Problems with shell-script while!

First of all, I'm a beginner on shell-script. This code I've done is not working.

I want to repeat a code for 30 seconds but it doesn't work. It just keep doing my logic indefinitely.

DIFF=0

while [ $DIFF < 30 ]; do

    START=$(date +%s)

######## My logic #开发者_如何学运维########

    DIFF=$(( $END - $START ))
    echo $DIFF
    cd ..
    sleep 5s

done

I think it's because I'm not doing the while clause properly?

Well, you definitely need to provide some values for $START and $END. They won't set themselves!

You may want to do something like

START = `date +%s` 

to set it to a time in seconds. Of course END will need to be set inside your loop to get it updated.

EDIT: cd .. is hopefully not really what you plan to run inside the loop. Within a few milliseconds your current directory will be the root directory, with little else accomplished. It would be cheaper to do a single cd / .

EDIT 2: This shouldn't be such a hard problem. For this edit, I've built and tested a one-line solution:

START=$(date +%s); DIFF=0; while [ $DIFF -lt 30 ]; do echo $DIFF; DIFF=$(($(date +%s)-$START)); done

That will correctly update its variables and display them... and it ends after 30 seconds.

((end = $(date +%s) + 30))
while (( $(date +%s) < end ))
do
    something
done

Or, using the builtin variable $SECONDS in Bash:

((end = SECONDS + 30))
while (( SECONDS < end ))
do
    something
done

use an infinite loop. an example pseudocode

DIFF=0
while true
do
  START=$(date +%s)
  END=.... #define your end
  DIFF=$((END-START))
  if [ "$DIFF" -gt 30 ] ;then
    break
  fi
  .....
done

It looks like you're using bash.

Try something like this perhaps:

START=...
while (($DIFF<30)); do
   #   ....
   DIFF=$((END-START))
done

(See Bash arithmetic evaluation and The Double-Parentheses Construct.)