Bash Configuration File	Type of Shell	Function
~/.bashrc	Non-login shells (Although login shells on most distributions read this file as well. It is usually called from one of the configuration files listed below.	Stores individual user’s shell preferences.
/etc/profile	Login shells	Contains configuration parameters that are applied system-wide
~/.bash_profile	Login shells	Stores individual users’ shell preferences
~/.bash_login	Login shells	Stores individual users’ shell preferences
~/.profile	Login shells	Stores individual users’ shell preferences
~/bash_logout	Login shells	Specifies actions to be performed when a user logs out from the shell.

 

 

Commonly Used
Environment Variables

Environment Variable	Stores	Default Values
BASH and SHELL	The full path to the shell executable	/bin/bash
CPU	The type of CPU installed in the system	Depends on your system. An Intel Pentium IV computer would have a value of i686.
DISPLAY	The location where your X Windows display should be sent	0.0 (the local video card and monitor)
ENV	The name of the file bash read to configure its environment	~/.bashrc
EUID	The user ID (UID) of the current user	The UID number of the current user
HISTFILE	The path to the bash command history file	~/.bash_history
HISTSIZE	The number of commands saved in the command history	1000
HOME	The path to the current user’s home directory	The current user’s home directory.
HOST and HOSTNAME	The hostname of the system	The hostname you assigned when you installed the system
INFODIR	The path to your system’s info program	/usr/local/info:/usr/share/info:/usr/info
LOGNAME	The username of the current user	The username of the current user
MAIL	The path to the current user’s mailbox file	/var/spool/mail/username
MANPATH	The path to your system’s man program	/usr/local/man:/usr/share/man:/usr/X11R6/man:/opt/gnome/share/man
OLDPWD	The path to the prior current directory	Depends on what your prior current directory was
OSTYPE	The type of operating system currently running	Linux
PATH	A list of directories to be searched when running a command from the shell prompt	Depends on your distribution
PS1	The characters used to create the shell prompt	Depends on your distribution
PWD	The path to the current working directory	Depends on what your current directory is

 

 

 

 

You can use the set command
to view all your variables in a linux system. You can also use the env command to see your system
environment variables.

 

You must type export
VARIABLE to use a new or changes variable on a new shell.

 

File descriptors:

Stdin – This file
descriptor stands for standard input. Standard input is the input provided to a
particular command to process. The stdin for a command is represented by the
number 0.

Stdout – This
file descriptor stands for standard output. Standard output is simply the
output from a particular command. For example, the directory listing generated
by the ls command is its stdout. The stdout for a command is represented by the
number 1.

Stderr – This
file descriptor stands for standard error. Standard error is the error code
generated, if any, by a command. The stderr for a command is represented by the
number 2.

 

Cat file > output
- will save stdout ( no errors )

Cat file 1> output
 - will save stdout  ( no errors )

Cat file 2>  output  - will save stderr ( ERRORS )

Cat file 1> output
2>&1 - This will put both stdout and stderr in the same file.

Cat file 1>>
output 2>&1 - This will output both stdout and stderr to the same
file.

 

 

Sed Commands

s – Replaces
instances of a specified text string with another text string. The syntax for
using the s command is sed
s/term1/term2/.

d – Deletes the
specified text. For example, to delete every line of text from the stdin that
contains the term “eos”, you would enter sed
/eos/d.

 

AWK character outputs

t - Inserts a
tab character

n - Adds a new
line character.

f - Adds a
formfeed character.

r - Adds a
carrieage return character.

 

Shell Scripts

All variables are interpreted as text unless defined
otherwise. To declare a variable to be an integer use the following code: declare -i VAR.

 

If/Then/Else
statement structure:

 

If condition then

 Commands

Else

 Commands

Fi

 

Check if a directory/file exists.

If [ -e $VAR ]; then

 Echo “exists”;

Fi

 

Case Syntax

Case variable in

            Response_1
) commands

                        ;;

            Response_2
) commands

                        ;;

            Response_3
) commands

                        ;;

Esac

 

Example:

Read NAME

Case $NAME in

            Tully )
echo “Your name is Tully!”

                        ;;

            Tullyr )
echo “Awesome Tullyr!”

                        ;;

            Rankin )
echo “You have my last name!”

                        ;;

            * ) echo
“Your name is not cool”

                        ;;

Esace

Exit0

 

While Loops

While condition

Do

            Script
commands

Done

 

Until loop

Until condition

Do

            Script
commands

Done