Chapter 15. Arithmetic Expansion

Arithmetic expansion provides a powerful tool for performing arithmetic operations in scripts. Translating a string into a numerical expression is relatively straightforward using backticks, double parentheses, or let.


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The use of backticks in arithmetic expansion has been superseded by double parentheses $((...)) or the very convenient let construction.

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

let z=z+3
let "z += 3"  # Quotes permit the use of spaces and special operators.
#  The 'let' operator actually performs arithmetic evaluation,
#+ rather than expansion.
All the above are equivalent. You may use whichever one "rings your chimes".

Examples of arithmetic expansion in scripts:

  1. Example 12-6

  2. Example 10-14

  3. Example 26-1

  4. Example 26-6

  5. Example A-18