04、条件控制.md

提交控制

if语句

if [[ expression ]]
then
   Statement(s) to be executed if expression is true
fi

有三种 if ... else 语句：

if ... fi 语句
if ... else ... fi 语句
if ... elif ... else ... fi 语句

示例

#!/bin/bash/

a=10
b=20
if [ $a == $b ]
then 
	echo "a is equal to b"
elif [ $a -gt $b ]
then
	echo "a is greater to b"
else
	echo "a is less to b"
fi

if ... else 语句也经常与 test 命令结合使用

#!/bin/bash/

a=10
b=20
if test $a == $b 
then 
	echo "a is equal to b"
else
	echo "a is not equal to b"
fi

分支控制：case语句

case ... esac 与其他语言中的 switch ... case 语句类似，是一种多分枝选择结构。 语法示例：

#!/bin/bash/

grade="B"

case $grade in 
	"A") echo "Very Good!";;
	"B") echo "Good!";;
	"C") echo "Come On!";;
	*) 
		echo "You Must Try!"
		echo "Sorry!";;
esac

比较全面的例子：

#!/bin/bash
option="${1}"
case ${option} in
   "-f") FILE="${2}"
      echo "File name is $FILE"
      ;;
   "-d") DIR="${2}"
      echo "Dir name is $DIR"
      ;;
   *) 
      echo "`basename ${0}`:usage: [-f file] | [-d directory]"
      exit 1 # Command to come out of the program with status 1
      ;;
esac

运行

./test.sh -f index.html
File name is index.html

下面结合getopts命令介绍下一个经典的例子：从命令行读取参数。

#!/bin/sh
usage()
{
    echo "Usage: $0 -s [start|stop|reload|restart] -e [online|test]"
    exit 1
}

if [ -z $1 ]; then
    usage
fi

while getopts 's:e:h' OPT; do
    case $OPT in
        s) cmd="$OPTARG";;
        e) env="$OPTARG";;
        h) usage;;
        ?) usage;;
    esac
done

echo $cmd
echo $env

运行示例： sh run.sh -s start -e test

for循环

语法

for 变量 in 列表
do
    command1
    command2
    ...
    commandN
done

示例：

#!/bin/bash/

for value in 1 2 3 4 5
do 
	echo "The value is $value"
done

示例2：遍历文件目录

#!/bin/bash
for FILE in *
do
   echo $FILE
done

示例3： 遍历文件内容： city.txt

beijing
tianjin
shanghai

shell

#!/bin/bash

citys=`cat city.txt`
for city in $citys
do
   echo $city
done

while循环

语法：

while command
do
   Statement(s) to be executed if command is true
done

示例：

#!/bin/bash

c=0;
while [ $c -lt 3 ]
do
	echo "Value c is $c"
	c=`expr $c + 1`
done

跳出循环

使用 break 和 continue 来跳出循环。

break:

#!/bin/bash

i=0
while [ $i -lt 5 ]
do
	i=`expr $i + 1`

	if [ $i == 3 ]
		then
			break
	fi
	echo -e $i
done

continue:
continue命令与break命令类似，只有一点差别，它不会跳出所有循环，仅仅跳出当前循环。

#!/bin/bash

i=0
while [ $i -lt 5 ]
do
	i=`expr $i + 1`

	if [ $i == 3 ]
		then
			continue
	fi
	echo -e $i
	
done