Shell learning while statement

while/do/done

The usage of while is similar to that of C language. For example, a script to verify passwords:

#! /bin/sh

echo "Enter password:"

read TRY

while [ "$TRY" != "secret" ]; do

echo "Sorry, try again"

read TRY

done

The following example controls the number of loops through arithmetic operations:

#! /bin/sh

COUNTER=1

while [ "$COUNTER" -lt 10 ]; do

echo "Here we go again"

COUNTER=$(($COUNTER+1))

done

break和continue

break[n] can specify several layers of loops to jump out of; continue skips this loop, but will not jump out of the loop.

That is, break to jump out, continue to skip.

Exercise: Modify the program for verifying the password above. If the user enters the wrong password five times, it will report an error and exit.

#! /bin/bash

count=4

echo "input a passwd"
read psd

while [ "$psd" != "tw123" ]; do
   
   count=$[count-1]
   printf "Enter again\n"
   read psd
   
   if [ "$psd" == "tw123" ]; then
      printf "success.\n"
      break;
   
    elif [ $count -le 0 ];then
      printf "break\n"
      break
   be
      
done