9.7. The Double-Parentheses Construct

   1 #!/bin/bash
   2 # c-vars.sh
   3 # Manipulating a variable, C-style, using the (( ... )) construct.
   4 
   5 
   6 echo
   7 
   8 (( a = 23 ))  #  Setting a value, C-style,
   9               #+ with spaces on both sides of the "=".
  10 echo "a (initial value) = $a"   # 23
  11 
  12 (( a++ ))     #  Post-increment 'a', C-style.
  13 echo "a (after a++) = $a"       # 24
  14 
  15 (( a-- ))     #  Post-decrement 'a', C-style.
  16 echo "a (after a--) = $a"       # 23
  17 
  18 
  19 (( ++a ))     #  Pre-increment 'a', C-style.
  20 echo "a (after ++a) = $a"       # 24
  21 
  22 (( --a ))     #  Pre-decrement 'a', C-style.
  23 echo "a (after --a) = $a"       # 23
  24 
  25 echo
  26 
  27 ########################################################
  28 #  Note that, as in C, pre- and post-decrement operators
  29 #+ have different side-effects.
  30 
  31 n=1; let --n && echo "True" || echo "False"  # False
  32 n=1; let n-- && echo "True" || echo "False"  # True
  33 
  34 #  Thanks, Jeroen Domburg.
  35 ########################################################
  36 
  37 echo
  38 
  39 (( t = a<45?7:11 ))   # C-style trinary operator.
  40 #       ^  ^ ^
  41 echo "If a < 45, then t = 7, else t = 11."  # a = 23
  42 echo "t = $t "                              # t = 7
  43 
  44 echo
  45 
  46 
  47 # -----------------
  48 # Easter Egg alert!
  49 # -----------------
  50 #  Chet Ramey seems to have snuck a bunch of undocumented C-style
  51 #+ constructs into Bash (actually adapted from ksh, pretty much).
  52 #  In the Bash docs, Ramey calls (( ... )) shell arithmetic,
  53 #+ but it goes far beyond that.
  54 #  Sorry, Chet, the secret is out.
  55 
  56 # See also "for" and "while" loops using the (( ... )) construct.
  57 
  58 # These work only with version 2.04 or later of Bash.
  59 
  60 exit