In this chapter, we will discuss in detail about process management in Unix. When you execute a program on your Unix system, the system creates a special environment for that program. This environment contains everything needed for the system to run the program as if no other program were running on the system.
Whenever you issue a command in Unix, it creates, or starts, a new process. When you tried out the ls command to list the directory contents, you started a process. A process, in simple terms, is an instance of a running program.
The operating system tracks processes through a five-digit ID number known as the pid or the process ID. Each process in the system has a unique pid.
Pids eventually repeat because all the possible numbers are used up and the next pid rolls or starts over. At any point of time, no two processes with the same pid exist in the system because it is the pid that Unix uses to track each process.
Starting a Process
When you start a process (run a command), there are two ways you can run it −
Foreground Processes
Background Processes
Foreground Processes
By default, every process that you start runs in the foreground. It gets its input from the keyboard and sends its output to the screen.
You can see this happen with the ls command. If you wish to list all the files in your current directory, you can use the following command −
This would display all the files, the names of which start with ch and end with .doc −
The process runs in the foreground, the output is directed to my screen, and if the ls command wants any input (which it does not), it waits for it from the keyboard.
While a program is running in the foreground and is time-consuming, no other commands can be run (start any other processes) because the prompt would not be available until the program finishes processing and comes out.
Background Processes
A background process runs without being connected to your keyboard. If the background process requires any keyboard input, it waits.
The advantage of running a process in the background is that you can run other commands; you do not have to wait until it completes to start another!
The simplest way to start a background process is to add an ampersand (&) at the end of the command.
This displays all those files the names of which start with ch and end with .doc −
Here, if the ls command wants any input (which it does not), it goes into a stop state until we move it into the foreground and give it the data from the keyboard.
That first line contains information about the background process — the job number and the process ID. You need to know the job number to manipulate it between the background and the foreground.
Press the Enter key and you will see the following −
The first line tells you that the ls command background process finishes successfully. The second is a prompt for another command.
Listing Running Processes
It is easy to see your own processes by running the ps (process status) command as follows −
One of the most commonly used flags for ps is the -f ( f for full) option, which provides more information as shown in the following example −
Here is the description of all the fields displayed by ps -f command −
Sr.No.
Column & Description
1
User ID that this process belongs to (the person running it)
Parent process ID (the ID of the process that started it)
CPU utilization of process
Process start time
Terminal type associated with the process
CPU time taken by the process
The command that started this process
There are other options which can be used along with ps command −
Sr.No.
Option & Description
1
Shows information about all users
Shows information about processes without terminals
Shows additional information like -f option
Displays extended information
Stopping Processes
Ending a process can be done in several different ways. Often, from a console-based command, sending a CTRL + C keystroke (the default interrupt character) will exit the command. This works when the process is running in the foreground mode.
If a process is running in the background, you should get its Job ID using the ps command. After that, you can use the kill command to kill the process as follows −
Here, the kill command terminates the first_one process. If a process ignores a regular kill command, you can use kill -9 followed by the process ID as follows −
Parent and Child Processes
Each unix process has two ID numbers assigned to it: The Process ID (pid) and the Parent process ID (ppid). Each user process in the system has a parent process.
Most of the commands that you run have the shell as their parent. Check the ps -f example where this command listed both the process ID and the parent process ID.
Zombie and Orphan Processes
Normally, when a child process is killed, the parent process is updated via a SIGCHLD signal. Then the parent can do some other task or restart a new child as needed. However, sometimes the parent process is killed before its child is killed. In this case, the «parent of all processes,» the init process, becomes the new PPID (parent process ID). In some cases, these processes are called orphan processes.
When a process is killed, a ps listing may still show the process with a Z state. This is a zombie or defunct process. The process is dead and not being used. These processes are different from the orphan processes. They have completed execution but still find an entry in the process table.
Daemon Processes
Daemons are system-related background processes that often run with the permissions of root and services requests from other processes.
A daemon has no controlling terminal. It cannot open /dev/tty. If you do a «ps -ef» and look at the tty field, all daemons will have a ? for the tty.
To be precise, a daemon is a process that runs in the background, usually waiting for something to happen that it is capable of working with. For example, a printer daemon waiting for print commands.
If you have a program that calls for lengthy processing, then it’s worth to make it a daemon and run it in the background.
The top Command
The top command is a very useful tool for quickly showing processes sorted by various criteria.
It is an interactive diagnostic tool that updates frequently and shows information about physical and virtual memory, CPU usage, load averages, and your busy processes.
Here is the simple syntax to run top command and to see the statistics of CPU utilization by different processes −
Job ID Versus Process ID
Background and suspended processes are usually manipulated via job number (job ID). This number is different from the process ID and is used because it is shorter.
In addition, a job can consist of multiple processes running in a series or at the same time, in parallel. Using the job ID is easier than tracking individual processes.
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Linux Directory Structure and Important Files Paths Explained
For any person, who does not have a sound knowledge of Linux Operating System and Linux File System, dealing with the files and their location, their use may be horrible, and a newbie may really mess up.
This article is aimed to provide the information about Linux File System, some of the important files, their usability and location.
Linux Directory Structure Diagram
A standard Linux distribution follows the directory structure as provided below with Diagram and explanation.
Linux Directory Structure
Each of the above directory (which is a file, at the first place) contains important information, required for booting to device drivers, configuration files, etc. Describing briefly the purpose of each directory, we are starting hierarchically.
/bin : All the executable binary programs (file) required during booting, repairing, files required to run into single-user-mode, and other important, basic commands viz., cat, du, df, tar, rpm, wc,history, etc.
/boot : Holds important files during boot-up process, including Linux Kernel.
/dev : Contains device files for all the hardware devices on the machine e.g., cdrom, cpu, etc
/etc : Contains Application’s configuration files, startup, shutdown, start, stop script for every individual program.
/home : Home directory of the users. Every time a new user is created, a directory in the name of user is created within home directory which contains other directories like Desktop, Downloads, Documents, etc.
/lib : The Lib directory contains kernel modules and shared library images required to boot the system and run commands in root file system.
/lost+found : This Directory is installed during installation of Linux, useful for recovering files which may be broken due to unexpected shut-down.
/media : Temporary mount directory is created for removable devices viz., media/cdrom.
/mnt : Temporary mount directory for mounting file system.
/opt : Optional is abbreviated as opt. Contains third party application software. Viz., Java, etc.
/proc : A virtual and pseudo file-system which contains information about running process with a particular Process-id aka pid.
/root : This is the home directory of root user and should never be confused with ‘/‘
/run : This directory is the only clean solution for early-runtime-dir problem.
/sbin : Contains binary executable programs, required by System Administrator, for Maintenance. Viz., iptables, fdisk, ifconfig, swapon, reboot, etc.
/srv : Service is abbreviated as ‘srv‘. This directory contains server specific and service related files.
/sys : Modern Linux distributions include a /sys directory as a virtual filesystem, which stores and allows modification of the devices connected to the system.
/tmp :System’s Temporary Directory, Accessible by users and root. Stores temporary files for user and system, till next boot.
/usr : Contains executable binaries, documentation, source code, libraries for second level program.
/var : Stands for variable. The contents of this file is expected to grow. This directory contains log, lock, spool, mail and temp files.
Exploring Important file, their location and their Usability
Linux is a complex system which requires a more complex and efficient way to start, stop, maintain and reboot a system unlike Windows. There is a well defined configuration files, binaries, man pages, info files, etc. for every process in Linux.
/boot/vmlinuz : The Linux Kernel file.
/dev/hda : Device file for the first IDE HDD (Hard Disk Drive)
/dev/hdc : Device file for the IDE Cdrom, commonly
/dev/null : A pseudo device, that don’t exist. Sometime garbage output is redirected to /dev/null, so that it gets lost, forever.
/etc/bashrc : Contains system defaults and aliases used by bash shell.
/etc/crontab : A shell script to run specified commands on a predefined time Interval.
/etc/exports : Information of the file system available on network.
/etc/fstab : Information of Disk Drive and their mount point.
/etc/lilo.conf : lilo bootloader configuration file.
/etc/hosts : Information of Ip addresses and corresponding host names.
/etc/hosts.allow : List of hosts allowed to access services on the local machine.
/etc/host.deny : List of hosts denied to access services on the local machine.
/etc/inittab : INIT process and their interaction at various run level.
/etc/issue : Allows to edit the pre-login message.
/etc/modules.conf : Configuration files for system modules.
/etc/motd : motd stands for Message Of The Day, The Message users gets upon login.
/etc/mtab : Currently mounted blocks information.
/etc/passwd : Contains password of system users in a shadow file, a security implementation.
/etc/printcap : Printer Information
/etc/profile : Bash shell defaults
/etc/profile.d : Application script, executed after login.
/etc/rc.d : Information about run level specific script.
/etc/rc.d/init.d : Run Level Initialisation Script.
/etc/resolv.conf : Domain Name Servers (DNS) being used by System.
/etc/securetty : Terminal List, where root login is possible.
/etc/skel : Script that populates new user home directory.
/etc/termcap : An ASCII file that defines the behaviour of Terminal, console and printers.
/etc/X11 : Configuration files of X-window System.
/usr/bin : Normal user executable commands.
/usr/bin/X11 : Binaries of X windows System.
/usr/include : Contains include files used by ‘c‘ program.
/usr/share : Shared directories of man files, info files, etc.
/usr/lib : Library files which are required during program compilation.
/usr/sbin : Commands for Super User, for System Administration.
/proc/cpuinfo : CPU Information
/proc/filesystems : File-system Information being used currently.
/proc/interrupts : Information about the current interrupts being utilised currently.
/proc/ioports : Contains all the Input/Output addresses used by devices on the server.
/proc/meminfo : Memory Usages Information.
/proc/modules : Currently using kernel module.
/proc/mount : Mounted File-system Information.
/proc/stat : Detailed Statistics of the current System.
/proc/swaps : Swap File Information.
/version : Linux Version Information.
/var/log/lastlog : log of last boot process.
/var/log/messages : log of messages produced by syslog daemon at boot.
/var/log/wtmp : list login time and duration of each user on the system currently.
That’s all for now. Keep connected to Tecmint for any News and post related to Linux and Foss world. Stay healthy and Don’t forget to give your value-able comments in comment section.
Linux Directory Structure
