Chapter 12. 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expansion has been superseded by double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:
Example 15-9
Example 10-14
Example 26-1
Example 26-11
Example A-17