Help File Library: Configuring the Bash Shell

Bash is an acronym for Bourne Again SHell. It is compatible with the original
Bourne shell and boasts some enhancments, like command line editing [TRANSLATION: The Bash shell can run program written for the Bourne shell, plus it offers so much more than the Bourne shell]. It is also the default Linux shell and
is the most widely used shell in Linux. For those who do not know, a shell
is a program that acts as an intermediary between the user and the kernel
[TRANSLATION: A shell is a program that takes commands from the
user and passes them into the kernel for processing]. Like most of the
software that comes with Linux, bash is highly configurable.

ASSUMPTIONS
This article makes a few assumptions about you; the reader:

• You know basic Linux commands, eg: ls, cd, mv, etc...
• You know how to use a text editor, eg: vi, emacs, pico, etc...
• You know how to read path names, eg: /etc/profile
• You know a little bash shell programming.

THE CONFIGURATION FILES
bash has five common configuration files. They are not always found on every
Linux distribution, but they are not hard to create. These files are as follows:

• /etc/profile
• /etc/bashrc
• ~/.bash_profile
• ~/.bashrc
• ~/.bash_logout

/etc/profile
/etc/profile is the global system configuration for bash which controls the
environmental variables and programs that are to be run when bash is executed
[TRANSLATION: /etc/profile contains variables and programs that are
executed for every user who runs bash]. If you are an MS-DOS user, /etc/profile
is equivalent to autoexec.bat.

variable_name=variable_value

export variable_name

xconsole$ echo $SHELL
/bin/bash

FOOBAR=/bin/foobar

# FOOBAR=/bin/foobar

# not sure what this variable does, so I decided to 
comment it out
# 1 May 1999
# FOOBAR=/bin/foobar

PATH=/bin:/usr/bin:/usr/local/bin
export PATH

PATH=$PATH:/usr/games
export PATH

\t	the current time in HH:MM:SS format
\d	the date in "Weekday Month Date" forma
t (eg, "Tue May 26")
\n	newline
\s	the name of the shell
\w	the current working directory
\W	the basename of the current working directory
\u	the username of the current user
\h	the hostname
\#	the command number of this command
\!	the history number of this command
\$	if the effective UID is 0, a #, otherwise a $

[xconsole@localhost /etc]$

[12:18:24 bash]$

PS1="`fortune` \$ "

He is no laywer who cannot take two sides. $

xconsole$ if [ -f /etc/profile ]; then

xconsole$ if [ -f /etc/profile ]; then

>

• HISTSIZE
• HISTFILE
• HISTFILESIZE

• MAIL
• MAILCHECK
• MAILPATH
• MAIL_WARNING

MAIL
When email is delivered to a user, the contents of the email are stored in a file. This
file is defined by MAIL. Its value is /var/spool/mail/$USER [TRANSLATION:
If your user name is xconsole, then your email is kept in /var/spool/mail/xconsole]
by default if MAIL is not set to another file. This is only if the variable MAILPATH is not
defined. Changing this value will change the file in which your email is stored in.

MAILCHECK
This variable defines how often bash will check for incoming email. The default value is 60.
This means that bash will check to see if you have any new emails every minute.

MAILPATH This variable defines the paths to check for emails. Normally this value is set
to /var/spool/maile. Each path is to be seperated by a colon: ":". You can also
modify this variable to print out a special message when you receive any new emails.
For instance, if you defined this variable as:

MAILPATH='/var/spool/mail/xconsole "Yeah! Incoming message!"'



When you receive an email, this is what happens:

Yeah! Incoming message!

This message can be tailored in anyway you like of course. MAILPATH will begin by
checking the path to /var/spool/mail/xconsole and then informing you of the
received email.

MAIL_WARNING
If MAIL_WARNING is set, bash will inform you if you attempt to read an email that you
have just read with the following message:

The mail in mailfile has been read

mailfile is the variable MAIL, which is the file in which a user's email is stored.
Setting this variable is a good way of knowing if you are reading the same email twice,
but quite useless if you are using an email client that informs you automaticall if you have
already read/replied to an email; like pine.

RECEIVING NOTIFICATION WHEN A JOB FINISHES
Depending upon how bash is configured, you may or may not receive a message that
a job you have run in the background is done [TRANSLATION: Programs run in the
background are called jobs. Running a program in the background is done simply by
adding an amperstand: "&" right at the end of the command, eg updatedb &]. When
the job ends, you may have to press ENTER before you receive a message like:

The variable notfiy will notify you the moment a job in the background is finished.
To unset this variable, the command is unset +o notify.

TIMEOUT
When opening up a new bash session, you can specify how long to wait before the
shell automatically terminates with the TMOUT variable. The value of this variable is to
be set in the number of seconds to wait for user input before the shell self terminates.
If you add this to your /etc/profile or to your /etc/.bash_profile, as so:

TMOUT=60

bash will terminate the session after one minute. Here is what happens after one minute
of no user input:

As you can see, the user is immediately logged out and a new login prompt is issue.
This can be a good security practice if you do not use password protected screen savers.

OVERWRITING PRECAUTIONS
Often, one has to work with redirection in bash [TRANSLATION: Redirection is
running a program, and then having its output redirected to another file, or program,
instead of to the monitor]. An example of redirection is:

The ">" operator is the redirect operator, and the ">>" operator is the append operator.
The first command saves the string Hello World into a file called ~/.test_file.
Hello World will not be echoed to the screen at all! It must be read from the file
~/.test_file. The second command appends to the file ~/.test_file. This means
that when ~/.test_file is read, this is what we get:

Now imagine that this is some very important file. While typing away on the keyboard,
you accidentally type the following command:

What happens here is that ~/.test_file is completely overwritten. Anything important, has
been deleted and replaced:

To prevent against this sort of accident, you can set the noclobber variable. This is done
with the command set -o noclobber. When set, here is what happens when we try
to overwrite a file:

As you can see, bash prevents you from overwriting the file. If you want to overwrite it,
you can do a force by using the ">|" operator:

Here we see that when we issue the second command, bash no longer complains and
performs the command. I suggest you set this variable on at all times. Have it in your
~/.bash_profile or /etc/profile. It is really easy to type ">" by accident when
you meant to type ">>". It is much harder to type ">|". On the other hand, this could
cause problems with shell programs that need to execute the ">" operator [TRANSLATION:
If the program you are running uses the ">" operator, and you have the noclobber variable
set, then you will get an error message when executing the shell program]. So it is really up
to you if you want to set it or not. To unset this variable, the command is unset +o noclobber

ALIASES
Aliases are what you can call, "shortcut commands". If you want something done quick,
you can do it with aliases. One example an alias can do is to allow you to type ls -aF
--color just by typing ls, or anything else you like. An alias is normally defined in
/etc/bashrc or ~/.bashrc. Aliases look like this:

alias ls='ls -aF --color'
alias haha='ls -aF --color'
alias sl='ls -aF --color'

In the above, the commands ls, haha, sl, will all perform the same action when typed.
That is: ls -aF --color. This is the usefulness of aliases. One particular use of aliases
is for safety precautions:

In the above example, the commands rm, mv, and cp will all ask for confirmation before
attempting to overwrite a file. This is a good thing so you do not have to type the long form
command with options. Another example:

You can now mount the CDROM with the command mycd, although a much better way
would be to actually edit /etc/fstab, but this example serves the purpose as well. To
find out what your current aliases are, you use the alias command:

Aliases can also be set and unset while the shell is running. Setting an alias on a shell is
done with the alias command. Example:

You can also unset aliases with the unalias command:

The unalias command has one option: -a. This option allows you to unalias all
currently set aliases.

FUNCTIONS
Functions normally go to /etc/bashrc and ~/.bashrc. Functions are pieces of code
that perform a special action when called upon from the shell. You will need to know
bash shell programming in order to use functions. Here is an example of a function that
unpacks all tarballs in the current directory:

Now if you wanted to unpack all tarballs in a current directory, all you would have to
do is to use the command unpack, and all tarballs will be unpacked. Of course this is
a really simple function, and could have been used as an alias instead:

However, the above function is very simple. You can write much more complex
functions, depending on the job to be accomplished. You can put entire shell
programs into the /etc/bashrc or the ~/.bashrc files and have them run just
by typing their function name.

EXAMPLES
The following are examples of what the bash configuration files can look like.
Feel free to create and/or modify your own configuration files. It is your system,
and you should decide how you want it to run.

/etc/profile

# /etc/profile: This file contains system-wide 
defaults used by
# all Bourne (and related) shells.

# Set the values for some environment variables:
export OPENWINHOME=/usr/openwin
export MINICOM="-c on"
export MANPATH=/usr/local/man:/usr/man/preformat:
/usr/man:/usr/X11R6/man:/usr/openwin/man
export HOSTNAME="`cat /etc/HOSTNAME`"
export LESSOPEN="|lesspipe.sh %s"
export LESS="-M"
#export MOZILLA_HOME=/usr/local/netscape
export HISTSIZE=20
export HISTFILESIZE=20

# Set notification when a job ends.
set -o notify

# Set the default system $PATH:
PATH="$PATH:/usr/X11R6/bin:$OPENWINHOME/bin:
/usr/games"

# I had problems using 'eval tset' instead of 
'TERM=', but you might want to 
# try it anyway. I think with the right /etc/termcap 

it would work great.
# eval `tset -sQ "$TERM"`
if [ "$TERM" = "" -o "$TERM" = "unknown" ]; then
TERM=linux
fi

# 'kvt' uses "xterm-color" which isn't recognized by 

programs like 'pine'
# or 'pico', so let's change it to the default "xterm":
if [ "$TERM" = "xterm-color" ]; then
TERM=xterm
fi

# Set a default shell prompt:
PS1='[\u@\h \W]\$ '
PS2='> '

ignoreeof=10
export PATH DISPLAY LESS TERM PS1 PS2 ignoreeof

# Default umask. A umask of 022 prevents new files 
from being created group
# and world writable.
umask 022

# Set up the LS_COLORS and LS_OPTIONS environment 
variables for color ls:
eval `dircolors -b`

# Notify user of incoming mail. This can be overridden 


in the user's

# local startup file (~/.bash.login or whatever, 

depending on the shell)

if [ -x /usr/bin/biff ]; then

biff y

fi 

# Print a fortune cookie for login shells:

echo

fortune /usr/games/lib/fortunes/fortunes /usr/games/lib/fortunes/fortunes2 


echo


# Environment variables for the Qt package:


 QTDIR=/usr/lib/qt

CPLUS_INCLUDE_PATH=$QTDIR/include:$CPLUS_INCLUDE_PATH

export QTDIR

export CPLUS_INCLUDE_PATH



 # KDE additions:

KDEDIR=/opt/kde

PATH=$PATH:$KDEDIR/bin

export KDEDIR PATH



# Make sure that the current directory is always last 

in the PATH variable. PATH="$PATH:." 

# /etc/bashrc

# System wide functions and aliases 

 # Environment 
stuff goes in /etc/profile 

# functions go here 

# create a new ~/.plan file at every log in

new_plan()

{

if [ -f `which fortune` ]; then

echo > ~/.plan

`which fortune -s` >> ~/.plan

echo >> ~/.plan

fi

}


# aliases go here 

# paranoia

alias rm='rm -i'

alias cp='cp -i'

alias mv='mv -i'

~/.bash_profile

# .bash_profile
  
  # Get the aliases and functions
  if [ -f ~/.bashrc ]; then
  . ~/.bashrc
  fi
  
  # user specific environment and startup programs
  PATH=$PATH:$HOME/bin:$HOME/garbage
  BASH_ENV=$HOME/.bashrc
  
  export PATH BASH_ENV

#.bashrc
  
  # User specific aliases and functions
  
  # Source global definitions
  if [ -f /etc/bashrc ]; then
  . /etc/bashrc
  fi
  
  # create a log of all users who logged in,
  # and those currently logged in
  see_all()
  {
  LOG_FILE=/var/log/see_all
  echo `date` >> $LOG_FILE
  echo "Recent users to log in: " >> $LOG_FILE
  last | head -10 >> $LOG_FILE
  echo "Users who are currently online: " >> $LOG_FILE
  who >> $LOG_FILE
  } 
  
  # no pine-debug[?]
  alias pine='pine -d0'
  
  # special ls
  alias ls='ls -aF --color'

# .bash_logout
  echo "You logged off at `date`"
  echo "See you again `whoami`!"
  sleep 2
  clear

CONCLUSION Hopefully this Help File has been able to guide you into making your own bash configuration files, and modifying your current ones. You are also encouraged to read the manual page for bash: man bash for those of you who do not know. The best way to learn is to read the manual and of course, to experiment.