Watch – A Useful Linux Command You May Have Never Heard Of
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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]:
NAME
watch – execute a program periodically, showing output fullscreen
SYNOPSIS
watch [-dhvt] [-n ] [--differences[=cumulative]] [--help]
[--interval=] [--no-title] [--version]
DESCRIPTION
watch runs command repeatedly, displaying its output (the first screen
full). 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
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.
EXAMPLES
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.
Artem Russakovskii is a San Francisco programmer, blogger, and future millionaire (that last part is in the works). Follow Artem on Twitter (@ArtemR) or subscribe to the RSS feed.
In the meantime, if you found this article useful, feel free to buy me a cup of coffee below.
beer planet is a blog about technology, programming, computers, and geek life. It is run by Artem Russakovskii - a local San Francisco geek who is currently pursuing his own projects and regularly enjoys hacking Android, PHP, CSS, Javascript, AJAX, Perl, and regular expressions, working on Wordpress plugins and tools, tweaking MySQL queries and server settings, administering Linux machines, blogging, learning new things, and other geeky stuff.
Artem loves commenting on other people's blogs and reading comments on his own, so go ahead and leave one.
Not sure what you mean by:
"Now if anyone knows how to make watch print the bottom part of the command output rather than the top, please post a comment."
But using the –no-title option eliminates the:
Every 5.0s: ls Thu Jul 24 09:53:45 2008
portion of the output, giving you only the result of the stuff in your quotes (at the specified interval and maintaining the specified options, of course).
Carlos, what I mean is, if you are trying to watch a long directory listing that doesn't fit the screen, screen would show you the top output of 'ls' for example, not the bottom.
Artem,
Just pipe it out to the tail function, try this:
watch -d 'ls -l /usr/bin | tail -n 50'
Works quite well. Now if it could only take an action when something changed, i.e., move files in a directory when we found changes to a directory.
Thanks, Guy!
I was looking for a way to watch a certain part of a long output and your tailpipe solution was perfect!
Actually i have heard of the command but i must admit that i have really used it
Nice post, though !
I use this quite frequently:
watch -n 1 -d /usr/bin/sensors
Learn something new everyday! Nice post.
While watch is pretty useful, a better solution in this case would be to use inotify / incron, which is like cron, but based on filesystem events instead of a schedule.
So you plan to become a millionaire without actually providing any value, as evidenced by this post, copied in haste from a man page. The best of luck with that.
Thanks for stopping by and commenting on a 3-year old article, and for your kind and not at all passive aggressive wish. I am in fact working on becoming a millionaire in this exact way. Moreover, I believe this very page will push me past the $999,999.
Love the reply. I hope my comment pushes you over $999,9999.00
Hi!
Am trying to redirect the o/p of the watch command to a file.
For ex — watch -n 10 ps >> output.txt
This is giving the o/p in the file BUT, is adding these junk characters in addition. ALSO, it is NOT separating the outputs at different intervals. Its just mixing it all up.
Help needed to separate the outputs at diff intervals and NOT print the special characters!!
Regards
Type the following:
watch 'tail -1 /var/log/secure >> /root/sechist'
to create a cumulative file of the last line of the
/var/log/secure file, sampling every two seconds.
Then execute the following to see only the changes
uniq /root/sechist
Do an su command in another window to generate
some changes for this command.
Nice and useful post. When I recover data from bad HDDs, I used frequent "df -h" in 2 minutes interval. So I think "watch -n 120 -d df -h" will make me free from frequent typing. Cheers!
Hi Artem,
I wonder how to have "watch" execute a command or an application for me after its detection of something "new" created or changed? For example, I issue the command $watch 'ls /home/user/' and want to execute a script call upload.sh every time a new file created and detected by "watch"?
Thank you!
Don't think you can do that with watch. If you want to detect file system changes, I suggest you look into lsyncd or inotify-tools.