watch - execute a program periodically, showing output fullscreen
repeatedly, displaying its output and errors
(the first screenfull). This allows you to watch the program output change
over time. By default, command
is run every 2 seconds and watch
will run until interrupted.
- -d, --differences [permanent]
- Highlight the differences between successive updates. Option will read
optional argument that changes highlight to be permanent, allowing to see
what has changed at least once since first iteration.
- -n, --interval seconds
- Specify update interval. The command will not allow quicker than 0.1
second interval, in which the smaller values are converted. Both '.' and
',' work for any locales.
- -p, --precise
- Make watch attempt to run command every interval
seconds. Try it with ntptime and notice how the fractional seconds
stays (nearly) the same, as opposed to normal mode where they continuously
- -t, --no-title
- Turn off the header showing the interval, command, and current time at the
top of the display, as well as the following blank line.
- -b, --beep
- Beep if command has a non-zero exit.
- -e, --errexit
- Freeze updates on command error, and exit after a key press.
- -g, --chgexit
- Exit when the output of command changes.
- -c, --color
- Interpret ANSI color and style sequences.
- -x, --exec
- command is given to sh -c which means that you may need to
use extra quoting to get the desired effect. This with the --exec option,
which passes the command to exec(2) instead.
- -h, --help
- Display help text and exit.
- -v, --version
- Display version information and exit.
- Various failures.
- Forking the process to watch failed.
- Replacing child process stdout with write side pipe failed.
- Command execution failed.
- Closing child process write pipe failed.
- IPC pipe creation failed.
- Getting child process return value with waitpid(2) failed, or
command exited up on error.
- The watch will propagate command exit status as child exit status.
POSIX option processing is used (i.e., option processing stops at the first
non-option argument). This means that flags after command
interpreted by watch
Upon terminal resize, the screen will not be correctly repainted until the next
scheduled update. All --differences
highlighting is lost on that update
Non-printing characters are stripped from program output. Use "cat -v"
as part of the command pipeline if you want to see them.
Combining Characters that are supposed to display on the character at the last
column on the screen may display one column early, or they may not display at
Combining Characters never count as different in --differences
the base character counts.
Blank lines directly after a line which ends in the last column do not display.
mode doesn't yet have advanced temporal distortion technology
to compensate for a command
that takes more than interval
seconds to execute. watch
also can get into a state where it
rapid-fires as many executions of command
as it can to catch up from a
previous executions running longer than interval
taking ages on a DNS lookup).
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 "'"'$$'"'"
To see the effect of precision time keeping, try adding -p
- watch -n 10 sleep 1
You can watch for your administrator to install the latest kernel with
- watch uname -r
(Note that -p
isn't guaranteed to work across reboots, especially in the
face of ntpdate
or other bootup time-changing mechanisms)