variables

Variables

Variables in bash.

Default Values

Bash provides the following ways to set a variable with a "default" value if the variable is unset. The following examples provide the same output, ie. print value of $lang; if $lang is unset, print 'en-us'.

Format	Example
${var:-default}	echo ${lang:-en-us}
${var=default}	echo ${lang=en-us}
${var:=default}	echo ${lang:=en-us}

Now default value substitution using = and := will not work on positional parameters such as function arguments. So the following code will fail and you will get an error message $1: cannot assign in this way.

function echo_lang() {
  echo "${1=en-us}"
  echo "${1:=en-us}"
}

# Fails
echo_lang

Use :- instead.

function echo_lang() {
  echo "${1:-en-us}"
}

# Works
echo_lang

Declare

declare is a bash construct that set a variable attributes and values.

declare options are:

Option	Description
+	Turn off a given attribute
-	Turn on a given attribute
-g	Create global varaibles when used in a shell
-a	Make variables indexed array (if supported)
-A	Make variables associative array (if supported)
-i	Make variables have integer attribute, similar to let and allows arithmetic
-l	Convert variables to lower case on assignment
-n	Make variable a reference to the variable named by its value
-r	Make variables readonly
-t	Make variables have the 'trace' attribute
-u	Convert variables to upper case on assignment
-x	To make variables export

• Declare variables