Variables in Bash

April 26, 2017 by cryan.com

Using variables in Bash is easy to learn if you're familiar with PHP programming.

Four Simple tips when using variables in Bash

1. Keep it simple
2. Use variable parameters to expand the script capabilities
3. Watch for extra spacing when using the command expansion ${
4. When using characters as values make sure to use double quotes.

Learn by Example

One of the first Web programming books that I found useful was "Perl by Example." It's a great book to learn various Perl functionality. ( I still have the book!)

I got one of the early editions which was a much smaller book and a lot easier to understand

So here are some Variable examples in BASH:

Simple HelloWorld.sh Example

Basics of assigning a text value to a BASH variable:

#!/bin/bash 
STR="Hello World!" 
echo $STR 

BASH script with Variable Parameters

You can pass additional parameters to you BASH script at run time:

#!/bin/sh
#
# Echo a couple of script input values
# usage:
#    name   
#
# The result will be an echo of the first and last
if [ $# != 3 ]; then
 echo "usage: name.sh first middle last"
 exit 0
fi
FIRST="$1"
MIDDLE="$2"
LAST="$3"
echo $FIRST . " " . $LAST

Execute Command Lines via BASH variables

You can assign the results of an command line to a BASH variable

#!/bin/bash
# Get the last day of March
lastday=$(date -v+3w -v1d -v+1m -v-1d +"%B %e, %Y")
echo $lastday

Simple Math in Bash

#!/bin/bash
num1=2017
num2=1975
total=$(($num1 - $num2))
echo "Did you know 1975 was $total years ago!"

Miscellaneous Notes

In Bash $$ is the process ID, as noted in the comments it is not safe to use as a temp filename for a variety of reasons.