watch - execute a program periodically, showing output
fullscreen
watch runs command 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.
- -b, --beep
- Beep if command has a non-zero exit.
- -c, --color
- Interpret ANSI color and style sequences.
- -C,
--no-color
- Do not interpret ANSI color and style sequences.
- -d,
--differences[=permanent]
- Highlight the differences between successive updates. If the optional
permanent argument is specified then watch will show all
changes since the first iteration.
- -e, --errexit
- Freeze updates on command error, and exit after a key press.
- -g, --chgexit
- Exit when the output of command changes.
- -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. The WATCH_INTERVAL environment can be
used to persistently set a non-default interval (following the same rules
and formatting).
- -p, --precise
- Make watch attempt to run command every --interval
seconds. Try it with ntptime (if present) and notice how the
fractional seconds stays (nearly) the same, as opposed to normal mode
where they continuously increase.
- -q, --equexit
<cycles>
- Exit when output of command does not change for the given number of
cycles.
- -r,
--no-rerun
- Do not run the program on terminal resize, the output of the program will
re-appear at the next regular run time.
- -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.
- -w, --no-wrap
- Turn off line wrapping. Long lines will be truncated instead of wrapped to
the next line.
- -x, --exec
- Pass command to exec(2) instead of sh -c which
reduces the need to use extra quoting to get the desired effect.
- -h, --help
- Display help text and exit.
- -v, --version
- Display version information and exit.
- 0
- Success.
- 1
- Various failures.
- 2
- Forking the process to watch failed.
- 3
- Replacing child process stdout with write side pipe failed.
- 4
- Command execution failed.
- 5
- Closing child process write pipe failed.
- 7
- IPC pipe creation failed.
- 8
- Getting child process return value with waitpid(2) failed, or
command exited up on error.
- other
- The watch will propagate command exit status as child exit status.
The behavior of watch is affected by the following
environment variables.
- WATCH_INTERVAL
- Update interval, follows the same rules as the --interval command
line option.
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.
Upon terminal resize, the screen will not be correctly repainted
until the next scheduled update. All --differences highlighting is
lost on that update as well. When using the --no-rerun option, no
output of will be visible.
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 all.
Combining Characters never count as different in
--differences mode. Only the base character counts.
Blank lines directly after a line which ends in the last column do
not display.
--precise 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 (for example, netstat(8) 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
to
- 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 (if present) or other bootup
time-changing mechanisms)