if

echo –n “input loginName:”
read user  #read user input varl

if
#多条指令,这些命令之间相当于“and”（与）
grep $user /etc/passwd >/tmp/null      
who -u | grep $user
then             #上边的指令都执行成功,返回值$?为0，0为真，运行then
 echo "$user has logged"
else             #指令执行失败，$?为1，运行else                            
 echo "$user has not logged"
fi