shell脚本：选择分支
选择分支可分为三类：1.单分支：if [ ] ;then
. .
fi
example: vim a.sh
#!/bin/bash
if[ -e/etc/issue] ; then
echo "/etc/issue's info : "
ls -l /etc/issue
fi
2.双分支：if [ ];then
..
else
..
fi
example:判断/etc/passwd下是否存在root，如果存在，输出“root exist”，否则输出“root not exist”
vim b.sh
#!/bin/bash
ifcut -d: -f1 /etc/passwd | grep "^$root$" &> /dev/null;then
echo "root exist."
else
echo "root not exist."
fi
3.多分支：if [ ];then
..
elif [ ];then
..
elif [ ];then
..
else
..
fi
example:vim c.sh
#!/bin/bash
if [ -e $1 ];then
if [ -f $1 ];then
echo "$1 is a common file."
elif [ -d $1 ];then
echo " $1 is a directory."
else
echo "$1 is unkown."
fi
else
echo "Please give me a correct file."
exit 3
fi
4.写一个脚本实现：密码过期，提示用户修改：
vim d.sh
#!/bin/basn
[ ! $user='root' ] && echo "Only root can execute this script." &&exit 1
myfile='/etc/shadow'
today=$[ `date + "%s"`/86400 ] （据元年1970到现在所经历的天数）
if [ -e $myfile ];then
while read line;do
username=`echo $line | cut -d: -f1`
let maxday=`echo $line | cut -d: -f5`
[ $[$maxday-$today]-lt 10 ] && echo "${username}'s passwd needs to change. "
done < $myfile
fi
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