Watch – A Useful Linux Command You May Have Never Heard Of

Posted by Artem Russakovskii on August 4th, 2007 in Linux

Updated: November 9th, 2007

How many times did I want to watch a directory waiting for a file to appear in it? Constant ls, for example, quickly got boring. A quick Perl script that would reload ls every 5 seconds… yeah it works but it takes a while to type up, and often enough I'm too lazy for that. And then I found 'watch' – a utility that comes with most *nix distros. Look at this beauty [man watch]:

watch – execute a program periodically, showing output fullscreen

watch [-dhvt] [-n ] [–differences[=cumulative]] [–help]
[–interval=] [–no-title] [–version]

watch runs command repeatedly, displaying its output (the first screen
This allows you to watch the program output change over time.
By default, the program is run every 2 seconds; use -n or –interval to
specify a different interval.

The -d or –differences flag will highlight the differences between
successive updates
. 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.

watch will run until interrupted.

Note that command is given to "sh -c" which means that you may need to
use extra quoting to get the desired effect.

Note that POSIX option processing is used (i.e., option processing
stops at the first non-option argument). This means that flags after
command don't get interpreted by watch itself.

To watch for mail, you might do

watch -n 60 from

To watch the contents of a directory change, you could use

watch -d ls -l

If you're only interested in files owned by user joe, you might use

watch -d 'ls -l | fgrep joe'

To see the effects of quoting, try these out

watch echo $$
watch echo '$$'
watch echo "'"'$$'"'"

You can watch for your administrator to install the latest kernel with

watch uname -r

(Just kidding.)

Now if anyone knows how to make watch print the bottom part of the command output rather than the top, please post a comment.

Edit: BSD users, "watch" out. watch does something completely different on BSD (it lets you snoop on other users' ttys). The corresponding BSD command is actually cmdwatch.

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Artem Russakovskii is a San Francisco programmer and blogger. Follow Artem on Twitter (@ArtemR) or subscribe to the RSS feed.

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