linux 控制结构

一.if

 注：

格式1、格式2：一个条件一个命令；

格式3：一个条件两个命令；

格式4：两个条件三个命令，注意条件的写法。

例1：

#!/bin/sh
#ifTest
#to show the method of if
echo -e "Enter the first integer:c"
read FIRST
echo -n "Enter the second integer:"
read SECOND
if [ "$FIRST" -gt "$SECOND" ]
then
echo "$FIRST is greater than $SECOND"
elif [ "$FIRST" -lt "$SECOND" ]
then
echo "$FIRST is less than $SECOND"
else
echo "$FIRST is equal to $SECOND"
fi

执行：

[root@cdh1 sh]# ./test.sh
Enter the first integer:7
Enter the second integer:7
7 is equal to 7
[root@cdh1 sh]# ./test.sh
Enter the first integer:8
Enter the second integer:7
8 is greater than 7
[root@cdh1 sh]#

 例2:

#!/bin/bash
#5.sh

#declare a,b ;

a=$1
b=$2
if [ "$a" = "$b" ] ;then
echo "a=b"
else
echo "a!=b"
fi

例3.

read -p "Please input (Y/N): " yn

if [ "$yn" = "Y" ] || [ "$yn" = "y" ]; then
echo "OK, continue"
exit 0
fi
if [ "$yn" = "N" ] || [ "$yn" = "n" ]; then
echo "Oh, interrupt!"
exit 0
fi
echo "I don't know what your choice is" && exit 0

执行

[root@cdh1 sh]# ./2.sh
Please input (Y/N): y
OK, continue

二.case

例1

#!/bin/sh
#caseTest
#to test the method of case
USER=`whoami`

case $USER in
root)echo "You can do all the operations"
;;
Dave)echo "You can do some operations"
;;
*)echo "Sorry,you can not do anything"
;;
esac

执行：

[root@cdh1 sh]# ./test.sh
You can do all the operations

三、for

例子:

#!/bin/sh
#forTest
#to test the method of for
COUNTER=0
for aa in *
do
echo ${aa}
COUNTER=`expr $COUNTER + 1`
done
echo "There are $COUNTER files in `pwd` "

执行

[root@cdh1 sh]# ./test.sh
2
2.sh
3.sh
4.sh
5.sh
function.sh
test.sh
There are 7 files in /opt/data/sh