Unix command watch runs the specified command repeatedly and displays the output on
stdout so you can watch it change over time. By default, the command
is run every 2 seconds, although this is adjustable with the -n secs argument. Since the command is passed to
sh -c, you may need to encase it in quotes for it to run correctly.
watch [options] command [command options]
Run the specified command repeatedly (by default, every two seconds) and display the output so you can watch it change over time.
The command and any options are passed to sh -c, so you may need to use quotes to get correct results.
For example you can use watch command with ping to see if the site is up.
Options
-d, --differences[=cumulative]
Highlight changes between iterations. If cumulative is specified, the highlighting remains on the screen throughout, giving
a cumulative picture of the changes.
-h, --help
Display help message and exit.
-nsecs, --interval=secs
Run the command every secs seconds.
-t, --no-title
Do not display the header or the blank line following the header.
This will generate a list of processes every 2 seconds, filter
for all lines that contain the word "PHP", and display the results on the screen. The output might look something like this:
Every 2s: ps aux | grep php Tue Jan 30 14:56:33 2007
reconst 30028 0.0 0.0 7044 2596 ? S Jan23 0:00 vim -r core/html_api.php
cinonet 28009 0.0 0.2 20708 11064 ? SN Jan25 0:30 php5.cgi
donoiz 23810 0.0 0.2 22740 10996 ? SN Jan27 0:30 php.cgi 43/pdf
The watch command is useful for viewing changes over time, like repeatedly running the
ls -l command to watch a file's size change, or running ps as in the above
example to monitor certain processes continuously.
Watch is a great utility that automatically refreshes data. Some of the more common uses for this command involve
monitoring system processes or logs, but it can be used in combination with pipes for more versatility.
Using watch command without any options will use the default parameter of 2.0 second refresh intervals.
As I mentioned before, one of the more common uses is monitoring system processes. Let's use it with the
free command
. This will give you up to date information about our system's memory usage.
watch free
Yes, it is that simple my friends.
Every 2.0s: free pop-os: Wed Dec 25 13:47:59 2019
total used free shared buff/cache available
Mem: 32596848 3846372 25571572 676612 3178904 27702636
Swap: 0 0 0
Adjust refresh rate of watch command
You can easily change how quickly the output is updated using the
-n
flag.
watch -n 10 free
Every 10.0s: free pop-os: Wed Dec 25 13:58:32 2019
total used free shared buff/cache available
Mem: 32596848 4522508 24864196 715600 3210144 26988920
Swap: 0 0 0
This changes from the default 2.0 second refresh to 10.0 seconds as you can see in the top left corner of our
output.
Remove title or header info from watch command output
watch -t free
The -t flag removes the title/header information to clean up output. The information will still refresh every 2
seconds but you can change that by combining the -n option.
total used free shared buff/cache available
Mem: 32596848 3683324 25089268 1251908 3824256 27286132
Swap: 0 0 0
Highlight the changes in watch command output
You can add the
-d
option and watch will automatically highlight changes for us. Let's take a
look at this using the date command. I've included a screen capture to show how the highlighting behaves.
<img src="https://i2.wp.com/linuxhandbook.com/wp-content/uploads/watch_command.gif?ssl=1" alt="Watch Command" data-recalc-dims="1"/>
Using pipes with watch
You can combine items using pipes. This is not a feature exclusive to watch, but it enhances the functionality of
this software. Pipes rely on the
|
symbol. Not coincidentally, this is called a pipe symbol or
sometimes a vertical bar symbol.
watch "cat /var/log/syslog | tail -n 3"
While this command runs, it will list the last 3 lines of the syslog file. The list will be refreshed every 2
seconds and any changes will be displayed.
Every 2.0s: cat /var/log/syslog | tail -n 3 pop-os: Wed Dec 25 15:18:06 2019
Dec 25 15:17:24 pop-os dbus-daemon[1705]: [session uid=1000 pid=1705] Successfully activated service 'org.freedesktop.Tracker1.Min
er.Extract'
Dec 25 15:17:24 pop-os systemd[1591]: Started Tracker metadata extractor.
Dec 25 15:17:45 pop-os systemd[1591]: tracker-extract.service: Succeeded.
Conclusion
Watch is a simple, but very useful utility. I hope I've given you ideas that will help you improve your workflow.
This is a straightforward command, but there are a wide range of potential uses. If you have any interesting uses
that you would like to share, let us know about them in the comments.
The Last but not LeastTechnology is dominated by
two types of people: those who understand what they do not manage and those who manage what they do not understand ~Archibald Putt.
Ph.D
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<img src="https://i2.wp.com/linuxhandbook.com/wp-content/uploads/Watch_Command.png?ssl=1" alt="Watch Command" srcset="https://i2.wp.com/linuxhandbook.com/wp-content/uploads/Watch_Command.png?w=800&ssl=1 800w, https://i2.wp.com/linuxhandbook.com/wp-content/uploads/Watch_Command.png?resize=300%2C169&ssl=1 300w, https://i2.wp.com/linuxhandbook.com/wp-content/uploads/Watch_Command.png?resize=768%2C432&ssl=1 768w" sizes="(max-width: 800px) 100vw, 800px" data-recalc-dims="1" />
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