Linux Admin - Variables

Variables in Bash are used like any other scripting language. The syntax may vary from languages such as Perl, Python and Ruby.

The first thing we will want to note is BASH variables comes in two basic varieties: Global or Environment Variables and Script or Local Variables.

Global or Environment Variables are set across all shells and scripts. Environment Variables can be displayed with either the env or printenv commands −

bash-3.2# printenv 
SHELL=/bin/bash 
TERM=xterm-256color 
USER=root 
SUDO_USER="rick cardon" 
SUDO_UID=501 
USERNAME=root 
MAIL=/var/mail/root 
PATH=/usr/local/pear/bin:/Library/Frameworks/Python.framework/Versions/3.5/bin:
/opt/local/bin:/opt/local/sbin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/ 
PWD=/home/rdc/Desktop 
LANG=en_US.UTF-8

Common syntax is to use uppercase for Environment variables and lowercase script for local variables. These are the variables used within your script, assuming the script will not be setting or changing the shell Environment Variables.

Script or Local Variables are only accessible to the current shell.

#!/bin/bash
num = 0
while [ $num -lt 100 ]
   do 
   num = $[$num+1] 
   echo $num 
   
   if [ $((num % 10)) = 0 ]; 
      then 
      sleep 5s 
   fi 
done

echo $PATH

The small script above simply increments to 20, pauses for five seconds every 5th iteration, then echoes an environmental variable: the current user's path −

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/usr/local/pear/bin:/Library/Frameworks/Python.framework/Versions/3.5/bin:/opt/
local/bin:/opt/local/sbin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin

If you'd like a variable to be seen outside your shell, the following export command must be used.

#!/bin/bash
export MY_NEW_GLOBAL = "I am Global"