The Double-Parentheses Construct

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# 8.3. The Double-Parentheses Construct

Similar to the let command, the (( ... )) construct permits arithmetic expansion and evaluation. In its simplest form, `a=$(( 5 + 3 ))` would set `a` to `5 + 3`, or `8`. However, this double-parentheses construct is also a mechanism for allowing C-style manipulation of variables in Bash, for example, `(( var++ ))`.

Example 8-5. C-style manipulation of variables

    #!/bin/bash
    # c-vars.sh
    # Manipulating a variable, C-style, using the (( ... )) construct.

    echo

    (( a = 23 ))  #  Setting a value, C-style,
                  #+ with spaces on both sides of the "=".
    echo "a (initial value) = $a"   # 23

    (( a++ ))     #  Post-increment 'a', C-style.
    echo "a (after a++) = $a"       # 24

    (( a-- ))     #  Post-decrement 'a', C-style.
    echo "a (after a--) = $a"       # 23

    (( ++a ))     #  Pre-increment 'a', C-style.
    echo "a (after ++a) = $a"       # 24

    (( --a ))     #  Pre-decrement 'a', C-style.
    echo "a (after --a) = $a"       # 23

    echo

    ########################################################
    #  Note that, as in C, pre- and post-decrement operators
    #+ have different side-effects.

    n=1; let --n && echo "True" || echo "False"  # False
    n=1; let n-- && echo "True" || echo "False"  # True

    #  Thanks, Jeroen Domburg.
    ########################################################

    echo

    (( t = a<45?7:11 ))   # C-style trinary operator.
    #       ^  ^ ^
    echo "If a < 45, then t = 7, else t = 11."  # a = 23
    echo "t = $t "                              # t = 7

    echo

    # -----------------
    # Easter Egg alert!
    # -----------------
    #  Chet Ramey seems to have snuck a bunch of undocumented C-style
    #+ constructs into Bash (actually adapted from ksh, pretty much).
    #  In the Bash docs, Ramey calls (( ... )) shell arithmetic,
    #+ but it goes far beyond that.
    #  Sorry, Chet, the secret is out.

    # See also "for" and "while" loops using the (( ... )) construct.

    # These work only with version 2.04 or later of Bash.

    exit

See also Example 11-13 and Example 8-4.

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