Repeat command automatically in Linux

Is it possible in Linux command line to have a command repeat every n seconds?

Say, I have an import running, and I am doing

ls -l

to check if the file size is increasing. I would like to have a command to have this repeat automatically.

Solution 1:

Watch every 5 seconds ...

watch -n 5 ls -l

If you wish to have visual confirmation of changes, append --differences prior to the ls command.

According to the OSX man page, there's also

The --cumulative option makes highlighting "sticky", presenting a running display of all positions that have ever changed. The -t or --no-title option turns off the header showing the interval, command, and current time at the top of the display, as well as the following blank line.

Linux/Unix man page can be found here

Solution 2:

while true; do
    sleep 5
    ls -l
done

Solution 3:

"watch" does not allow fractions of a second in Busybox, while "sleep" does. If that matters to you, try this:

while true; do ls -l; sleep .5; done

Solution 4:

sleep already returns 0. As such, I'm using:

while sleep 3 ; do ls -l ; done

This is a tiny bit shorter than mikhail's solution. A minor drawback is that it sleeps before running the target command for the first time.

Solution 5:

If the command contains some special characters such as pipes and quotes, the command needs to be padded with quotes. For example, to repeat ls -l | grep "txt", the watch command should be:

watch -n 5 'ls -l | grep "txt"'