shell脚本基础知识梳理<三>：条件测试与比较、流程控制 if

一、条件测试与比较
1、test 测试表达式 常用
test -f file && echo true||echo false
2、[ 测试表达式 ] 特别常用
[ -f file ]&& echo true||echo false
3、[[ 测试表达式 ]]
二、流程控制

1、流控制语句 if
实例1
#!/bin/bash
#获取uid=0(root)中的0；
id=id | awk -F ‘[=(]‘ ‘{print $2}‘
echo "your user id is:$id"
if [ $id -eq 0 ]
then
echo "root"
else
echo "not root"
fi
实例2：判断登录的用户
#!/bin/bash
#$#是添加到shell 参数个数
if [ $# -eq 1 ] #或 [[ $#==1 ]] 或 (($#==1))
then
if who|grep $1 >/dev/null
then
echo $1 is active.
else
echo $1 is not active.
fi
else
echo "Usage: $0 <username>"
exit 1
fi
~
执行后的结果
[[email protected] shell]# sh if-if.sh root
root is active.
[[email protected] shell]# sh if-if.sh zabbix
zabbix is not active.
[[email protected] shell]# sh if-if.sh
Usage: if-if.sh <username>
[[email protected] shell]#
实例3 if-elif..else-fi
#!/bin/bash
##if-elif..else-fi
read -p "how lod are you? " age
#使用shell算数运算符(())进行条件测试
if ((age<0||age>120));then #[[ age < 0 || age > 120 ]]
echo "out of range ！"
exit 1
fi
if ((age>=0&&age<13));then
echo "child！"
elif ((age>=13&&age<20));then
echo "callan!"
elif ((age>=20&&age<30));then
echo "P iii"
elif ((age>=30&&age<40));then
echo "P IV I"
else
echo "Sorry I asked."
fi
实例4
#!/bin/bash
##if 语句可以嵌套使用
file=$1
[ $# -ne 1 ] && echo "Usage: $0 <filename>" && exit 1
#错误的写法 [ $# -ne 1 ] && echo "Usage: $0 <filename>" ; exit 1 这样";" 不管前面的判断是否正确都会执
行
if [ -d $file ]
then
echo "$file is a directory"
elif [ -f $file ]
then
if [ -r $file -a -w $file -a -x $file ];then
echo "you have (rwx) permissioon on $file."
else
echo "$file is file."
fi
else
echo "$fles is neither a file nor a directory."
fi
执行结果
[[email protected] shell]# vim if-elif-if-else.sh
[[email protected] shell]# sh if-elif-if-else.sh liu
you have (rwx) permissioon on liu.
[[email protected] shell]# sh if-elif-if-else.sh liub
liub is a directory
[[email protected] shell]# touch qq
[[email protected] shell]# sh if-elif-if-else.sh qq
qq is file.
[[email protected] shell]#

原文地址：https://blog.51cto.com/14294148/2434481