Linux Commands and arguments
Introduction
In this session, we have covered the shell expansion by taking a close look at commands and arguments. Knowing shell expansion is important because many commands on your Linux system are processed and most likely changed by the shell before they are executed.
The command line interface or shell used on most Linux systems is called bash, which stands for Bourne again shell. The bash shell incorporates features from sh (the original Bourne shell), csh (the C shell), and ksh (the Korn shell).
This chapter frequently uses the echo command to demonstrate shell features. The echo command is very simple: it echoes the input that it receives.
arguments
One of the primary features of a shell is to perform a command line scan. When you enter a command at the shell’s command prompt and press the enter key, then the shell will start scanning that line, cutting it up in arguments. While scanning the line, the shell may make many changes to the arguments you typed.
This process is called shell expansion. When the shell has finished scanning and modifying that line, then it will be executed.
white space removal
Parts that are separated by one or more consecutive white spaces (or tabs) are considered separate arguments, any white space is removed. The first argument is the command to be executed, the other arguments are given to the command. The shell effectively cuts your command into one or more arguments.
This explains why the following four different command lines are the same after shell expansion.
The echo command will display each argument it receives from the shell. The echo command will also add a new white space between the arguments it received.
single quotes
You can prevent the removal of white spaces by quoting the spaces. The contents of the quoted string are considered as one argument. In the screenshot below the echo receives only one argument.
double quotes
You can also prevent the removal of white spaces by double quoting the spaces. Same as above, echo only receives one argument.
Later in this tutorial, when discussing variables we will see important differences between single and double quotes.
echo and quotes
Quoted lines can include special escaped characters recognized by the echo command (when using echo -e). The screenshot below shows how to use \n for a newline and \t for a tab (usually eight white spaces).
The echo command can generate more than white spaces, tabs, and newlines. Look at the man page for a list of options.
commands
external or builtin commands ?
Not all commands are external to the shell, some are builtin. External commands are programs that have their own binary and reside somewhere in the file system. Many external commands are located in /bin or /sbin. Builtin commands are an integral part of the shell program itself.
type
To find out whether a command given to the shell will be executed as an external command or as a builtin command, use the type command.
As you can see, the cd command is builtin and the cat command is external.
You can also use this command to show you whether the command is aliased or not.
running external commands
Some commands have both builtin and external versions. When one of these commands is executed, the builtin version takes priority. To run the external version, you must enter the full path to the command.
which
The which command will search for binaries in the $PATH environment variable (variables will be explained later). In the screenshot below, it is determined that cd is builtin, and ls, cp, rm, mv, mkdir, pwd, and which are external commands.
aliases
create an alias
The shell allows you to create aliases. Aliases are often used to create an easier to remember the name for an existing command or to easily supply parameters.
abbreviate commands
An alias can also be useful to abbreviate an existing command.
default options
Aliases can be used to supply commands with default options. The example below shows how to set the -i option default when typing rm.
Some distributions enable default aliases to protect users from accidentally erasing files (‘rm -i’, ‘mv -i’, ‘cp -i’)
viewing aliases
You can provide one or more aliases as arguments to the alias command to get their definitions. Providing no arguments gives a complete list of current aliases.
unalias
You can undo an alias with the unalias command.
displaying shell expansion
You can display shell expansion with the set -x, and stop displaying it with set +x. You might want to use this further on in this tutorial, or when in doubt about exactly what the shell is doing with your command.
Exercise, Practice and Solution:
1. How many arguments are in this line (not counting the command itself).
touch ‘/etc/cron/cron.allow’ ‘abc123.txt’ «abc456.txt»
Code:
2. Is tac a shell builtin command ?
Code:
3. Is there an existing alias for rm ?
Code:
4. Read the man page of rm, make sure you understand the -i option of rm. Create and remove a file to test the -i option.
Code:
5. Execute: alias rm=’rm -i’ . Test your alias with a test file. Does this work as expected ?
Code:
6. List all current aliases.
Code:
7. Create an alias called ‘city’ that echoes your hometown.
Code:
8. Use your alias to test that it works.
Code:
9. Execute set -x to display shell expansion for every command.
Code:
10. Test the functionality of set +x by executing your city and rm aliases.
Code:
11. Execute set +x to stop displaying shell expansion.
Code:
12. Remove your city alias.
Code:
13. What are the location of the cat and the passwd commands?
14. Explain the difference between the following commands :
echo
/bin/echo
Code:
15. Explain the difference between the following commands :
echo Hello
echo -n Hello
Code:
16. Display A B C with two spaces between B and C.
(optional)
Code:
17. Complete the following command (do not use spaces) to display exactly as soon below
The solution is to use tabs with \t.
18. Use echo to display the following exactly:
Code:
19. Use one echo command to display three words on three lines.
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Linux Shell Commands
The Shell is the command interpreter on Linux systems. This document intoduces some of the basic features of the Shell and lists many of the commands or programs available on the Linux computers in Cardiff School of Computer Science & Informatics.
The Shell
The Linux command interpreter or shell is the program users interact with in a terminal emulation window. The terminal emulation window can be one in the workstation’s Graphical User Interface mate-terminal on Linux. Alternatively, it can be an application such as SSH secure shell client or PuTTY on a Windows PC that’s logged into Linux over the network.
The shell used in the School of Computer Science & Informatics is bash Bourne Again Shell. There are other shells available such as the Bourne Shell, the C-Shell and the TC-Shell, and you can choose to use a different shell if you prefer. They all have similar characteristics but each has its own particular features. This document assumes you are using bash .
Bash has the following features:
A command prompt which may be configured by the user. The default prompt is a dollar symbol preceded by «bash» and the bash program’s version number.
/bin containing the following.
/bin/countc
bash-2.05$ countc
45
516 ls -lrt
517 ps -ef
518 pdflatex myfile.tex
See the manual page on bash for more details (type man bash ).
Bash has an additional mechanism which allows you to recall and edit previous commands using the keyboard up-arrow key. If you press up-arrow, the last command re-appears on the terminal. Press up-arrow again to get earlier commands. To rerun the command, press RETURN . To amend the command before rerunning it, use the delete key to remove characters from the end or use the back-arrow key to reposition the cursor to delete or insert characters within the command.
Shell Commands
Here is a summary of some of the commands available. For more details refer to the manual page of each command. You can see these on-line by using the man command. Just type man followed by the name of the command you want to see.
Logging out
| Command | Description |
| logout | log out of a Linux terminal |
Note, on a Linux workstation you will need to exit the Desktop Environment instead.
Files and Directories
These commands allow you to create directories and handle files.
| Command | Description |
|---|---|
| cat | concatenate and print data |
| lpr | spool file for line printing |
| cd | change current directory |
| lprm, cancel | remove jobs from line printer queue |
| chgrp | change file group |
| ls | list and generate statistics for files |
| chmod | change file mode |
| mkdir | make a new directory |
| cp | copy file data |
| more, page | display file data at your terminal |
| Command | Description |
|---|---|
| file | determine file type |
| mv | move or rename files |
| find | find files |
| pwd | print working directory |
| grep | search file for regular expression |
| rm, rmdir | remove (unlink) files or directories |
| head | give first few lines |
| tail | print last lines from file |
| just | text justification program |
| touch | update access and modification times of a file |
| lpq | spool queue examination program |
File Editors
Editors are used to create and amend files.
| Command | Description |
|---|---|
| emacs | GNU project Emacs |
| xemacs | emacs with mouse action |
| ex, edit | line editor |
| Command | Description |
|---|---|
| pico | easy text editor for vdus |
| pluma | Mate GUI text editor |
| gedit | GNOME text editor |
| vi, vim | standard text editor |
Vi , pico and emacs are screen-based editors which run on a vdu or in a workstations terminal emulation window; pluma , gedit and xemacs are graphical user interface (GUI) based editors with cut and paste and mouse-controlled cursor positioning.
Manipulating data
The contents of files can be compared and altered with the following commands.
| Command | Description |
| awk | pattern scanning and processing language |
| perl | data manipulation language |
| cmp | compare the contents of two files |
| paste | merge file data |
| comm | compare sorted data |
| sed | stream text editor |
| cut | cut out selected fields of each line of a file |
| sort | sort file data |
| diff | differential file comparator |
| Command | Description | split | split file into smaller files |
|---|---|
| expand, unexpand | expand tabs to spaces, and vice versa |
| tr | translate characters |
| gawk | pattern scanning and processing language |
| uniq | report repeated lines in a file |
| join | join files on some common field |
| look | find lines in sorted data |
| wc | count words, lines, and characters |
Compressed files
Files may be compressed to save space. Compressed files can be created and examined.
| Command | Description |
|---|---|
| gzip | compress files |
| zmore | file perusal filter for crt viewing of compressed text |
| uncompress | uncompress files |
| Command | Description |
|---|---|
| zcat | cat a compressed file |
| gunzip | uncompress gzipped files |
| zcmp, zdiff | compare compressed files |
Information
Manuals and documentation are available on-line. Go to our web site www.cs.cf.ac.uk/systems for web-based documentation. The following Shell commands give information.
| Command | Description |
| apropos | locate commands by keyword lookup |
| man | displays manual pages online |
| Command | Description |
|---|---|
| info | displays command information pages online |
| yelp | GNOME help viewer |
Status
These commands list or alter information about the system.
| Command | Description |
| ps | print process status statistics |
| date | print the date |
| quota -v | display disk usage and limits |
| reset | reset terminal mode |
| du | print amount of disk usage |
| script | keep script of terminal session |
| stty | set terminal options |
| groups | show group memberships |
| time | time a command |
| homequota | show quota and file usage |
| iostat | report I/O statistics |
| tty | print current terminal name |
| Command | Description |
|---|---|
| kill | send a signal to a process |
| uptime | display system status |
| last | show last logins of users |
| users | print names of logged in users |
| lun | list user names or login ID |
| vmstat | report virtual memory statistics |
| netstat | show network status |
| w | show what logged in users are doing |
| who | list logged in users |
| printenv | display value of a shell variable |
Printing
Files can be printed using shell commands, using the GUI print manager, or direct from some applications.
You must specify a printer by name. Printers are called
| Printer Name | Location |
|---|---|
| tl1_lw | Teaching Lab 1 (C/2.04) laser printer |
| tl3_lw | Teaching Lab 3 (C/2.08) laser printer |
| Printer Name | Location |
|---|---|
| tl2_lw | Teaching Lab 2 (C/2.05) laser printer |
| tl4_lw | Teaching Lab 4 (C/2.10) laser printer |
Most commands which can be used to print files, expect the printer name to be given following a -P argument.
Files may be sent to the printers as simple text files or they may be processed in various ways for the laser printers.
| Command | Description |
|---|---|
| lpr -P printer | send a file to a printer |
| dvips -P printer | postprocess TeX file into Postscript and print on laser printer |
| a2ps -P printer | format text file in PostScript and print on laser printer |
Messages between Users
The Linux systems support on-screen messages to other users and world-wide electronic mail.
| Command | Description |
|---|---|
| write | send a message to another local user |
| wall | send a message to all local users |
| Command | Description |
|---|---|
| pine | vdu-based mail utility |
| simple send or read mail program | |
| thunderbird | GUI mail handling tool on Linux |
Networking
The School of Computer Science & Informatics is connected to the JANET Internet Protocol Service (JIPS), the UK Universities’ network.
These commands are used to send and receive files from Campus Linux hosts and from other hosts on JIPS and the Internet, that permit such connections, around the world.
| Command | Description |
|---|---|
| ftp | file transfer program |
| tftp | trivial file transfer program |
| sftp | secure shell file transfer program |
| rcp | remote file copy |
| scp | secure shell remote file copy |
| wget | non-interactive network downloader |
| Command | Description |
|---|---|
| telnet | make terminal connection to another host |
| ssh | secure shell terminal or command connection |
| rlogin | remote login to a Linux host |
| rsh | remote shell |
| curl | transfer data from a url |
| firefox | web browser |
| google-chrome | web browser |
These commands work only where the remote host permits such connections.
Programming
The following programming tools and languages are available.
General
| Command | Description |
|---|---|
| make | maintain groups of programs |
| size | print program’s sizes |
| Command | Description |
|---|---|
| nm | print program’s name list |
| strip | remove symbol table and relocation bits |
| Command | Description |
|---|---|
| cb | C program beautifier |
| gcc | GNU ANSI C Compiler |
| Command | Description |
|---|---|
| ctrace | C program debugger |
| indent | indent and format C program source |
| cxref | generate C program cross reference |
| Command | Description |
|---|---|
| g++ | GNU C++ Compiler |
| Command | Description |
|---|---|
| appletviewer | JAVA applet viewer |
| javac | JAVA compiler |
| eclipse | Java integrated development environment on Linux |
FORTRAN
| Command | Description |
| f95 | GNU Fortran 95 compiler |
Other Languages
(Not available on all systems).
| Command | Description |
| bc | interactive arithmetic language processor |
| matlab | maths package |
| gcl | GNU Common Lisp |
| perl | general purpose language |
| Command | Description |
|---|---|
| python | object-oriented programming language |
| squeak | smalltalk |
| php | web page embedded language |
| mathematica | symbolic maths package |
| asp | web page embedded language |
Text Processing
TeX is a typesetting language used extensively in Linux and other operating systems for producing high-quality printed documents. Another set of programs based on Troff is the standard Linux text formatting family used, for example to format manual pages.
General Commands
| Command | Description |
|---|---|
| fmt | simple text formatter |
| evince | GNOME PostScript previewer |
| Command | Description |
|---|---|
| acroread | PDF viewer |
| spell | check text for spelling error |
| aspell | interactive spelling checker |
Troff
| Command | Description |
|---|---|
| eqn | mathematical preprocessor for troff |
| tbl | prepare tables for nroff or troff |
| grap | pic preprocessor for drawing graphs |
| troff | text formatting and typesetting language |
| Command | Description |
|---|---|
| nroff | text formatting language |
| groff | GNU troff interface for laserprinting |
| pic | troff preprocessor for drawing pictures |
| Command | Description |
|---|---|
| tex | text formatting and typesetting |
| latex | latex formatter |
| Command | Description |
|---|---|
| pdflatex | latex formatter with PDF output |
| xdvi | dvi previewer |
| dvips | convert a DVI file to POSTSCRIPT |
Word Processing
LibreOffice is available on the School’s Linux systems and attempts compatibilty with Microsoft Office.
| Command | Description |
|---|---|
| libreoffice | start LibreOffice applications |
Database Management
MySQL and Oracle are available.
| Command | Description |
|---|---|
| sqlplus | run the Oracle SQL interpreter |
| mysql | run the mysql SQL interpreter |
| sqldeveloper | Oracle SQL Developer GUI interface |
| mysql-workbench | GUI interface for MySQL |
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