7.2. Your text environment

7.2.1. Environment variables

We already mentioned a couple of environment variables, such as PATH and HOME. Until now, we only saw examples in which they serve a certain purpose to the shell. But there are many other Linux utilities that need information about you in order to do a good job.

What other information do programs need apart from paths and home directories?

A lot of programs want to know about the kind of terminal you are using; this information is stored in the TERM variable. In text mode, this will be the linux terminal emulation, in graphical mode you are likely to use xterm. Lots of programs want to know what your favorite editor is, in case they have to start an editor in a subprocess. The shell you are using is stored in the SHELL variable, the operating system type in OS and so on. A list of all variables currently defined for your session can be viewed entering the printenv command.

The environment variables are managed by the shell. As opposed to regular shell variables, environment variables are inherited by any program you start, including another shell. New processes are assigned a copy of these variables, which they can read, modify and pass on in turn to their own child processes.

There is nothing special about variable names, except that the common ones are in upper case characters by convention. You may come up with any name you want, although there are standard variables that are important enough to be the same on every Linux system, such as PATH and HOME.

The following table gives an overview of the most common predefined variables:

A lot of variables are not only predefined but also preset, using configuration files. We discuss these in the next section.

7.2.2. Shell setup files

When entering the ls -al command to get a long listing of all files, including the ones starting with a dot, in your home directory, you will see one or more files starting with a . and ending in rc. For the case of bash, this is .bashrc. This is the counterpart of the system-wide configuration file /etc/bashrc.

When logging into an interactive login shell, login will do the authentication, set the environment and start your shell. In the case of bash, the next step is reading the general profile from /etc, if that file exists. bash then looks for ~/.bash_profile, ~/.bash_login and ~/.profile, in that order, and reads and executes commands from the first one that exists and is readable. If none exists, /etc/bashrc is applied.

When a login shell exits, bash reads and executes commands from the file ~/.bash_logout, if it exists.

This procedure is explained in detail in the login and bash man pages.

7.2.3. A typical set of setup files

Let's look at some of these config files. First /etc/profile is read, in which important variables such as PATH, USER and HOSTNAME are set:

debby:~>cat /etc/profile
# /etc/profile

# System wide environment and startup programs, for login setup
# Functions and aliases go in /etc/bashrc


# Path manipulation
if [ `id -u` = 0 ] && ! echo $PATH | /bin/grep -q "/sbin" ; then
    PATH=/sbin:$PATH
fi

if [ `id -u` = 0 ] && ! echo $PATH | /bin/grep -q "/usr/sbin" ; then
    PATH=/usr/sbin:$PATH
fi

if [ `id -u` = 0 ] && ! echo $PATH | /bin/grep -q "/usr/local/sbin" ; then
    PATH=/usr/local/sbin:$PATH
fi

if ! echo $PATH | /bin/grep -q "/usr/X11R6/bin" ; then
    PATH="$PATH:/usr/X11R6/bin"
fi

These lines check the path to set: if root opens a shell (user ID 0), it is checked that /sbin, /usr/sbin and /usr/local/sbin are in the path. If not, they are added. It is checked for everyone that /usr/X11R6/bin is in the path.

# No core files by default
ulimit -S -c 0 > /dev/null 2>&1

All trash goes to /dev/null if the user doesn't change this setting.

USER=`id -un`
LOGNAME=$USER
MAIL="/var/spool/mail/$USER"

HOSTNAME=`/bin/hostname`
HISTSIZE=1000

Here general variables are assigned their proper values.

if [ -z "$INPUTRC" -a ! -f "$HOME/.inputrc" ]; then
    INPUTRC=/etc/inputrc
fi

If the variable INPUTRC is not set, and there is no .inputrc in the user's home directory, then the default input control file is loaded.

export PATH USER LOGNAME MAIL HOSTNAME HISTSIZE INPUTRC

All variables are exported together.

for i in /etc/profile.d/*.sh ; do
    if [ -r $i ]; then
    	. $i
    fi
done
unset i

All readable shell scripts from the /etc/profile.d directory are read and executed. These do things like enabling color-ls, aliasing vi to vim, setting locales etc. The temporary variable i is unset to prevent it from disturbing shell behavior lateron.

Then bash looks for a .bash_profile in the user's home directory:

debby:~>cat .bash_profile 
#################################################################
#                                                               #
#   .bash_profile file                                          #
#                                                               #
#   Executed from the bash shell when you log in.               #
#                                                               #
#################################################################

DO_FORTUNE=1
source ~/.bashrc
source ~/.bash_login
case "$OS" in
  IRIX)
    stty sane dec
    stty erase 
    ;;
#  SunOS)
#    stty erase 
#    ;;
  *)
    stty sane
    ;;
esac
if test "$DO_FORTUNE" -a ! -f "$HOME/.hushlogin"; then
  "$DO_FORTUNE"
fi

First, a variable is set that is used in the end to determine whether or not to display a fortune cookie. If the user has a ~/.hushlogin file, which silences the login process (see man login), nothing is done. Then (this is a general config file for use on multiple architectures) terminal line settings are defined for operating systems that might make a fuss about them.

The ~/.bash_login file defines default file protection by setting the umask value. The ~/.bashrc is used to define a bunch of user-specific aliases and functions and personal environment variables. It first reads /etc/bashrc, which describes the default prompt (PS1) and the default umask value. After that, you can add your own settings. If no ~/.bashrc exists, /etc/bashrc is read by default:

debby:~>cat /etc/bashrc
# /etc/bashrc

# System wide functions and aliases
# Environment stuff goes in /etc/profile

# by default, we want this to get set.
# Even for non-interactive, non-login shells.
if [ `id -gn` = `id -un` -a `id -u` -gt 99 ]; then
	umask 002
else
	umask 022
fi

# are we an interactive shell?
if [ "$PS1" ]; then
  if [ -x /usr/bin/tput ]; then
    if [ "x`tput kbs`" != "x" ]; then # We can't do this with "dumb" terminal
      stty erase `tput kbs`
    elif [ -x /usr/bin/wc ]; then
      if [ "`tput kbs|wc -c `" -gt 0 ]; then # We can't do this with "dumb"\
      terminal
        stty erase `tput kbs`
      fi
    fi
  fi
  case $TERM in
	xterm*)
	if [ -e /etc/sysconfig/bash-prompt-xterm ]; then
		PROMPT_COMMAND=/etc/sysconfig/bash-prompt-xterm
	else
   PROMPT_COMMAND='echo -ne "\033]0;${USER}@${HOSTNAME%%.*}:${PWD/$HOME/~}\007"'
	fi
    ;;
	*)
   [ -e /etc/sysconfig/bash-prompt-default ] && PROMPT_COMMAND=/etc/sysconfig/\
   bash-prompt-default
	    ;;
    esac
    [ "$PS1" = "\\s-\\v\\\$ " ] && PS1="[\u@\h \W]\\$ "
    
    if [ "x$SHLVL" != "x1" ]; then # We're not a login shell
        for i in /etc/profile.d/*.sh; do
	    if [ -x $i ]; then
	        . $i
	    fi
	done
    fi
fi

Upon logout, the commands in ~/.bash_logout are executed, which can e.g. clear the terminal.

Let's take a closer look at how these scripts work in the next section. Keep man bash close at hand.

7.2.4. The Bash prompt

[jerry@nowhere jerry]$ MYPROMPT=$PS1

[jerry@nowhere jerry]$ echo $MYPROMPT
[\u@\h \W]\$

[jerry@nowhere jerry]$

7.2.5. Shell scripts

A shell script is, as we saw in the shell configuration examples, a text file containing shell commands. When such a file is used as the first non-option argument when invoking Bash, and neither the `-c' nor `-s' option is supplied, Bash reads and executes commands from the file, then exits. This mode of operation creates a non-interactive shell. When Bash runs a shell script, it sets the special parameter `0' to the name of the file, rather than the name of the shell, and the positional parameters are set to the remaining arguments, if any are given. If no additional arguments are supplied, the positional parameters are unset.

A shell script may be made executable by using the chmod command to turn on the execute bit. When Bash finds such a file while searching the PATH for a command, it spawns a sub-shell to execute it. In other words, executing

filename ARGUMENTS

is equivalent to executing

bash file_name ARGUMENTS

if `filename' is an executable shell script. This sub-shell reinitializes itself, so that the effect is as if a new shell had been invoked to interpret the script, with the exception that the locations of commands remembered by the parent (see hash in the Info pages) are retained by the child.

Most versions of UNIX make this a part of the operating system's command execution mechanism. If the first line of a script begins with the two characters `#!', the remainder of the line specifies an interpreter for the program. Thus, you can specify bash, awk, perl or some other interpreter or shell and write the rest of the script file in that language.

The arguments to the interpreter consist of a single optional argument following the interpreter name on the first line of the script file, followed by the name of the script file, followed by the rest of the arguments. Bash will perform this action on operating systems that do not handle it themselves.

Bash scripts often begin with `#! /bin/bash' (assuming that Bash has been installed in `/bin'), since this ensures that Bash will be used to interpret the script, even if it is executed under another shell.

#!/bin/bash
echo "Hello $USER"

#!/bin/bash
who | cut -d " " -f 1 | sort -u

tille:~>cat /usr/local/bin/wav2mp3 
#!/bin/bash
# convert *.wav into *.mp3
LIST=$(ls *.wav)
for i in $LIST; do
	ORIG=$i
	DEST=$(ls $i | cut -d "." -f 1).mp3 
	lame -h $ORIG $DEST
	echo "done converting $i, removing.."
	rm $i
done

#!/bin/sh
#
# Run control script for Interchange
# http://interchange.redhat.com/
#
# chkconfig: 345 96 4
# description: Interchange is a database access and HTML templating system focused on ecommerce
# processname: interchange
# pidfile: /var/run/interchange/interchange.pid
# config: /etc/interchange.cfg
# config: /var/lib/interchange/*/catalog.cfg

# Source function library.
. /etc/rc.d/init.d/functions

# See how we were called.
case "$1" in
	start)
		echo -n "Starting Interchange: "
		daemon interchange -q
		echo
		touch /var/lock/subsys/interchange
		;;
	stop)
		echo -n "Shutting down Interchange: "
		killproc interchange
		echo
		rm -f /var/lock/subsys/interchange
		rm -f /var/run/interchange/interchange.pid
		;;
	status)
		status interchange
		;;
	restart)
		$0 stop
		$0 start
		;;
	*)
		echo "Usage: $0 {start|stop|restart|status}"
		exit 1
esac

exit 0