# 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The use of backticks (backquotes) in arithmetic expansion has been superseded by double parentheses -- ((...))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: