When I run:
watch 'cmd >> output.txt' &
the job gets suspended by the system:
3569 Stopped (tty output)
Is there a workaround?
The purpose of
watch is to show the results of a command full-screen and update continuously; if you're redirecting the output into a file and backgrounding it there's really no reason to use watch in the first place.
If you want to just run a command over and over again with a delay (
watch waits two seconds by default), you can use something like this:
while true; do cmd >> output.txt sleep 2 done
Here's a way:
watch -n 1 'date' &>/dev/null &
Since you background the process, we can assume you don't need the terminal display and you are fine redirecting to a file. If you do that you will be able to background
watch without issue.
sleep, as suggested by Michael Mrozek, will slowly lag which can be undesirable. Aside from a convoluted shell script that monitors system time and executes a command based upon elapsed time,
watch -p can be a good option for precise timings.
For precise timings:
watch -n 1 -p 'date' &>/dev/null &
I'm not sure about your motivations, but maybe this would be enough?
while true; do sleep 2; cmd >>output.txt; done &
Otherwise, please explain why you really need
tee is your friend. For example, here's how you could show the number of files in the current directory, and append it to count.txt every 2 seconds:
watch 'find . -maxdepth 1 -printf " " | wc -c | tee --append count.txt'
Depends on your motivations:
You want to run something periodically. Then you could use directly cron. (If a time granularity of 1 minute is enough for your purposes).
You want to just run something repeatedly:
while sleep 2 ; do cmd done >& log.txt
(You can run this from the crontab for example, although using a script is highly recommended).
You want to run periodically a command and have access to the output of its last execution.
You can write the output to a ramfs (modifying the script before):
while sleep 2 ; do cmd >& last_execution.log done
or even better:
while sleep 2 ; do cmd >& last_execution.log.tmp mv last_execution.log.tmp last_execution.log ## atomically done
You can combine
watch, so that you can always check the latest status, as long as it fits in one screenful:
screen watch -n 3660 ./make-backup.sh
You can use screen command.
If you must use
watch - Just combine answers from mike dangelo and l0b0
watch -n 1 'cmd | tee -a output.txt' &>/dev/null &
watch -n 1 'date | tee -a output.txt' &>/dev/null &
then watch the seconds go by
tail -f output.txt
Watch is designed to output to the screen, but it's output can be redirected (both stdout and stderr) then it will run in the background.
watch 'date >>fa' >/dev/null 2>/dev/null &