watch Man page

WATCH(1) User Commands WATCH(1)

NAME

watch – execute a program periodically, showing output fullscreen

SYNOPSIS

watch [options] command

DESCRIPTION

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, the program is run every 2 seconds. By
default, watch will run until interrupted.

OPTIONS

-d, –differences [permanent] Highlight the differences between successive updates. Option
will read optional argument that changes highlight to be perma‐
nent, 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 con‐
verted.

-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 continu‐
ously 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.

-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.

NOTE
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 “‘”‘$$'”‘”

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 or other bootup time-changing mechanisms)

BUGS

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).

EXIT STATUS
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.
AUTHORS
The original watch was written by Tony Rems ⟨rembo@unisoft.com⟩ in
1991, with mods and corrections by Francois Pinard. It was reworked
and new features added by Mike Coleman ⟨mkc@acm.org⟩ in 1999. The beep,
exec, and error handling features were added by Morty Abzug ⟨morty@
frakir.org⟩ in 2008. On a not so dark and stormy morning in March of
2003, Anthony DeRobertis ⟨asd@suespammers.org⟩ got sick of his watches
that should update every minute eventually updating many seconds after
the minute started, and added microsecond precision. Unicode support
was added in 2009 by Jarrod Lowe ⟨procps@rrod.net⟩

procps-ng June 2011 WATCH(1)