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

 if  cut -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