#! /bin/bash
#a function of
 echo "P: To display current directory"
 echo "S: To display the name of running file"
 echo "D: To display today's date and present time"
 echo "L: To see the listing of files in your working directory"
 echo "W: To see who is logged in"
 echo "Q: To quit this program"
 echo "Enter your option and hit:"
 read char
 until [$char = 'Q']
 do
# echo "Enter your option and hit:"
# read char
 case $char in
 P|p) pwd
 ;;
 S|s) ps
 ;;
 D|d) date
 ;;
 L|l) ls
 ;;
 W|w) who
 ;;
 *) echo "Please input a correct command!"
 esac
 echo "Enter your option and hit:"
 read char
 done

其中 until [$char = 'Q']错在哪，麻烦解释清楚