History

• Written by Brian Fox of Free Software Foundation
• An attempt to create the best of all shells
• Completely backward compatible with Bourne Shell
• Tries to include some features of both C shell and Korn Shell
• Freely available for all flavors and all distros of Linux

Performing Operations ((()))

Perform operations by putting the operation inside a set of double parenthesis.

Example: Arithmetic with double parenthesis

a=$(( 4 + 5 ))
echo $a
# 9
a=$((3+5))
echo $a
# 8
b=$(( a + 3 ))
echo $b
# 11
b=$(( $a + 4 ))
echo $b
# 12
(( b++ ))
echo $b
# 13
(( b += 3 ))
echo $b
# 16
a=$(( 4 * 5 ))
echo $a
# 20

Length

${#a} prints the length of the variable $a

Arrays

Indexed array is automatically created if any variable is assigned with the syntax:

name[subscript]=value

Arrays can also be explicitly declared with:

declare -a name[subscript]

Referencing an array without a subscript is the same as referencing with a subscript of 0.

Any element of an array may be referenced using ${name[subscript]}. The braces are required to avoid conflicts with the shell’s filename expansion operators. If the subscript is ‘@’ or ‘*’, the word expands to all members of the array name.