terminal window

• nohub sleep 60 & ;creates a background job just like before, but it will stay on even if you close the terminal window
• ps aux | grep sleep ;allow you to see the background jobs on a different window

disown ;detach from a terminal window a background job

pgrep ;look up for a signal process base on its name or other attributes

103.6 Modify process execution priorities (2)

In the Linux world, the priority level of a particular process is refer to as the «nice» level, and a process can have a priority level anywhere from -20 (takes a lot of the pizza of the CPU) to +19 (only takes a tiny slice of the CPU pizza), being 0 (zero) the default «nice» level of a process. The reason that the high priority level is in negative, is because the process that fall into negative are not «nice» to the system because they use lots of CPU

renice ;it changes the already assigned priority level of a process that was started with nice. A user DO NOT have the access to renice any process to higher priority, they only can lower the priority of the given processes

103.7 Search text files using regular expressions (2)

grep ;search for string inside a file, here we have some examples of using the grep command:

egrep ;extender version of the grep command

• egrep ‘^[a-m]’ file1.txt ; find all words at the beginning of a line that start with either «a» or «m»
• egrep ‘^[P|p]’ file1.txt ; find the words at the beginning of the line that start with either capital P or lower p

fgrep ;similar to grep but faster (for f) as it doesn’t evaluate any expressions like $$ or special character

• . (dot) — a single character
• ? — the preceding character matches 0 or 1 times only
• * — the preceding character matches 0 or more times
• + — the preceding character matches 1 or more times
• — the preceding character matches exactly n times
• — the preceding character matches at least n times and not more than m times.
• [agd] — the character is one of those included within the square brackets.
• [^agd] — the character is not one of those included within the square brackets.
• [c-f] — the dash within the square brackets operates as a range. In this case it means either the letters c, d, e or f.
• () — allows us to group several characters to behave as one.
• | (pipe symbol) — the logical OR operation.
• ^ — matches the beginning of the line.
• $ — matches the end of the line.

Character Sets ;they are included within square brackets [], if you put an hyphen it means that is range 7 otherwise it will treat the character individually. If you put an exclamation mark inside the set, that means not to match whichever character will folow after [!0-9] ;the character classes matches one character of a certain type [:digit:], [:lower:],[:alpha:], etc, meanint that 3 is equal to [:digit:]

Extended globs ;ensure extglob is enable by typting shopt -s extglob, you can created more defined queries and group matches like this !(+(photo|file)*(.jpgl.gig)) = all files that do no have photo or file and don’t end with jpef or gif.

Single quote ‘ tell the shell to present the character inside literally, and all special characters inside ‘ have no meaning

Double quote » tell the shell to interpret waht is insiss

103.8 Perform basic file editing operations using vi (3)

vi ; use the keys H-J-K-L (K for kites go up) to navigate through vi if needed. When starting vi you always start in Command mode, press INSERT to switch to Insert mode, you can also press «i» and vi will allow you to insert text just before the cursor; shift + i let you insert text at the beginning of the line. Refer to this for more info: http://www.lagmonster.org/docs/vi.html

• :wq = write and quit
• :wFile2.txt = write to a new file called File2.txt
• :q! ;quit without saving
• :e! ;reads the original file back again and starts all over
• :sh ;open the shell so you can enter commands while the file is open, press Crt+D to return to the file
• :g/heart/p ;displays all lines containing the word heart
• :!ls -l ;this will run the command ls -l while maintaining the file open
• ZZ ;writes the file if it has been modified and then exit the application
• /string ;that will search for the string «string» forward in the document
• :%s/Love/Dragoste ;substitute the string «Love» for «Dragoste» in the entire document (because you have used the % operator); if you add the «/gic» at the end it will do g (global, will find all matches), i (makes case in-sensitive) and c (will confirm for each replacement)
• . ;the dot command repeat the last command
• a ;insert text After the character at the current position
• b ;navigates to the Beginning of a word
• c ;insert a character Before the cursor
• B ;navigates to the beginning of a White-Space delimiting word
• dw ;deletes the word immediately after the cursor
• d$ ;deletes from the insertion point to the end of the line
• dl ;deletes a character
• d5l ;deletes 5 characters
• dd ;deletes the entire current line
• D ;deletes the rest of the current line from the cursor position
• cw ;deletes the current word, and automatically enter INSERT mode so you can type a replacement word
• e ;navigates to the End of a word
• E ;navigates to the end of a White-Space delimiting word
• u ;undo the most recent command in the vi editor
• U ;(capital u) redo the last recent operation
• h ;moves left one character
• j ;moves down one line
• J ;join two lines
• k ;moves up one line (kites go up!)
• l ;moves right one character
• 0 ;(zero) moves to the start of the current line
• o ;opens a new line under the current line
• p ;inserts the text deleted in the last deletion operation after current cursor location
• yy ;copy one single line to clipboard (yank) the current line, if you want to copy 4 x lines use 4yy
• y5y ;yank five lines, if you press «p» after it will copy whichever lines/s you copied
• x ;deletes a character at the cursor
• X ;deletes a character before the cursor
  • I = navigate right one character
  • $ = navigate to the end of the current line
  • H = navigate to the first line of the text
  • M = navigate to the middle of the text
  • L = navigate to the last line of the text
  • G = move to a specific line to perform an editing operation
  • :sh allows you to go back to the shell to write commands, use Crtl+D to return to the file, so cool!
    • Crtl + U = moves half a screen up
    • Crtl + D = moves half a screen down
    • Crtl + F = moves down almost a full screen
    • Crtl + B = moves up almost a full screen
    • Crtl + E = scroll down one line and Crtl + Y scroll up one line
• To go to a specific line, pres ESC, then enter the line number you want to go to (you’ll see nothing on the screen) and finally press Shift+g to go to the line

nano ;user friendly text editor, crtl+a moves you to start of a line, crtl+e moves you at the end

Thanks to Langmonster for his/her great «vi Cheat Sheet»: http://www.lagmonster.org/docs/vi.html

104.1 Create partitions and filesystems (2)

Extended file systems are these:

• ext2 ;max partition 32TB, no journal but good for lower write actions (like USB drives)
• ext3 ;max partition 32TB, journaled
• ext4 ;max partition 1EB, journaled
• BTRFS ;snapshot, compression and auto-defrag
• ReiserFS ;very space efficient with small files, but the developer apparently is in jail so the filesystem is not in use that much
• XFS ;good for large partitions

When running fdisk, do first «o» to create an MBR, then n to start creating partitions. Press «L» to list the code (83 = Linux partition, 82 = Linux swap partition)

mkswap ;creates a swap partition

mkfs.ext2 ;creates a ext2 partition, use mkfs.ext3 to create a ext3 partition, use -t to define the type mkfs -t ext4 /dev/sda1

mke2fs ;creates a ext2 partition, use mke2fs -j to create a ext3 partition; the valid settings for file systems with the mke2fs are small, news (creates 1 x inode per 4-kb block) largefile (creates 1 x inode per megabyte) and largefile-4 (creates 1 x inode per 4MB block)

• mkfs ;create most filesystems
• mkfs, msdos, mkdosfs or mkfs.vfat ;creates a DOS FAT filesystem
• mkfs.ext2 or mke2fs ;creates an ext2 filesystem
• mkfs.ext3 or mke2fs -j creates an ext3 filesystem
• mkisofs ;creates a CD-ROM filesystem
• mkfs,reiserfs or mkreiserfs ;creates a reiserFS filesystem

e2fsck ;ensures that a ext2 file system that was improperly shutdown is error-free

parted ;shows you the partitions that you have on your system

xfs_growfs ;this command is used to expand an existing XFS file system

• xfs_growfs -n ;returns information of the file system geometry
• xfs_info /dev/sda1 ;also returns information of the file system geometry

lsblock ;shows you the attached block devices of your system (hard drives), very handy

fdisk is for MBR while gdisk is for GPT partition tables

104.2 Maintain the integrity of filesystems (2)

Use the fsck (pronounced ‘fisk’) to check for errors on a file system (ensure the drive is unmounted), you can do fsck.ext2 (but you’re screw if the filesystem is ‘ext3), so thanks God fsck will auto-detect the file system. Note that fsck should always run in single-user mode.

• Use the -N switch and it will show you the file format it has detected on the partition

Use the xfsprogs (on Ubuntu) to check for error of XFS file format, like xfs_repair, xfs_check (not recommended, run repair first)

For the xfs file system you have to use the xfs command to check it, a kind of beast on his own way

tune2fs ;change ext2 to ext3, so you add journal, remember to modify the /etc/fstab to reflect the new file system, changes are effective after a reboot. Tune2fs also allows you to view block counts for a file system, allowing you to adjust the number of mounts or days (-i for interval) after which a volume will be checked by e2fsck

• tune2fs -0 has_journal /dev/sda1 ;another way of converting an ext2 to an ext3 file system
• tune2fs -0 ^has_journal /dev/sda1 ;this will do the opposite, converting ext3 to ext2

dumpe2fs ;shows all superblock info

debugfs ;interactive filesystem editor, if you use this you’re definitely a professional

Use du / -hd1 (h for human, to show detail level 1 only) to see the space of different folders

df ;shows disk free, use the -h for human, use the option -T to show the file type system

104.3 Control mounting and unmounting of filesystems (3)

Use mount or umount to mount hard drives into empty folders, if you want the mount to be permanent then add it to the /etc/fstab

Use smbmount to mount a samba filesystem

The first partition of the first SCSI drive is /dev/sda1

The first SCSI tape drives load with /dev/st0

The first Parallel port is /dev/lp0 while the 2nd parallel port would be /dev/lp1

The first RAID volume is referred as /dev/md0

mount -t iso9660 image.img /dev/mnt/image ;this mount the .img file as if it was a partition

104.4 Manage disk quotas (1)

If we want to have quotas on a drive, we need to add the options » usrquota,grpquota » on the entry in /etc/fstab for the drive, quite easy, add both entries if you want to have quotas for both users and groups

Once the quota is enable on the drive, it will display the «aquota.group» and «aquota.user» files on the root of the drive

104.5 Manage file permissions and ownership (3)

In Linux the listing of permissions are the user-owner (u) of the file, the group the owner belongs to (g) and the group others-everyone (o), the string (a) applies permission to ALL

• chmod ugo+w file1.txt ;u(users) g(grouip) o(other), set every to write for the given file
• chmod u+rw,g+rw,o-rwx ;set user and the group to be rw but remove the rw access from the other

Octal Notation, most common values:

• 7 = rwx 777 = a+rwx = rwxrwxrwx
• 6 = rw 755 = u+rwx,g=rx,o=rx = rwxr-xr-x
• 5 = rx 644 = u=rw,g=r,o=r = rw-r—r—
• 4 = r 700 = u=rwx,g-rwx,o-rwx = rwx——

-(file) d(directory) | owner | group permissions | other, anybody who is not the owner or does not belong to the group

The umask is the default permission that are assigned when you create a file or directory ( umask -S to see values), the umask is set in /etc/profile /etc/bashrc or /etc/login.defs and the way it works is by subtracting the value from the default all-permissions, for example if you execute the umask 077, the effective permissions created during the session would be:

• 600 for files (rw only, as-)
• 777 for directories (rwx)

The special permissions are set on the «executable» bit of the relevant user, group or everybody (other). The sticky bit is set ONLY in the other field. When you see the lower case «s» it means that the executable bit has also been set, while if you see only the capital «S» the executable bit is not set

SUID ;[set user id](value of 4) executes with permissions of the file owner, like for example ping, or the /usr/bin/passwd file, that runs with the root credentials even if you are a user (it will display with an «s» e.g. passwd = -rwsr-xr-x.

GUI ;(value of 2) the file is executed with the permission of the group ID, this option is not used very much, however any new files created on the folder will be executed with the permission of the group

Sticky bit (value of 1);nowadays the sticky bit is set on a folder, and that means that the files inside that folder can only be deleted by the owner

setfacl ;use this command to configure extended ACL permissions to a file (chmod cannot configure ACL), use getfacl filename to get current info of a file/directory, using getfacl -t filename gives you a different view. Linux permission have got some limitations, only 1 x user and only 1 x group can own the file/directory, while other could be anybody; this is where ACL comes into place

• setfacl -m user:bob:rwx filename ;this will set tbob as a user in addition to whichever user is the user of the ‘filename’ file. if you type ls-l that will show you a ‘+’ at the end, inficationg ACL permissions have been set
• setfacl -m u:bob:rwx,g:accouting:rw filename ;another way of doing it, the -m switch is for modify
• ACL are turn on on CentOS but only on the / partition!
• To remove ACL use these switches
  • -x remove specific ACLs; setfacl -x bob filename or setfacl -x group:marketing filename
  • -k remove all default ACLs
  • -b remove all ACLs

chown ;change the ownership of files chown user:group -R /myfolder will assign the ownership to the user and the group, use it without the -R if you want to take ownership of a file only

chattr ;if you set the immutable feature on, those files cannot even be deleted by the root chattr -i scritp.sh

104.6 Create and change hard and symbolic links (2)

Hard links are files pointing to the inode of the hard drive, you cannot create hard links in between 2 x disks

Soft links are just shortcuts in between files, you update the master and the shortcut gets updated. If you move the original file (mv command) the soft link will be broken

104.7 Find system files and place files in the correct location

locate ;similar to find, a bit quicker but works by creating a database which is updated once a day ( sudo updatedb updates de locate database)

• sudo vi /etc/updated.conf ;contain the setting for the locate database

which ;use to find binary files (#which pint)

whereis ;similar to «which», also find the binary file together with the man pages

105.1 Customize and use the shell environment (4)

Working on the command line, Alt+b moves one character back, while Alt+f moves it forward. The shell allows you to execute OS command by sending them to the kernel; when a command is entered in the terminal, the command execution order is the following:

1. The Shell will check if it’s a built-in command
2. The Shell will check if it’s an alias of a command configured by the OS or another user
3. The Shell will check if the command is on the hard drive, and will display command not found if all of the above is false

There is a standard to write scripts, so they can be portable, called POSIX-compliant shell ;bash shell is currently version 4 but there are other popular shells like cash (for debian, using less memory and faster execution than bash), fish -friendly interactive shell- is designed for interactivity and only available in third-party EPEL software repository, Zsh is very similar to bash but with a wonderful command completion. Most of the shells can be installed using yum install -y zsh / ksh /csh To switch shell use chsh then type the path /bin/zsh for the desired shell, then logout and log back in again

• Environment variables are defined for the current shell and inherited by any child shells or process, printenv to see those ones
• Shell variables are contained exclusively within the shell in which they were define; use set to see shell variables as well as functions, to see them without the functions use set -o posix and then set | less ;to include a variable as part of the environment use the export command (use the export -n to unset it); if what you want is to unset shell variables use the unset command instead. You can see all the sell options by typing set -o (or set +o) and if you like you can for example set posix off by typing set -o posix ,however the changes of this bash variables values only last for your current logon, once you log out all is back to normal, to make these configuration persistent we need to add them to one of the bash files

Logon Process ;when a user logs into an interactive shell, the Linux OS process the logon as follows:

/etc/profile ;this file loas, which in term load all the scripts under /etc/profile.d/*.sh ; both contain System environment and Shell variables

/.bash_profile ;this file loads, containing the user environment and shell variables; in CentOS this file also execute the

/bashrc that contain the user aliases and shell functions specific to the user

/etc/bashrc ;this file loads, containing the System wide aliases and Shell functions

When you need to add aliases to the

/.bashrc file you must add them to the end of the file, after /etc/bashrc has been called so that you override systemwide aliases and functions

The commands in Linux normally are Command + Options + Arguments. When a login shell launches, it reads the files in this order:

/etc/profile first ;either it exist or not

Non-login shell will read only the /etc/bash.bashrc and then the

/.bashrc files. To change shells in the middle of your session use the dot (.) command . bashrc will switch you to the bashrc shell. The PS1=» \$» environment variable can be set in either /etc/profile or /etc/bash_profile, this will cause the prompt to look like user \u @ host \h with \s and \w representing shell name and pwd respectively

Any content found in the /etc/skel directory will be copied to the home directories of newly created users; the file

/.bash_logout runs when you exit from a session

Run echo $BASH_VERSION to find out what release of bash you have (4.2.46 r2 at the time of writing). Bash stands for Bourne Again BAsh, since 1989 https://tiswww.case.edu/php/chet/bash/bashtop.html

#!/interpreter ;this is called the «Shebang», (the command interpreter) meaning that this is the interpreter (executable) that will run that particular script; if you don’t specify a «Shebang» interpreter, the script will be execute on whichever bash you are

Conditionals and loops

• if [conditional statement] then else fi
• for VAR in [some sequence] do done ;in this loop the sequence statement gets accumulative to the VAR, if 1 then 2, then 3, etc

While loops ; while [conditional loop] do done ;conditional loops can be dangerous and it may loop in the infinite if not properly coded

declare ;when you declare variables you can use -i (specify it is an integer, therefore can do math) or -r (read-only)

alias as=»ls -lha;cat timezone» ;this alias will perform those two commands, to remove it use unalias as .Functions are a bit more complicated, like for example function listd() < ls -lha $1>;to remove an alias use unalias and to remove a function use unset

105.2 Customize or write simple scripts (4)

Testing values, notice the operator NOT equal to ( != ), you can use other ones like lt (less than) gt (greater than), etc

The if statement works well if there are only 2 choices to choose from, but if you have 3 or more choices, you better use the «case» statement (notice that it ends with the clause esac, meaning case in reverse)

Loops are scripts that required a repeated action like while or until a certain condition is met

This next example is an «until» script; «while» continues until the condition is TRUE, and until continues «until» the condition IS NOT true

for loop ;a loop constructed with the for will take all the values returned by a command and act on them one at a time

seq ;the sequence command produces number in sequence, for example seq 5 14

exec ;the exec command is use to launch Linux shell commands written in other languages like C, PHP or Perl, for example exec(«dir», $output, $return);

105.3 SQL data management (2)

Run the following to install MySQL in Ubuntu (do not forget the ‘sudo’ in front of the commands):

#MySQL -u sa -p food ;this will use the username «sa» to connect to a database «food» and will prompt for a password «-p» to login to it

In MySQL interactive mode you can use the following commands:

• mysqlL> SHOW TABLES;
• mysql> SELECT * FROM fruit;
• mysql> SELECT name FROM fruit; [select the column name from table fruit]
• mysql> SELECT * FROM fruit WHERE color = «yellow»;
• mysql> SELECT * FROM fruit JOIN vegetable ON fruit.color = vegetable.color;

106.1 Install and configure X11 (2)

xorg.conf file is a historically file that existed in /etc/X11 and by default is not in the new distributions, nowadays everything is automatically detected. Back on the day computers used to be so slow that there was a font server to load the fonts and renders them on the computer. If in the xorg.conf file the driver for the card is «visa», most likely means that the card is not supported.

• xwininfo ;tool that give you info about the monitor/graphics setting
• xdpyinfo ;another tool with tons of information
• xhost ;if you type «xhost +» that means that the X11 can listen to connections, however the nowadays X11 server do not longer listen to connections!
  • xhost +192.168.0.11 ;if the X Server was listening for external connections, the computer 192.168.0.11 will be able to connect
  • export DISPLAY=192.168.0.130:0.0 ;old stuff, how everything used to work, but this used to display your screen to the IP 192.168.0.130
• ssh -X manuel@uranus ;this command is used nowadays! once you logon to URANUS you go and type «xeyes» and the graphical display in Uranus will be displayed on your computer

Display Managers; there are a few (lightdm ,KDM, XDM, GDM). with Ubuntu you get lightdm, the config files for it are in «/etc/lihgtdm» and «/etc/lightdm.conf.d». To make changes to the configuration edit «/etc/lightdm/lidghdm.conf.do/10-xubuntu.conf»

• apt-cache search lightdm | grep greeter ;shows you the greeter that you can use in Ubuntu
• Edit «/etc/default/grup» and set to ‘text’ the GRUB_CMDLINE_LINUX_DEFAULT=»text» , then issue «sudo update-grub» and the Ubuntu will default to text mode. For CentoOS you just edit the default.target

106.2 Setup a display manager (1)

In CentOS only GNOME3 (default desktop) and KDE 4.10 are supported, though you can install many more desktop by using repositories. Cinamon desktop comes from GNOME2; other popular is Xfce which uses less memory and CPU cycles than other desktops. Go to http://www.kde.org/applications to get new goodies, but ensure you donwload them using yum so you’re using the right repos

GNOME3 uses hot corners, you put your mouse in a corner and the «Overview» is launched, this is a feature that GNOME Classic does not have. To install MATE Desktop you need to add a third-party repository, like this:

106.3 Accessibility (1)

Description: Demonstrate knowledge and awareness of accessibility technologies.

Key Knowledge Areas: Basic knowledge of keyboard accessibility settings (AccessX), Basic knowledge of visual settings and themes, Basic knowledge of assistive technology (ATs)

• Sticky/Repeat Keys
• Slow/Bounce/Toggle Keys
• Mouse Keys
• High Contrast/Large Print Desktop Themes
• Screen Reader
• Braille Display
• Screen Magnifier
• On-Screen Keyboard
• Gestures (used at login, for example GDM)
• Orca
• GOK
• emacspeak

107.1 Manage user and groups accounts and related system files (5)

Execute su (substitute ) to change the user (su -l root). It is obviously NOT GOOD PRACTISE to logon with the root account.

• sudo -k ;give up the privileges after using sudo, logging out and leaving the user logon
• sudo -s ;switch over to fully root account, notice how the prompt changes to the hash sign (#) so you know you are root, type ‘exit’ to go back the ($) user-sign
• Just remember when using su that the echo $PATH would be pointing to the logged on user

Type «passwd» to change your own password. Type «sudo passwd bob» to change bob password.

lastb ;display a list of the most recent bad login attempts

useradd ; -m (create home directory), -d /home/user (specify home directory) -G group1,group2 (add user to groups) -c string (comment, puts the name of the user); type useradd alone to see all the options

usermod ;almost as above, but modify the user like -L (lock account) -U (unlock account) -aG (add user to additional group)

userdel ; with the -r you delete the files in home folder along with the account

cat /etc/passwd ;all users need to be able to read this file; the password will appear as «x», meaning that you need to look at the shadow file to see the password. The /etc/shadow file is only readable by the root user, the password will show as encrypted, but this is where the system looks for the passwords.

/etc/passwd

user:x:1001:2001:userComments:/home/user:/bin/bash

/etc/shadow

Password hash; ! if the password has not been set or the account is locked

1=md5; 21 or 2y =Blowfish; 5=SHA256; 6=SHA512

Column
1	Username	Username
2	Password enconding file, an ‘x’ indicates the password is stored in /etc/shadow
3	Numeric userID (0-900 for System Accounts, 100+ for user accounts)	Days between 1-Jan-1970 and the most recent password change
4	Numeric primary groupID	Number of days before the password can be change; 0 means it can be change at anytime
5	Comment Field to store comments	Number of days before the password must be change, 99999 is about 274 years
6	Home directory	Number of days to warn the users of an expiring password, default 7 days
7	Default login shell	Number of days after the password has expired that the account is disabled, default 7 days
8	Number of days between 1-Jan-1970 and the date the account was disabled

Accounts with the shell /bin/false cannot login, this normally relates to deamon or service accounts; cat etc/shells shows you the shells installed on the system. The hashtag of the /etc/shadow file can be changed using the /etc/login.defs or by using the autoconfig command. Salt is a set of ramdon characters that are appended to the password before it is hash, thus making it impossible for diccionary attacks, note that the portion to the left-hand-side of the dollar sign is the salt; use the command gpasswd to set a password for a group

yum install shadow-utils ;just in case the shadow utilities are not installed; the commands pwconv and pwunconv will allow you to convert the passwd to shadow file; grpconv and grpunconv for the gpasswd file

chage (for CHange AGe), tool use to change the password expiration for users

cat /etc/group ;shows the group memebership. The /etc/gshadow is similar to /etc/shadow ; the third column gives you the groupID, while the 4th columns shows you the users that belong to that group. Groups can have password, to prevent users from adding themselves to the group

cat userpassword | chpasswd ;changes all the users password with whichever password is in the «userpassword» file, that file must be in the format username:password, and you can use the -e switch to tell chpasswd that the file is encrypted

etc/security/pwquality.conf ;configure the password requirements using this file

User Limits

ulimit ;type with -a to see all the limits that have been set, how many open files, etc

To make limits changes that are global for everybody edit /etc/security/limits.conf so you can comment out and set limits per users or groups (nproc is number of processes that are allowed)

who ;it shows you who is login into the system (working in the console) whoami ;tells you who you are, but use the logname to find out who logs on to the system. id ; shows you more info about the user, like group memebership, etc, type groups to see the groups of the current user

w ;it shows what the users are doing in the system

last ;shows you what users have been doing in the system, as well as the reboot times, etc, great command, use it with the -F to see Full info

lastlog ;shows you last logged on users; lsof ;list open files, use it with the -i switch

visudo ;edit this file to manage the access users have when using the sudo commands, the lines with % are active, and notice that members of the group wheel have got ALL access

Enter user’s password

Elevates privileages for one command

All command execution is logged

Granular permission system; only allows certain people certain privileages

Enter root’s password

Elevates privileages for whole session

No accountability; all users who switch are root

All-powerful for anyone who has the password

107.2 Automate system administration tasks by scheduling jobs (4)

Cron jobs could be specific to the user or run by the system. Both types of Cron jobs use the same format of the 5 digits-fields, which is as follows: [minute 0-59] [hour 0-23] [day of the month 1-31] [month 1-12] [day of the week, 0-7, with both 0 and 7 being Sunday], leave all asterisk [*] to apply for all digits. The sixth field is the job to run while the seventh stipulates the cron.daily, hourly, monthly or weekly. If you add the @reboot the job will run every time the system reboots, if you type @daily it will do it daily

• 0 15 3 2 3 = 15:00 3rd February Sunday
• */ * * * * = execute the tasks every 5′
• 6 3 * * 1-5 = weekedays at 03:06

The cron jobs users are stored in /var/spool/cron/username You can add users to the security files of cron to allow or denied access accordingly (follow the principle of most-restrictive always, deny everybody and allow only the users you want). Create these files accordingly to provide security, and note that if the cron.allow file exist but nobody is on it, then nobody will be able to run cron jobs; if none of those files exists, no users can run crontabs

There is a online cron job generator at http://crontab-generator.org to help you out. The file /etc/security/access.conf controls access to the crond sevice using PAM -Pluggeable Authentication Modules — The example below entry on the access.conf file allow all users to create cron jobs with the exception of root

System crontab; edit the system-wise crontab by editing the file (same name as the program) located in /etc/crontab, where you can specify the user under which credentials the task will run. Applications that have schedules task are automatically saved in the folder /etc/crontab/crontab.d under their own files. Note that inside the ‘cron.daily’, ‘cron.weekly’, etc there are just executable files that run at these regular intervals.

Anacron ;in case there is a power cut, etc, and the schedule tasks don’t run, the anacron command takes charge. It works by taking a timestamp (located in /var/spool/anacron ) of the tasks and checks to see if there is a miss entry, if there is it runs the schedule job. The anacron program is kept in /usr/sbin/anacron, and it needs to be monitoring the schedules on cron.daily, etc by adding an entry on the /etc/contab system-wide file, if the anacron program is not there monitoring those folders it will not keep track of the timestamps

at ;is a service use only for jobs that run at certain times (for recurring jobs use crond) or in the case of a batch job when the CPU average drops below 0.8 ;at supports general syntax like tomorrow, midday or now, even teatime (specifically 4pm)

batch ;batch jobs only run when the system is not busy, when the CPU average is below 0.8

107.3 Localisation and internationalization (3)

The told «tzconfig» command is not longer use. The command tzselect is used to set the time for the user, appending the resulting entry into the profile file of the user. To make the time zone globally for the system, do as follows (this works for any system, Debian, Ubuntu, etc):

locale ;shows you the language setting of the system. To modify any of the entries type:

ASCII, 128 characters in English (all text), but it had lots of limitations, so it progressed to ISO-8859 (more characters and supported more languages), but also has lots of issues with «sets» of characters, so it got evolved to UTF-8 (Universal Transformation Format, 8 bits), which is the daddy of them old, basically universal!

iconv ;convert back and forth from ASCII to UTF-8, ISO, etc

108.1 Maintain system time (3)

Visit /etc/adjtime to check if the computer is using the hardware clock or time zone (UTC)

The hwclock -w OR hwclock -systohc sets the Hardware clock from the current system time

The hwclock -s OR hwclock —hctosys set the System time from the hardware clock, you can also use hwclock -hctosys

Do «apt-get install ntp» or «yum install ntp» depending on the distro. Edit /etc/ntp.conf and remove the Ubuntu/centos pools and replace them by your time zone ones found at www.pool.ntp.org Then execute «service ntp restart» to make the change effective

sudo yum install chrony ;install the NTP client

108.2 System logging (3)

Any program that happens to log messages will send its messages to the logger, and identify these messages with these categories (so that every message sent to the SysLog is tagged with these 2 categories):

• Facilities ;auth,user,kerm,cron,daemon,mail,local1,local2
• Priorities ;emer,panic,alert,crit,errr, warn, notice, info, debug

facility.priority > action ; for example a user.info > /var/log/user.log but a kern.panic > @root (screen notification) OR *.* > /var/log/messages. If you execute «mail.info» all the priorities above info (emer,panic,etc) are going to be logged

In ubuntu edit the conf file under /etc/rsyslog.d to see the seeting regarding facilities+priorities for services. To make your linux machine a syslog server, edit /etc/defaults/rsyslog and add -r to RSYSLOGD_OPTIONS_»-r» (remote) and it will listen for incoming messages

In CentOS systemd has its own binary logging program called journalctl , where the logs are kept in binary code, run journalctl MESSAGE= to see if there is any message there; configure it by /etc/systemd/journald.conf

The file /etc/rc.d/init.d/syslog is used to start the System Log Daemon (syslog)

yum install gnome-system-monitor ;install something like task manager for the GUI interface

108.3 Mail Transfer Agent (MTA) basics (3)

sendmail ;the oldest program to send mail, there are new ones now (qmail, exim, postfix) but all of them are backwards compatible with sendmail

See the /etc/aliases to see the alias of the system, and if you make any changes to the aliases run newaliases so the system detects the changes

If you create a file .forward in the home directory of the user and just type the name of the user, mail will be forwarded from user to user. The purpose of the

./forward file in a user’s home directory is to redirect mail for a single user

108.4 Manage printers and printing (2)

The CUPS (developed by Apple) is done on the web browser, to visit it for the system go to http://localhost:631 and logon with the 1st user created on the system, or a user who is a member of the «LP Admin Group»

Edit the file /etc/cups/cupsd.conf if you like to make the changes on the cmd instead that on the browser; the printers.conf contain the configuration of the printers, to edit it first ensure the cups service is stopped

You can use cupsenable printername , cupsdisable printername, and for the jobs that you want to deal in the queue you can use cupsreject printername, cupsaccept

109.1 Fundamentals of internet protocols (4)

ifconfig is now obsolete, use ip (iproute in RHEL) instead. Predictable Network Interface Names, is like ens033 (en=ethernet, wl=wireless, p=pci, u=usb, s33, is slot number). The interface needs to be set as manageable before Network Manager can actually manage it, issue nmcli device and checkout the STATE

ethtool ;query or control network driver and hardware setting , wol, etc

Network Manager ;unless told not to, it will always manage all interfaces under the network-scripts folder, to tell it not no add the line NM_CONTROLLED=no to the relevant interface, then restart the network.service. For Ubuntu edit the /etc/NetworkManager/NetworkManager.conf

Knowledge of the differences between private and public “dotted quad” IP addresses. Knowledge about common TCP and UDP ports and services (20, 21, 22, 23, 25, 53, 80, 110, 123, 139, 143, 161, 162, 389, 443, 465, 514, 636, 993, 995)
Knowledge about the differences and major features of UDP, TCP and ICMP. Knowledge of the major differences between IPv4 and IPv6. Knowledge of the basic features of IPv6

• /etc/services
• IPv4, IPv6
• Subnetting
• TCP, UDP, ICMP

This is an example of a ifcfg-ens192 config file in the network-scripts folder on a CentOS 7, for a static IP

To use DHCP just remove the last 3 fields and set the BOOTPROTO=dhcp

In both cases (dhcp or none) don’t forget to edit the /etc/resolv.conf file, and set the «nameserver 8.8.8.8» or whichever your DNS server is

109.2 Basic network configuration (4)

Cat the /etc/services to see the entirely list of ports the system recognize, some of these ports might include: port 53 (DNS), port 123 (NTP), port 139 (NetBIOS), port 143 (IMAP), port 161-162 (SNMP), por 636 (LDAPS), port 993 (IMAPS), port 995 (POP35)

Historically, the DNS entries have been kept in /etc/resolv.conf

On a CentOS configuration, visit the folder/etc/sysconfig/network-scripts/ and see the files in there, edit ifcfg-eth0 to change the IP on permanent basis. To modify the gateway use /etc/sysconfig/network

On an Ubuntu visit /etc/network/interfaces where all the stuff related to the nics (how many nics you have) and gateway are kept, lovely! Ensure however there is only 1 x «iface» stanza for each interface in the /etc/network/interfaces file, just like this for example:

• Edit /etc/hostname (and then reboot or logout) to change the hostname, use hostnamectl if the system has systemctl, you can also use nmcli general hostname to query it or nmcli general hostname server1 to change it
• Edit /etc/hosts to add additional DNS entries, like local intranet etc, that will be query before asking the legitimate DNS servers
• The command route shows you gateway information, execute route add default gw 192.168.0.2 to add default gateway; user traceroute and/or tracepath
• User ping6 to execute ICMP packets by the ipv6 protocol
• Use dig to query the DNS entries, similar to nslookup; execute dig @172.19.0.32 google.com to query a different server for the answer
• Edit the file /etc/nsswitch.conf to arrange the order on which the Linux query the DNS servers, useful if you don’t want the System to look at the host file first

To restart the network serive on different systems, do as follows:

• For Debian: /etc/init.d/networking restart
• For Red Hat: either service network restart OR /etc/init.d/network restart OR systemctl restart network
  
• For Ubuntu: sudo /etc/init.d/networking restart

Routing ;to find your gateway type route -n and to delete use route del default gw 192.0.0.1

tcpchk ;this command will test your TCP wrappers configuration, it will verify the syntax of /etc/hosts.allow and /etc/hosts.deny

iptables ;tool for packet filtering and NAT; use -A or —append to append one or more rules at the end of the selected chain; the rules are evaluated from TOP to BOTTOM so put the more specific ones at the top

109.3 Basic network troubleshooting (4)

We know that «ls -l /bin/ping» will show that the ping command has the user bit set, so that program can run with root credentials to matter which user invokes it

netstat -tuna ;you definitely won’t forget this command, it tells you the ports that are listening, not big problem is the local address show as the loopback

nmap ;use to scan a gateway to server to check the firewall, you probably need to install it on the 1st use

sudores ;to edit the file /etc/sudoers you actually need to type visudo ,in Ubuntu the 1st user created on the system is automatically added to the sudo group

ss ;utility to check sockets, issue ss -tl to see the LISTEN ports, use ss -tln to see the port number; ss -tn for ESTABLISHED connections; check the man pages for this tool, it’s great

109.4 Configure client side DNS (2)

Query remote DNS servers. Configure local name resolution and use remote DNS servers. Modify the order in which name resolution is done

110.1 Perform security administration tasks (3)

110.2 Setup host security (3)

Awareness of shadow passwords and how they work. Turn off network services not in use. Understand the role of TCP wrappers

110.3 Securing data with encryption (3)

You create a Private Key and make it all secure (lock it away), while the Public Key can be shown to absolutely everyone; users can encrypt a file with your Public Key and only you can open it with your Private Key. To verify identity, users can decrypt files with your Public Key, so they know you have encrypt them (and only you) with your Private Key

/home/username/.ssh ;this folder contains the keys use by ssh, check the file «known_hosts»

/etc/ssh ;contain the different keys of the System, the file ssh_host_rsa_key.pub is the public key for ssh, send publically to the user when he/she tries to connect (the ssh_host_rsa_key file is actually the private key)

gpg ;tool to create encrypted keys that can be share among users. To create a key starts the wizard by executing gpg —gen-key to create a public key with your real details, it may take 5′ minutes or so to generate the key

After the above, username2 can create a file (touch file3) and encrypt it using gpg —out file4 recipient username1 —encrypt file3 and that file4 can only be open by username1, if you then try to cat the file4 it will all garbish. To decrypt the file at username1 use gpg —out file5 —decrypt file4 and you’ll be prompted for the passphrase to decrypt it

Источник

sudo	su