17-Apr-2022 in Command-line Interface

Execute command in timed intervals

Running a command (script, program, et cetera) on a regularly timed interval can be accomplished a few different ways. One way, is to leverage the crontab utility, which schedules runs to run one or more times per day.

While a fantastic utility, crontab can be a bit heavy at times, especially if you just need to run something for a finite amount of time.

In those types of scenarios, you can use the watch command, which works the same way that crontab does, but right from your command-line.

Let's say you want to check the contents of a directory every 5 seconds, you can run the following:

watch --interval 5 ls

Or more concisely:

watch -n 5 ls

Once you're done monitoring things, you can simply hit Ctrl+C and watch will cease execution!

To take things a bit further, watch also has an argument that will tell it to highlight the differences between runs:

# Highlight the differences
watch -n 5 --differences ls -al
# Also, highlight the differences
watch -n 5 -d ls -al
# Highlight the differences, persisting the highlight between runs
watch -n 5 --differences=permanent ls -al