shell 流程控制

分支语句

• if else-if else

#！ /bin/bash

a=10
b=20
if [ $a == $b ]　　#此处用[[]]也是可以的，shell标准手册中推荐使用 [[]]
then
   echo "a 等于 b"
elif [ $a -gt $b ]
then
   echo "a 大于 b"
elif [ $a -lt $b ]
then
   echo "a 小于 b"
else
   echo "没有符合的条件"
fi

• case

#! /bin/bash

a=4

case $a in 
    1) echo 'a is 1'
        ;;
    2) echo 'a is 2'
        ;;
    3|4) echo 'a is 3 or 4'
        ;;
    *) echo 'a is others'
        ;;
esac

 

循环语句

• for 循环

　　shell for 循环

• while 循环

#! /bin/bash

i=1
while [ $i -le 5 ] 
do
    echo $i
    let "i++"
done

• until 循环

#! /bin/bash

a=0

until [ ! $a -lt 10 ]
do
    echo $a
    a=`expr $a + 1`
done

 

跳出循环

• break
• continue

总结

• case 语句中的 ;; 是不能省略的，不会出现C代码中的，不加break，跳到下一个case中继续执行的写法
• break、continue 只会用于跳出循环

拓展

• Conditional expressions are used by the [[ compound command and the test and [ builtin commands.
• The shell allows arithmetic expressions to be evaluated, as one of the shell expansions or by using the (( compound command, the let builtin, or the -i option to the declare builtin

  $ cat demo.sh 
  #! /bin/bash
  int=1
  while (( $int <= 5))　　   #for ((;;)) 循环也可以这么写
  do
      echo $int
      let "int++"
  done
  
  rivsidn@rivsidn:~/demo/bash/test$ ./demo.sh 
  1
  2
  3
  4
  5