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*.

- 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*backticks*(*backquotes*) in arithmetic expansion has been superseded by*double parentheses*--and**((...))**-- and also by the very convenient let construction.**$((...))**z=$(($z+3)) z=$((z+3)) # Also correct.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: