linux shell if语句

1、测试1

[root@centos7 test2]# ls
a.txt
[root@centos7 test2]# if [ -e a.txt ]; then echo "exist";else echo "no nxist"; fi
exist
[root@centos7 test2]# if [ -e b.txt ]; then echo "exist";else echo "no nxist"; fi
no nxist

2、测试2

[root@centos7 test2]# seq 3 > a.txt
[root@centos7 test2]# seq 5 > b.txt
[root@centos7 test2]# anum=$(wc -l a.txt|awk '{print $1}')
[root@centos7 test2]# bnum=$(wc -l b.txt|awk '{print $1}')
[root@centos7 test2]# if [ $anum -gt $bnum ]; then echo "a > b"; else echo "b > a"; fi
b > a

3、测试3

[root@centos7 test2]# cat test.sh
#!/bin/bash
read -p "please input your age: " age
if [[ $age =~ [^0-9] ]]; then
echo "please input an integer."
exit 10
elif [ $age -gt 150 ]; then
echo "your age is wrong."
exit 20
elif [ $age -gt 20 ]; then
echo "good good work, day day up"
else
echo "good good study, day day up"
fi
[root@centos7 test2]# bash test.sh
please input your age: abcde
please input an integer.
[root@centos7 test2]# bash test.sh
please input your age: 200
your age is wrong.
[root@centos7 test2]# bash test.sh
please input your age: 35
good good work, day day up
[root@centos7 test2]# bash test.sh
please input your age: 14
good good study, day day up

4、

[root@centos7 test2]# cat test.sh
#!/bin/bash
read -p "please input your score: " score
if [[ $score =~ [^0-9] ]]; then
echo "please input an integer."
exit 10
elif [ $score -gt 100 ]; then
echo "your score is wrong."
exit 20
elif [ $score -gt 85 ]; then
echo "your score is very good."
elif [ $score -ge 60 ]; then
echo "your score is soso."
else
echo "you are loser."
fi
[root@centos7 test2]# bash test.sh
please input your score: abcde
please input an integer.
[root@centos7 test2]# bash test.sh
please input your score: 200
your score is wrong.
[root@centos7 test2]# bash test.sh
please input your score: -13
please input an integer.
[root@centos7 test2]# bash test.sh
please input your score: 99
your score is very good.
[root@centos7 test2]# bash test.sh
please input your score: 78
your score is soso.
[root@centos7 test2]# bash test.sh
please input your score: 34
you are loser.

参考：https://mp.weixin.qq.com/s/rTsLIb2p-AE3oDPQXMQmLw