How can I see the change that caused watch -g to exit

Question

I'm using watch -g to monitor the output of a command and exit as soon as it changes. The problem is that all the UI elements of watch (including the output of the command that I'm monitoring) disappear as soon as it exits. I want to know what the output changed to before watch exited. Is this possible?

Try TERM=linux watch -g …. I got this by trial and error, so it's voodoo, therefore not an answer. Hopefully someone will give you an educative answer. — Kamil Maciorowski, Jan 04 '23 at 21:27
@KamilMaciorowski, I like your approach. If OP doesn't need to compare last and the second last output this is preaty nice solution I would say. — Damir, Jan 04 '23 at 23:03
@Kamil - looks like that TERM doesn't use the alternate buffer... Anyway, I'm marking this as duplicate, if the OP wants to see the output of watch he can switch to the alternate buffer per the answer to the duplicate, i.e. run printf \\33\[\?47h — don_crissti, Jan 04 '23 at 23:46
Does this answer your question? How to configure screen-restore in a terminal? — don_crissti, Jan 04 '23 at 23:47

Damir · Answer 1 · 2023-01-04T22:46:43.780

watch -g "ls | tee -a log_file"

In log_file you will get the output of your command. ls is just example, of course you will use your own command or if more of them you should put them in parentheses like in answer below. Credit to this answer.

For easier recognition of differences between last and second last output you could add empty new line and some markation:

watch -g "(echo; echo 'New output below:'; echo '------------------'; ls) | tee -a log_file"

Anthony Albertina · Answer 2 · 2023-01-05T23:26:37.963

If you're willing to not use watch, try this and it will show the most recent output when the command failed. Replace somecommand with your command.

logFile='/var/log/somecommand.log'
somecommand &> $logFile &
expectedOutput=$(cat $logFile)
while true; do
  somecommand &> $logFile &
  actualOutput=$(cat $logFile)
  if [[ "$actualOutput" != "$expectedOutput" ]]; then
    echo "Process failed, here is the most recent output..."
    echo "$actualOutput"
    exit 1
  fi
  sleep 2
done

How can I see the change that caused watch -g to exit

2 Answers2