Introduction to Linux/Unix Shell Commands

The purpose of this document is to provide the reader with a fast and simple introduction on using the Linux command shell and some of its basic utilities. It is assumed that the reader has zero or very limited exposure to the Linux command prompt.


Command Shell

• A program that interprets commands
• Allows a user to execute commands by typing them manually at a terminal, or automatically in programs called shell scripts
• A shell is not an operating system. It is a way to interface with the operating system and run commands

BASH

• Stands to Bourne Again SHell
• Bash is a shell written a free replacement to the standard Bourne Shell (/bin/sh) originally written by Steave Bourne for UNIX systems 
• It has all of the features of the original Bourne Shell, plus additions that make it easier to program with and use from the command line
• Since it is Free Software, it has been adopted as the default shell on most Linux systems

The difference between BASH and DOS command prompt

• Case Sensitivity - In Linux/UNIX, commands and filenames are case sensitive, meaning that typing "EXIT" instead of the proper "exit" is a mistake.
• "\" vs. "/"  - In DOS, the forward-slash "/" is the command argument delimiter, while the backslash "\" is a directory separator. In Linux/UNIX, the "/" is the directory separator, and the "\" is an escape character. More about these special characters in a minute!
• Filenames - The DOS world uses the "eight dot three" filename convention, meaning that all files followed a format that allowed up to 8 characters in the filename, followed by a period ("dot"), followed by an option extension, up to 3 characters long (e.g FILENAME.TXT). In UNIX/Linux, there is no such thing as file extension. Periods can be placed at any part of the filename, and "extensions" may be interpreted differently by all programs, or not at all. 

Special Characters

Before we continue to learn about Linux shell commands, it is important to know that there are many symbols and characters that the shell interprets in special ways. 

This means that certain typed characters: 

• cannot be used in certain situations, 
• may be used to perform a special operations, or, 
• must be "escaped" if you want to use them in a normal way. 

Character Description	Description
/	Escape character. If you want to reference a special character, you must "escape" it with a backslash first. Example: touch /tmp/filename \*
\	Directory separator, used to separate a string of directory names. Example: /usr/src/linux
.	Current Directory. Can also "hide" files when it is the first character in a filename
..	Parent directory
~ (tilde)	User's home directory
*	Represents 0 or more characters in a filename, or by itself, all files in a directory. Example: pic*2002 can represent the files pic2002, picJanuary2002, picFeb292002, etc.
?	Represents a single character in a filename Example: hello?.txt can represent hello1.txt, helloz.txt, but not hello22.txt
[ ]	Can be used to represent a range of values, e.g. [0 - 9], [A-Z], etc. Example: hello[0-2].txt represents the names hello0.txt, hello1.txt, and hello2.txt
|	"Pipe". Redirect the output of one command into another command. Example: ls | more
>	Redirect output of a command into a new file. If the file already exists, over-write it. Example: ls > myfiles.txt
>>	Redirect the output of a command onto the end of an existing file Example: echo "Mary 555-1234" >> phonenumbers.txt
<	Redirect a file as input to a program. Example: more < phonenumbers.txt
;	Command separator. Allows you to execute multiple commands on the single line Example: cd /var/log ; less messages
&&	Command separator as above, but only runs the second command if the first one finished without errors. Example: cd /var/logs && less messages
&	Execute a command in the background, and immediately get your shell back. Example: find / -name core > /tmp/corefiles.txt &