高级Bash脚本编程指南(17)：测试和分支(case与select结构)

高级Bash脚本编程指南(17)：测试和分支(case与select结构)

成于坚持，败于止步

测试和分支(case与select结构)

case和select结构在技术上说并不是循环, 因为它们并不对可执行代码块进行迭代.但是和循环相似的是, 它们也依靠在代码块顶部或底部的条件判断来决定程序的分支.

case (in) / esac

在shell中的case结构与C/C++中的switch结构是相同的. 它允许通过判断来选择代码块中多条路径中的一条. 它的作用和多个if/then/else语句的作用相同,是它们的简化结构, 特别适用于创建菜单.

case "$variable" in

 "$condition1" )
 command...
 ;;

 "$condition2" )
 command...
 ;;

esac

注意四点：

1、对变量使用""并不是强制的, 因为不会发生单词分割.

2、每句测试行, 都以右小括号)来结尾.

3、每个条件判断语句块都以一对分号结尾 ;;.

4、case块以esac (case的反向拼写)结尾.

看一个实例，看看case是如何使用的

#!/bin/bash
# 测试字符串范围.

while echo; echo "Hit a key, then hit return, q(Quit)"
do 
	read Keypress
	if [ "$Keypress" = "q" ]
	then
		exit 0
	fi

	case "$Keypress" in
	  [[:lower:]]   ) echo "Lowercase letter";;
	  [[:upper:]]   ) echo "Uppercase letter";;
	  [0-9]         ) echo "Digit";;
	  *             ) echo "Punctuation, whitespace, or other";;
	esac      #  允许字符串的范围出现在[中括号]中,
		      #+ 或者出现在POSIX风格的[[双中括号中.
done
#  在这个例子的第一个版本中,
#+ 测试大写和小写字符串的工作使用的是
#+ [a-z] 和 [A-Z].
#  这种用法在某些特定场合的或某些Linux发行版中不能够正常工作.
#  POSIX 的风格更具可移植性.
#  感谢Frank Wang指出了这点.

exit 0

结果：

root@ubuntu:~/resource/shell-study/0613-2013# ./test4.sh 

Hit a key, then hit return, q(Quit)
w
Lowercase letter

Hit a key, then hit return, q(Quit)
1
Digit

Hit a key, then hit return, q(Quit)
E
Uppercase letter

Hit a key, then hit return, q(Quit)
q
root@ubuntu:~/resource/shell-study/0613-2013#

接着看一个例子，使用case来创建菜单：

#!/bin/bash

clear # 清屏.

echo "          Contact List"
echo "          ------- ----"
echo "Choose one of the following persons:"
echo
echo "[E]vans, Roland"
echo "[J]ones, Mildred"
echo "[w]yu, Wang"
echo

read person

for para in $person
do
	case "$para" in
	"E" | "e" )
		# 接受大写或者小写输入.
		echo
		echo "Roland Evans"
		echo "4321 Floppy Dr."
		echo "Hardscrabble, CO 80753"
		echo "(303) 734-9874"
		echo "(303) 734-9892 fax"
		echo "[email protected]"
		echo "Business partner & old friend"
		;;
		# 注意, 每个选项后边都要以双分号;;结尾.

	"J" | "j" )
		echo
		echo "Mildred Jones"
		echo "249 E. 7th St., Apt. 19"
		echo "New York, NY 10009"
		echo "(212) 533-2814"
		echo "(212) 533-9972 fax"
		echo "[email protected]"
		echo "Ex-girlfriend"
		echo "Birthday: Feb. 11"
		;;

	"W" | "w" )
		echo
		echo "yu wang"
		echo "1987-07-30"
		echo "New York, NY 10009"
		echo "(212) 533-2814"
		echo "(212) 533-9972 fax"
		echo "[email protected]"
		echo "Ex-girlfriend"
		;;

	  * )
	   # 默认选项.
	   # 空输入(敲回车RETURN), 也适用于这里.
	  echo
	  echo "Not yet in database."
	  ;;

	esac
done

echo

exit 0

结果：

          Contact List
          ------- ----
Choose one of the following persons:

[E]vans, Roland
[J]ones, Mildred
[w]yu, Wang

j w

Mildred Jones
249 E. 7th St., Apt. 19
New York, NY 10009
(212) 533-2814
(212) 533-9972 fax
[email protected]
Ex-girlfriend
Birthday: Feb. 11

yu wang
1987-07-30
New York, NY 10009
(212) 533-2814
(212) 533-9972 fax
[email protected]
Ex-girlfriend

root@ubuntu:~/resource/shell-study/0613-2013#

上面两个实例同时使用了while和for循环的方式对书中代码进行了少许修改，书中给了个很聪明的实例，那就一定要看看了，O(∩_∩)O~

#! /bin/bash

case "$1" in
"")					# 没有命令行参数,或者第一个参数为空.
	echo "Usage: ${0##*/} "
	exit $E_PARAM
	;;  

# 注意: ${0##*/} 是 ${var##pattern} 的一种替换形式. 得到的结果为$0.

-*)
	FILENAME=./$1   #  如果传递进来的文件名参数($1)以一个破折号开头,
					#+ 那么用./$1来代替.
					#+ 这样后边的命令将不会把它作为一个选项来解释.
	echo $FILENAME
	;;

* )
	FILENAME=$1     # 否则, $1.
	echo $FILENAME
	;;
esac

结果：

root@ubuntu:~/resource/shell-study/0613-2013# ./test6.sh test1.sh 
test1.sh
root@ubuntu:~/resource/shell-study/0613-2013# ./test6.sh -ehello 
./-ehello
root@ubuntu:~/resource/shell-study/0613-2013# 

不过呢，可能你并不理解，这个脚本到底聪明在哪里，要理解这个程序，你首先看一下下面的过程：

root@ubuntu:~/resource/shell-study/0613-2013# touch -ehello
touch: invalid option -- 'e'
Try `touch --help' for more information.
root@ubuntu:~/resource/shell-study/0613-2013# ls
nested-loop.sh  test1.sh  test2.sh  test3.sh  test4.sh  test5.sh  test6.sh
root@ubuntu:~/resource/shell-study/0613-2013# touch ./-ehello
root@ubuntu:~/resource/shell-study/0613-2013# ls
-ehello  nested-loop.sh  test1.sh  test2.sh  test3.sh  test4.sh  test5.sh  test6.sh

可以看到，当我使用touch -ehello去新建一个名叫“-ehello”的空文件时，提示错误，因为默认把-后面的字符解析为命令，要想避免shell把-解析为命令，就需要使用下面的第二种方式，touch ./-ehello，所以现在可以看出这个脚本的聪明之处了吧，放在把以“-”开头的文件名解析为命令

还有一个一个命令行参数处理的更容易理解的例子:

#! /bin/bash

while [ $# -gt 0 ]; do    # 直到你用完所有的参数 . . .
  case "$1" in
    -d|--debug)
		# 是 "-d" 或 "--debug" 参数?
		DEBUG=1
		echo "open the debug mode"
		;;
    -c|--conf)
		CONFFILE="$2"
		shift
		if [ ! -f $CONFFILE ]; then
			echo "Error: Supplied file doesn't exist!"
			exit $E_CONFFILE     # 错误: 文件未发现.
		else
			echo "command: -c $CONFFILE"
		fi
		;;
  esac
  shift       # 检查剩余的参数.
done

结果：

root@ubuntu:~/resource/shell-study/0613-2013# ./test7.sh -d
open the debug mode
root@ubuntu:~/resource/shell-study/0613-2013# ./test7.sh -c test1.sh 
command: -c test1.sh
root@ubuntu:~/resource/shell-study/0613-2013# 

使用命令替换来产生case变量

#!/bin/bash
# case-cmd.sh: 使用命令替换来产生"case"变量.

case $( arch ) in   # "arch" 返回机器体系的类型.
                    # 等价于 'uname -m' ...
i386 ) echo "80386-based machine";;
i486 ) echo "80486-based machine";;
i586 ) echo "Pentium-based machine";;
i686 ) echo "Pentium2+-based machine";;
*    ) echo "Other type of machine";;
esac

exit 0

结果：

root@ubuntu:~/resource/shell-study/0613-2013# chmod +x test8.sh 
root@ubuntu:~/resource/shell-study/0613-2013# ./test8.sh 
Pentium2+-based machine
root@ubuntu:~/resource/shell-study/0613-2013# 

简单的字符串匹配

#!/bin/bash

match_string ()
{
  MATCH=0
  NOMATCH=90
  PARAMS=2     # 此函数需要2个参数.
  BAD_PARAMS=91

  [ $# -eq $PARAMS ] || return $BAD_PARAMS

  case "$1" in
  "$2") return $MATCH;;
  *   ) return $NOMATCH;;
  esac

}


a=one
b=two
c=three
d=two


match_string $a     # 参数个数错误.
echo $?             # 91

match_string $a $b  # 不匹配
echo $?             # 90

match_string $b $d  # 匹配
echo $?             # 0


exit 0

结果：

root@ubuntu:~/resource/shell-study/0613-2013# chmod +x test9.sh 
root@ubuntu:~/resource/shell-study/0613-2013# ./test9.sh 
91
90
0
root@ubuntu:~/resource/shell-study/0613-2013# 

检查输入字符是否为字母

#!/bin/bash
# isalpha.sh: 使用"case"结构来过滤字符串.

SUCCESS=0
FAILURE=-1

isalpha ()  # 检查输入的 *第一个字符* 是不是字母表上的字符.
{
if [ -z "$1" ]                # 没有参数传进来?
then
  return $FAILURE
fi

case "$1" in
[a-zA-Z]*) return $SUCCESS;;  # 以一个字母开头?
*        ) return $FAILURE;;
esac
}             # 同C语言的"isalpha ()"函数比较一下.


isalpha2 ()   # 测试 *整个字符串* 是否都是字母表上的字符.
{
  [ $# -eq 1 ] || return $FAILURE

  case $1 in
  *[!a-zA-Z]*|"") return $FAILURE;;
               *) return $SUCCESS;;
  esac
}

isdigit ()    # 测试 *整个字符串* 是否都是数字.
{             # 换句话说, 就是测试一下是否是整数变量.
  [ $# -eq 1 ] || return $FAILURE

  case $1 in
  *[!0-9]*|"") return $FAILURE;;
            *) return $SUCCESS;;
  esac
}



check_var ()  # 测试isalpha().
{
if isalpha "$@"
then
  echo "\"$*\" begins with an alpha character."
  if isalpha2 "$@"
  then        # 不需要测试第一个字符是否是non-alpha.
    echo "\"$*\" contains only alpha characters."
  else
    echo "\"$*\" contains at least one non-alpha character."
  fi
else
  echo "\"$*\" begins with a non-alpha character."
              # 如果没有参数传递进来, 也是"non-alpha".
fi

echo

}

digit_check ()  # 测试isdigit().
{
if isdigit "$@"
then
  echo "\"$*\" contains only digits [0 - 9]."
else
  echo "\"$*\" has at least one non-digit character."
fi

echo

}

a=23skidoo
b=H3llo
c=-What?
d=What?
e=`echo $b`   # 命令替换.
f=AbcDef
g=27234
h=27a34
i=27.34

check_var $a
check_var $b
check_var $c
check_var $d
check_var $e
check_var $f
check_var     # 没有参数传递进来, 将会发生什么?

digit_check $g
digit_check $h
digit_check $i


exit 0        # S.C改进了这个脚本.

结果：

root@ubuntu:~/resource/shell-study/0613-2013# chmod +x test10.sh 
root@ubuntu:~/resource/shell-study/0613-2013# ./test10.sh 
./test10.sh: line 16: return: -1: invalid option
return: usage: return [n]
"23skidoo" begins with a non-alpha character.

"H3llo" begins with an alpha character.
./test10.sh: line 26: return: -1: invalid option
return: usage: return [n]
"H3llo" contains at least one non-alpha character.

./test10.sh: line 16: return: -1: invalid option
return: usage: return [n]
"-What?" begins with a non-alpha character.

"What?" begins with an alpha character.
./test10.sh: line 26: return: -1: invalid option
return: usage: return [n]
"What?" contains at least one non-alpha character.

"H3llo" begins with an alpha character.
./test10.sh: line 26: return: -1: invalid option
return: usage: return [n]
"H3llo" contains at least one non-alpha character.

"AbcDef" begins with an alpha character.
"AbcDef" contains only alpha characters.

./test10.sh: line 11: return: -1: invalid option
return: usage: return [n]
./test10.sh: line 16: return: -1: invalid option
return: usage: return [n]
"" begins with a non-alpha character.

"27234" contains only digits [0 - 9].

./test10.sh: line 36: return: -1: invalid option
return: usage: return [n]
"27a34" has at least one non-digit character.

./test10.sh: line 36: return: -1: invalid option
return: usage: return [n]
"27.34" has at least one non-digit character.

root@ubuntu:~/resource/shell-study/0613-2013# 

select结构是建立菜单的另一种工具, 这种结构是从ksh中引入的.

select variable [in list]
do
	command...
	break
done

提示用户输入选择的内容(比如放在变量列表中)，注意: select命令使用PS3提示符, 默认为(\#?), 当然, 这可以修改.

#!/bin/bash

PS3='Choose your favorite vegetable: ' # 设置提示符字串.

echo

select vegetable in "beans" "carrots" "potatoes" "onions" "rutabagas"
do
  echo
  echo "Your favorite veggie is $vegetable."
  echo "Yuck!"
  echo
  break  # 如果这里没有 'break' 会发生什么?
done

exit 0

结果：

root@ubuntu:~/resource/shell-study/0613-2013# chmod +x test11.sh 
root@ubuntu:~/resource/shell-study/0613-2013# ./test11.sh 

1) beans
2) carrots
3) potatoes
4) onions
5) rutabagas
Choose your favorite vegetable: 2

Your favorite veggie is carrots.
Yuck!

root@ubuntu:~/resource/shell-study/0613-2013# 

如果注释掉其中的break，结果也许会出于你的意料吧：

#!/bin/bash

PS3='Choose your favorite vegetable: ' # 设置提示符字串.

echo

select vegetable in "beans" "carrots" "potatoes" "onions" "rutabagas"
do
  echo
  echo "Your favorite veggie is $vegetable."
  echo "Yuck!"
  echo
  #break  # 如果这里没有 'break' 会发生什么?
done

exit 0

结果：

root@ubuntu:~/resource/shell-study/0613-2013# ./test11.sh 

1) beans
2) carrots
3) potatoes
4) onions
5) rutabagas
Choose your favorite vegetable: 1

Your favorite veggie is beans.
Yuck!

Choose your favorite vegetable: 2

Your favorite veggie is carrots.
Yuck!

Choose your favorite vegetable: 3

Your favorite veggie is potatoes.
Yuck!

Choose your favorite vegetable: 4

Your favorite veggie is onions.
Yuck!

Choose your favorite vegetable: 5

Your favorite veggie is rutabagas.
Yuck!

Choose your favorite vegetable: ^C
root@ubuntu:~/resource/shell-study/0613-2013# 

直到你使用ctrl+c才会退出

如果忽略了in list列表, 那么select命令将会使用传递到脚本的命令行参数(\$@), 或者是函数参数(当select是在函数中时).

与忽略in list的for variable [in list]结构比较一下.

#!/bin/bash

PS3='Choose your favorite vegetable: '

echo

choice_of()
{
select vegetable
# [in list]被忽略, 所以'select'使用传递给函数的参数.
do
  echo
  echo "Your favorite veggie is $vegetable."
  echo "Yuck!"
  echo
  break
done
}

choice_of beans rice carrots radishes tomatoes spinach
#         $1    $2   $3      $4       $5       $6
#         传递给choice_of()的参数

exit 0

结果：

root@ubuntu:~/resource/shell-study/0613-2013# chmod +x test112.sh 
root@ubuntu:~/resource/shell-study/0613-2013# ./test112.sh 

1) beans
2) rice
3) carrots
4) radishes
5) tomatoes
6) spinach
Choose your favorite vegetable: 2

Your favorite veggie is rice.
Yuck!

root@ubuntu:~/resource/shell-study/0613-2013# 

分支用法就说到这里

先到这里了，O(∩_∩)O~

我的专栏地址：http://blog.csdn.net/column/details/shell-daily-study.html

待续。。。。。