9.12 Environment Variables

Shell environment variables provide temporary storage of data and configuration settings. The shell itself sets up a number of environment variables that may be changed by the user to modify the behavior of the shell. A listing of currently defined variables may be obtained using the env command:

$ env

SSH_CONNECTION=192.168.0.19 61231 192.168.0.28 22

MODULES_RUN_QUARANTINE=LD_LIBRARY_PATH

LANG=en_US.UTF-8

HISTCONTROL=ignoredups

HOSTNAME=demo-pc.ebookfrenzy.com

XDG_SESSION_ID=15

MODULES_CMD=/usr/share/Modules/libexec/modulecmd.tcl

USER=demo

ENV=/usr/share/Modules/init/profile.sh

SELINUX_ROLE_REQUESTED=

PWD=/home/demo

HOME=/home/demo

SSH_CLIENT=192.168.0.19 61231 22

SELINUX_LEVEL_REQUESTED=

.

.

.

Perhaps the most useful environment variable is PATH. This defines the directories in which the shell will search for commands entered at the command prompt, and the order in which it will do so. The PATH environment variable for a user account on a newly installed Ubuntu system will likely be configured as follows:

$ echo $PATH

/home/demo/.local/bin:/home/demo/bin:/usr/share/Modules/bin:/usr/local/bin:/usr/bin:/usr/local/sbin:/usr/sbin

Another useful variable is HOME which specifies the home directory of the current user. If, for example, you wanted the shell to also look for commands in the scripts directory located in your home directory, you would modify the PATH variable as follows:

$ export PATH=$PATH:$HOME/scripts

The current value of an existing environment variable may be displayed using the echo command:

$ echo $PATH

You can create your own environment variables using the export command. For example:

$ export DATAPATH=/data/files

A useful trick to assign the output from a command to an environment variable involves the use of back quotes (`) around the command. For example, to assign the current date and time to an environment variable called NOW:

$ export NOW=`date`

$ echo $NOW

Tue Apr 2 13:48:40 EDT 2020

If there are environment variable or alias settings that you need to be configured each time you enter the shell environment, they may be added to a file in your home directory named .bashrc. For example, the following .bashrc file is configured to set up the DATAPATH environment variable and an alias:

# .bashrc

  

# Source global definitions

if [ -f /etc/bashrc ]; then

        . /etc/bashrc

fi

 

# User specific environment

PATH="$HOME/.local/bin:$HOME/bin:$PATH"

export PATH

 

# Uncomment the following line if you don't like systemctl's auto-paging feature:

# export SYSTEMD_PAGER=

 

# User specific aliases and functions

export DATAPATH=/data/files

alias l="ls -lt"