XXX Chapter 13. Arithmetic Expansion¶

Arithmetic expansion provides a powerful tool for performing (integer)

arithmetic operations in scripts. Translating a string into a numerical expression is relatively straightforward using backticks , double parentheses , or let .

** Variations**

Arithmetic expansion with backticks

(often used in conjunction with expr )

z=`expr $z + 3`          # The 'expr' command performs the expansion.
Arithmetic expansion with double parentheses , and

using let

The use of backticks ( backquotes ) in arithmetic expansion has been superseded by double parentheses – `` ((...)) `` and `` $((...)) `` – and also by the very convenient let construction.

z=$(($z+3))
z=$((z+3))                                  #  Also correct.
                                            #  Within double parentheses,
                                            #+ parameter dereferencing
                                            #+ is optional.

# $((EXPRESSION)) is arithmetic expansion.  #  Not to be confused with
                                            #+ command substitution.



# You may also use operations within double parentheses without assignment.

  n=0
  echo "n = $n"                             # n = 0

  (( n += 1 ))                              # Increment.
# (( $n += 1 )) is incorrect!
  echo "n = $n"                             # n = 1


let z=z+3
let "z += 3"  #  Quotes permit the use of spaces in variable assignment.
              #  The 'let' operator actually performs arithmetic evaluation,
              #+ rather than expansion.

Examples of arithmetic expansion in scripts: