| Chapter 13. Arithmetic ExpansionArithmetic expansion provides apowerful tool for performing (integer) arithmeticoperations in scripts. Translating a string into anumerical expression is relatively straightforward usingbackticks, doubleparentheses, 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The use of backticks (backquotes) in arithmetic expansion has been superseded by double parentheses -- ((...))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 = 1let z=z+3let "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: Example 16-9 Example 11-14 Example 27-1 Example 27-11 Example A-16
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