linux shell script程序之选择结构简介

        很多人都是先学习C/C++语言， 然后再学习shell script的。 如果大家有一点点编程基础， 那么就很容易理解所谓的顺序、选择和循环。顺序结构其实没什么好说的， 之前早就接触过了， 在本文中， 我们来介绍一下选择结构。 虽然简单， 但还是要熟练正确地使用， 毕竟和C/C++的语法还是有一些出入的。 一不小心， 就容易出错。

1. 先看个入门级别的：

[taoge@localhost learn_shell]$ cat a.sh
#! /bin/bash
echo "enter your bank password"
read passwd

if [ $passwd = "3.1415" ]; then
echo yes
else
echo no
fi
[taoge@localhost learn_shell]$
[taoge@localhost learn_shell]$
[taoge@localhost learn_shell]$
[taoge@localhost learn_shell]$ ./a.sh
enter your bank password
1
no
[taoge@localhost learn_shell]$ ./a.sh
enter your bank password
3.1415
yes
[taoge@localhost learn_shell]$ ./a.sh
enter your bank password
12  34
./a.sh: line 5: [: too many arguments
no
[taoge@localhost learn_shell]$

     上面的程序， 有如下几点值得注意：

     (1) $passwd两边最好加上引号， 免得用户输入12 34的时候， 出现 12 34 = "3.1415"这样的错误。

     (2) if后面必须有空格， 好恶心的规定啊。

     (3) [ ]里面该加空格的地方一定要加， 否则会有错误。

     (4) 如果then和if处在同一行， 那么then前面的那个分号绝对不能少。 当然， then可以放到下面独立成行

     (5) 在比较相等的时候， 最好用==, 而不是=,  尽管后者也可以， 但是会让很多c/c++程序猿感觉难以适应， 感觉不舒服。

      下面， 我来改一下， 使得上面程序更好：

[taoge@localhost learn_shell]$ cat a.sh
#! /bin/bash
echo "enter your bank password"
read passwd

if [ "$passwd" == "3.1415" ]  # pay special attention to this line, my friends!
then
echo yes
else
echo no
fi
[taoge@localhost learn_shell]$
[taoge@localhost learn_shell]$
[taoge@localhost learn_shell]$
[taoge@localhost learn_shell]$ ./a.sh
enter your bank password
1
no
[taoge@localhost learn_shell]$ ./a.sh
enter your bank password
3.1415
yes
[taoge@localhost learn_shell]$ ./a.sh
enter your bank password
12  34
no
[taoge@localhost learn_shell]$

      和C/C++一样， linux shell script也会进行短路求值， 如下（我认为这个程序非常精妙）：

[taoge@localhost learn_shell]$ cat a.sh
#! /bin/bash

#! /bin/bash
echo "enter your bank password"
read passwd

[ "$passwd" == "3.1415" ] && echo yes
[ "$passwd" == "3.1415" ] || echo no
[taoge@localhost learn_shell]$
[taoge@localhost learn_shell]$
[taoge@localhost learn_shell]$
[taoge@localhost learn_shell]$ ./a.sh
enter your bank password
3.1415
yes
[taoge@localhost learn_shell]$ ./a.sh
enter your bank password
3
no
[taoge@localhost learn_shell]$

    2. 看个稍微复杂一点点的， 其实也很简单：

[taoge@localhost learn_shell]$ cat a.sh
#! /bin/bash
echo "enter your login password"
read passwd

if [ "$passwd" == "yaoming" ]
then
echo "hello yaoming, you are so tall"
elif [ "$passwd" == liuxiang ]
then
echo "hello liuxiang, you are so fast"
elif [ "$passwd" == wangliqin ]
then
echo "hello wangliqin, you play pingpong so well"
else
#no then in this line
echo "you do not belong here"
fi
[taoge@localhost learn_shell]$
[taoge@localhost learn_shell]$
[taoge@localhost learn_shell]$
[taoge@localhost learn_shell]$ ./a.sh
enter your login password
yaoming
hello yaoming, you are so tall
[taoge@localhost learn_shell]$ ./a.sh
enter your login password
liuxiang
hello liuxiang, you are so fast
[taoge@localhost learn_shell]$ ./a.sh
enter your login password
wangliqin
hello wangliqin, you play pingpong so well
[taoge@localhost learn_shell]$ ./a.sh
enter your login password
zhangyining
you do not belong here
[taoge@localhost learn_shell]$

     3. 再看case:

[taoge@localhost learn_shell]$ cat a.sh
#! /bin/bash
echo "enter your login password"
read passwd

case "$passwd" in
"yaoming")
echo "hello yaoming, you are so tall"
;;

"liuxiang")
echo "hello liuxiang, you are so fast"
;;

"wangliqin")
echo "hello wangliqin, you play pingpong so well"
;;

*)
echo "you do not belong here"
;;
esac
[taoge@localhost learn_shell]$
[taoge@localhost learn_shell]$
[taoge@localhost learn_shell]$
[taoge@localhost learn_shell]$ ./a.sh
enter your login password
yaoming
hello yaoming, you are so tall
[taoge@localhost learn_shell]$ ./a.sh
enter your login password
liuxiang
hello liuxiang, you are so fast
[taoge@localhost learn_shell]$ ./a.sh
enter your login password
wangliqin
hello wangliqin, you play pingpong so well
[taoge@localhost learn_shell]$ ./a.sh
enter your login password
zhangyining
you do not belong here
[taoge@localhost learn_shell]$

     需要注意： ;;就类似于C/C++语言中的break, 只不过， 在此处绝不可少， 否则运行shell的时候， 会出问题。

                         另外， 最后的*)位置相当重要， 必须放在最后， 因为linux shell script中的case是逐条前后逐条匹配的， 而C/C++中的default放置的位置则没有限制， 放在最开始的位置也没有任何关系。

     下面，我们来一起验证一下， *)放在前面会导致逻辑错误， 如下：

[taoge@localhost learn_shell]$ cat a.sh
#! /bin/bash
echo "enter your login password"
read passwd

case "$passwd" in
*)
echo "you do not belong here"
;;

"yaoming")
echo "hello yaoming, you are so tall"
;;

"liuxiang")
echo "hello liuxiang, you are so fast"
;;

"wangliqin")
echo "hello wangliqin, you play pingpong so well"
;;
esac
[taoge@localhost learn_shell]$
[taoge@localhost learn_shell]$
[taoge@localhost learn_shell]$
[taoge@localhost learn_shell]$ ./a.sh
enter your login password
yaoming
you do not belong here
[taoge@localhost learn_shell]$

    

      4. test命令和[ ]命令

       先说明一下， test命令完全等价于[ ]命令， 可以任选其一， 当然， 每个人有不同的爱好， 我个人就比较喜欢[ ]命令， 下面， 我们来一起看一下：

[taoge@localhost learn_shell]$ cat a.sh
#! /bin/bash
echo "enter your login password"
read passwd

if test "$passwd" == "linux" ; then
echo yes
else
echo no
fi
[taoge@localhost learn_shell]$
[taoge@localhost learn_shell]$
[taoge@localhost learn_shell]$
[taoge@localhost learn_shell]$ ./a.sh
enter your login password
linux
yes
[taoge@localhost learn_shell]$ ./a.sh
enter your login password
LINUX
no
[taoge@localhost learn_shell]$

     我看着test就感觉挺别扭的， 还是用[ ]吧。在用[ ]的时候一定要注意： [后面如果没有空格， 那将是语法错误， 但是， 如果 ==左右没有空格， 那就是致命的逻辑错误， 此时if会永远为真， 千千万万要注意啊。 linux shell script就是这样， 到处都是蛇， 小心它咬你。

     最后， 我们以一个稍微复杂一点的程序来结束本文：

[taoge@localhost learn_shell]$ cat a.sh
#! /bin/bash

# compare string
name=""
if [ "$name" == "" ]; then
echo log1
else
echo log2
fi

if [ -z "$name" ]; then
echo log3
else
echo log4
fi

name="taoge"
if [ -z "$name" ]; then
echo log5
else
echo log6
fi

if [ -n "$name" ]; then
echo log7
else
echo log8
fi

if [ "$name" == "taoge" ]; then
echo log9
else
echo log10
fi

# compare number
if [ "$#" -eq 0 ]; then
echo log11
else
echo log12
fi

if [ "$#" -lt 1 ]; then
echo log13
else
echo log14
fi

if [ 1 -lt 2  -a  2 -lt 3 ]; then
echo log15
else
echo log16
fi

if [ 1 -lt 2 ] && [ 2 -lt 3 ]; then
echo log17
else
echo log18
fi

# test directory or file
pwd
if [ -e /home/taoge/Desktop/learn_shell ]; then
echo log19
else
echo log20
fi

if [ -e /home/taoge/Desktop/learn_shell/test.html ]; then
echo log21
else
echo log22
fi

touch /home/taoge/Desktop/learn_shell/test.html
if [ -e /home/taoge/Desktop/learn_shell/test.html ]; then
echo log23
else
echo log24
fi

[taoge@localhost learn_shell]$
[taoge@localhost learn_shell]$
[taoge@localhost learn_shell]$
[taoge@localhost learn_shell]$ ./a.sh
log1
log3
log6
log7
log9
log11
log13
log15
log17
/home/taoge/Desktop/learn_shell
log19
log22
log23
[taoge@localhost learn_shell]$

      好吧， 先说到这里。