（1）if语句

root@ubuntu:/mnt/shared/shellbox/shellif# cat shellif.sh 
#!/bin/bash

#判断字符串
if [ "$1" = "hello" ]
then
        echo "\$1=$1"
fi

#判断数字,if()方式只能在bash下用，在sh下不行
if ((  $1 > 20 ))
then
        echo "\$1: $1 > 20"
elif (( $1 == 20 ))
then
        echo "\$1 == 20"
elif (( $1 < 20 ))
then
        echo "\$1 < 20"
fi

#方括号判断语句
if [ $1 -lt 20 ]
then
        echo "\$1 < 20"
elif [ $1 -ge 20 -a $1 -le 30 ]
then
        echo "\$1 >= 20 && \$1 <= 30 "
elif [ $1 -gt 30 ]
then
        echo "\$1 > 30"
fi

root@ubuntu:/mnt/shared/shellbox/shellif# ./shellif.sh 10
$1 < 20
$1 < 20
root@ubuntu:/mnt/shared/shellbox/shellif# ./shellif.sh 20
$1 == 20
$1 >= 20 && $1 <= 30 
root@ubuntu:/mnt/shared/shellbox/shellif# ./shellif.sh 30
$1: 30 > 20
$1 >= 20 && $1 <= 30 
root@ubuntu:/mnt/shared/shellbox/shellif# ./shellif.sh 40
$1: 40 > 20
$1 > 30

（2）for语句

root@ubuntu:/mnt/shared/shellbox/shellfor# cat shellfor.sh 
#!/bin/bash

for i in $*
do
        echo $i
done

for char in {a..c}
do
        echo $char
done


for int in {1..3}
do 
        echo $int
done

root@ubuntu:/mnt/shared/shellbox/shellfor# ./shellfor.sh 
a
b
c
1
2
3

（3）while语句：

root@ubuntu:/mnt/shared/shellbox/shellwhile# cat shellwhile.sh 
#!/bin/bash

#注意: (( ))这种方式只能在bash中使用，而不能在sh中使用
i=0
while (( i < $1 ))
do
    echo "i=$i"
    let i+=1
done

#赋值时"="前后不能有空格
num=0
while [[ $num != $1 ]]
do
        echo "num=$num, num != \$1"
        let num+=1
done

while true
do
        echo "here in while true ..."
        sleep 2
done

root@ubuntu:/mnt/shared/shellbox/shellwhile# ./shellwhile.sh 5
i=0
i=1
i=2
i=3
i=4
num=0, num != $1
num=1, num != $1
num=2, num != $1
num=3, num != $1
num=4, num != $1
here in while true ...
here in while true ...

shell 脚本之if、for、while语句