用shell如何写递增的程序

gzhllj

我想实现一个如i++的功能，
用shell如何写呢？
我自己写了个简单的加1的程序，可是总是报错。
我的代码如下：
$ loop = 0
$ loop = `expr $loop+1`

错误如下：
=: not found
expr: syntax error
=: not found

我的系统是aix 5.1

[ 本帖最后由 gzhllj 于 2006-4-17 19:01 编辑 ]

寂寞烈火

注意等号以及expr运算符两边的空格问题
a=0  
a=`expr $a + 1`

gzhllj

非常感谢哈！

yjh777

原帖由 gzhllj 于 2006-4-17 18:54 发表
我想实现一个如i++的功能，
用shell如何写呢？
我自己写了个简单的加1的程序，可是总是报错。
我的代码如下：
$ loop = 0
$ loop = `expr $loop+1`

错误如下：
=: not found
expr: syntax error
=: no ...

用这个(())吧：

$ ((loop = 0))
$ ((loop = $loop + 1))

gzhllj

不行,你的（（）） 是什么作用阿

yjh777

原帖由 gzhllj 于 2006-4-18 16:09 发表
不行,你的（（）） 是什么作用阿

不行？你用的什么shell，看提示符象是bash，什么版本的？

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-type manipulation of variables in Bash.

Example 9-30. C-type manipulation of variables

   1 #!/bin/bash
   2 # Manipulating a variable, C-style, using the ((...)) construct.
   3
   4
   5 echo
   6
   7 (( a = 23 ))  # Setting a value, C-style, with spaces on both sides of the "=".
   8 echo "a (initial value) = $a"
   9
  10 (( a++ ))     # Post-increment 'a', C-style.
  11 echo "a (after a++) = $a"
  12
  13 (( a-- ))     # Post-decrement 'a', C-style.
  14 echo "a (after a--) = $a"
  15
  16
  17 (( ++a ))     # Pre-increment 'a', C-style.
  18 echo "a (after ++a) = $a"
  19
  20 (( --a ))     # Pre-decrement 'a', C-style.
  21 echo "a (after --a) = $a"
  22
  23 echo
  24
  25 ########################################################
  26 #  Note that, as in C, pre- and post-decrement operators
  27 #+ have slightly different side-effects.
  28
  29 n=1; let --n && echo "True" || echo "False"  # False
  30 n=1; let n-- && echo "True" || echo "False"  # True
  31
  32 #  Thanks, Jeroen Domburg.
  33 ########################################################
  34
  35 echo
  36
  37 (( t = a<45?7:11 ))   # C-style trinary operator.
  38 echo "If a < 45, then t = 7, else t = 11."
  39 echo "t = $t "        # Yes!
  40
  41 echo
  42
  43
  44 # -----------------
  45 # Easter Egg alert!
  46 # -----------------
  47 #  Chet Ramey apparently snuck a bunch of undocumented C-style constructs
  48 #+ into Bash (actually adapted from ksh, pretty much).
  49 #  In the Bash docs, Ramey calls ((...)) shell arithmetic,
  50 #+ but it goes far beyond that.
  51 #  Sorry, Chet, the secret is now out.
  52
  53 # See also "for" and "while" loops using the ((...)) construct.
  54
  55 # These work only with Bash, version 2.04 or later.
  56
  57 exit 0

[ 本帖最后由 yjh777 于 2006-4-18 16:22 编辑 ]