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.
  bash-2.05$
• The shell, like other programs on Linux has an associated current directory. Programs running on Linux use the current directory as the starting point when locating files. The shell command cd is used to change the current directory to a different location in the Linux file system.
• Commands are invoked by naming them. Most Linux commands are simply programs which are executed by the shell. For example, to run the ls command which reads the the current directory and lists the names of its files the following would be used.
  bash-2.05$ ls
• When you type a command name, the shell will check to see if the command is built-in and will otherwise search a set of directories until it finds the program. This set is known as the search path. The search path includes the current directory, your home directory and its subdirectory "bin". You can write your own programs and invoke them simply by typing their names. If you store such a program in the directory ``bin'' it will be found and run no matter what your current directory is.
• Commands often have argument strings which may, for instance, represent filenames. For example, the below command changes the current directory to "bin" in your home directory. The tilde character means your home directory to the shell.
  bash-2.05$ cd ~/bin
  Some commands need more than one argument. For example, the copy command takes two arguments the file to copy and it's destination. This is shown below where fileA is copied to a new file, fileB.
  bash-2.05$ cp fileA fileB
  Some commands have flag or option argument strings usually beginning with ``-'' or ``-''. The flags modify the behaviour of the program being invoked. The below command when invoked makes ls give a long listing of files sorted by time of creation.
  bash-2.05$ ls -lt
• The shell will expand wildcards to match filenames in the current directory. For example, to give a directory listing of the file with names "anything.c" use the following.
  bash-2.05$ ls -l *.c
• Most Linux commands and programs adhere to a concept of standard input and standard output. The standard input is a stream of data which the program reads and the standard output is a stream of output written by the program. Often these are both attached to the terminal so that input comes from your keyboard and output goes to your screen. The shell lets you redirect the standard input and output.
  bash-2.05$ cat < fileA
  bash-2.05$ cat < fileA > fileB
• The Shell has the facility to pipe the output of one program to the input of another. The pipe symbol is "|". For example to count the number of words in fileA we can cat the file then pipe the output in the wc program.
  bash-2.05$ cat fileA | wc -w
  405
• You may assign aliases for commands or groups of commands that you may execute frequently or find cumbersome to enter. For example we could assign an alias "countc" to count the number of C program source files in the current directory using ls to list the files and wc to count the number of lines output.
  alias countc="ls -l *.c | wc -l"
• The shell has string and numeric valued variables.
  bash-2.05$ x="Hello World!"
  bash-2.05$ echo $x
  Hello World!
  Some variables are pre-set, e.g. $HOME is your home directory. Type set to see a list of assigned variables.
• Bash is an interpretive programming language with while loops, for loops, if-then-else statements and many more. See the Linux on-line documentation for more details by typing the following command.
  bash-2.05$ man bash
• Scripts of shell commands can be written. These can be invoked in the same way as compiled programs (i.e. just by naming them). For example, to create a script that counts the number of C program files in the current directory we first create a file in ~/bin containing the following.
  #! /bin/bash
  ls -l *.c | wc -l
  We must then make the file executable using the chmod command before we can run it like normal.
  bash-2.05$ chmod +x ~/bin/countc
  bash-2.05$ countc
  45
• The shell has ``job control''. Programs which don't require any terminal interaction can be run in the background.
  bash-2.05$ sort bigfile > sortedfile &
  [1] 3470
  The above puts the program sort in the background and the shell is available immediately for other commands. The shell prints the job control number ("1" in this case) and the process identity number ("3470"). The special character Ctrl + z can be used to suspend a program which is running in the foreground. Once stopped the bg command can be used to put the program in the background or fg can be used to continue it in the foreground. If you have more than one job running in the background or suspended, you can refer to them by their job number. To see your jobs and their job numbers use the jobs command to list the status of all stopped or background jobs.
• The shell has a history mechanism, it remembers the last few commands. The history command can be used to list the last few commands executed along with a reference number.
  bash-2.05$ history
  515 cd ~
  516 ls -lrt
  517 ps -ef
  518 pdflatex myfile.tex
  In a workstation's terminal emulation windows, you can cut and paste from the history to rerun a command. You can also use the symbol ``!'' to rerun any command from the history.
  bash-2.05$ !518 # rerun command number 518 from the history
  bash-2.05$ !ps # rerun the last command starting "ps"
  bash-2.05$ !! # rerun the last command

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.

File Editors

Editors are used to create and amend files.

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.

Compressed files

Files may be compressed to save space. Compressed files can be created and examined.

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.

Status

These commands list or alter information about the system.

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

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 -Pprinter	send a file to a printer
dvips -Pprinter	postprocess TeX file into Postscript and print on laser printer
a2ps -Pprinter	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.

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.

These commands work only where the remote host permits such connections.

Programming

The following programming tools and languages are available.

General

C

C++

Command	Description
g++	GNU C++ Compiler

JAVA

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).

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

Troff

TeX

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