watch - execute a program periodically, showing output fullscreen
watch [
options]
command
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.
-
-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.
-
-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.
-
-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.
-
-q, --equexit <cycles>
- Exit when output of command does not change for the
given number of cycles.
-
-c, --color
- Interpret ANSI color and style sequences.
-
-x, --exec
- Pass command to exec(2) instead of sh
-c which reduces the need to use extra quoting to get the desired
effect.
-
-w, --no-wrap
- Turn off line wrapping. Long lines will be truncated
instead of wrapped to the next line.
-
-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 behaviour 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.
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 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)