Suggested improvements to stream-recording script: timeout?

Question

I'm trying to adapt the script below (taken from https://superuser.com/questions/181517/how-to-execute-a-command-whenever-a-file-changes) to cause my system to record a video stream whenever a particular file changes on the system:

#!/bin/bash

# Set initial time of file
LTIME=`stat -c %Z /path/to/monitored/file`

while true    
do
   ATIME=`stat -c %Z /path/to/monitored/file`

   if [[ "$ATIME" != "$LTIME" ]]
   then    
       echo "RUN COMMAND"
       LTIME=$ATIME
   fi
   sleep 5
done

As is evident, the script polls the time stamp of the monitored file every 5 seconds to see whether it has changed and, if it has, echoes something to the terminal. It will also be clear that the echo command is just a sort of proof-of-concept and that just about any other command could take its place in the script.

Here's what I've got so far that almost works as desired for my purposes (recordingscript.sh uses mplayer -dumpstream -dumpfile mystream URL to record a stream for 70 minutes):

#!/bin/bash

# Set initial time of file
LTIME=`stat -c %Z /path/to/monitored/file`

while true    
do
   ATIME=`stat -c %Z /path/to/monitored/file`

   if [[ "$ATIME" != "$LTIME" ]]; then
      /path/to/my/recordingscript.sh
        break
      LTIME=$ATIME
   fi
   sleep 5
done

The problem that remains is that I would like for this script to run as a cron job each weekday within, say, a 5 hour time frame. If no change is made to the monitored file within that time frame, I'd simply like for the script to abort/exit until the next time cron starts it. How can I modify my adapted script so that it will only run for about 5 hours, then exit? I realize I could probably do this by invoking the script with timeout 300m but I thought there might be other, possibly better, solutions for doing this

Answer 1

I would suggest the following script, in which I've made a few changes:

• used lower-case variable names
• assigned a "give up" variable with a spelled-out expression of "5 hours in seconds"
• parameterized the file to monitor in the filetomonitor variable
• changed the while true loop to be one that loops for 5 hours, based on the behavior of the SECONDS bash variable, which counts the number of seconds since the shell was started. This starts at zero when the shell script stars.
• changed the backticks to the newer $( ... ) form

The updated script:

#!/bin/bash
giveup=$((5 * 60 * 60))
filetomonitor='/tmp/file-to-monitor'

ltime=$(stat -c %Z "$filetomonitor")
while [[ "$SECONDS" -lt "$giveup" ]]
do
  atime=$(stat -c %Z "$filetomonitor")
  if [[ "$atime" -ne "$ltime" ]]
  then
        echo Take action
        break           ## if we're only supposed to act once
        ltime=$atime
  fi
  sleep 5
done

I've left the break in there, but commented out. If you want the script to exit (before the 5 hours) after executing the action, then uncomment the break line; otherwise, the current version of the script will execute for 5 hours (possibly longer, if the 70-minute action starts at 4:59), possibly calling the action multiple times.