Shell Variable Basic

Variable naming conventions:

Only letters (a to z or A to Z), numbers (0 to 9), and the underscore character ( _) can be used in a variable’s name in Linux scripting.

Example of some valid variable names:

name, Name, fullName, full_name, _name, name0, name1, name01, NAME, VAR, _VAR

Example of some invalid variable names:

!name, -name, full-name, name!, name*, 1name, 1_name

Here one thing to mention that Bash/Shell variables are case sensitive. Name, name, or NAME will be treated as three different variable names.

Define and use variables:

<variable-name>=<variable-value>  

Example: VALUE=10

Use/access a defined variable, dollar sign ($) is used :

newValue=$VALUE 
echo $VALUE

Note: Space is not allowed in either sides of the equal sign.

Save this code as variable_eample.sh

#!usr/bin/env bash
#space is not allowed in either sides of the equal sign
 name=Olive
 colour=Green
 echo Colour of $name is $colour.

#String cannot have space inside, use quatation mark to degine a string
 full_name="Green Olive"
 echo $full_name

Run in a Linux terminal and you will see the output like:

$ bash variable_eample.sh

Output:
Colour of Olive is Green.
Green Olive

Read-only variable

If a variable is marked as “readonly” its value cannot be changed/updated later. We can use readonly variables to define constants. Example:

#!/bin/sh

# readonly variable
FIXED_VAL=10000
readonly FIXED_VAL
echo value is: $FIXED_VAL
FIXED_VAL=20000 # Cannot be changed, will give error : FIXED_VAL: readonly variable

Unset variable

We can unset a variable using the “unset” keyword. If we use unset on a variable, the shell removes it from the list of variables it keeps track of. We can’t access the value in a variable after it’s been unset. Example:

!/bin/sh

# unsetting variable
OLD_NAME="Akbar"
unset OLD_NAME
echo $OLD_NAME  # this will print nothing as it was unset