WATCH(1) Linux User's Manual WATCH(1)NAMEwatch - execute a program periodically, showing output fullscreen
SYNOPSISwatch [-bdehpvtx] [-n seconds] [--beep] [--differences[=cumulative]]
[--errexit] [--exec] [--help] [--interval=seconds] [--no-title] [--pre‐
cise] [--version] command
DESCRIPTIONwatch 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; use -n or
--interval to specify a different interval. Normally, this interval is
interpreted as the amout of time between the completion of one run of
command and the beginning of the next run. However, with the -p or
--precise option, you can 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 increase.
The -d or --differences flag will highlight the differences between
successive updates. Using --differences=cumulative makes highlighting
"sticky", presenting a running display of all positions that have ever
changed. The -t or --no-title option turns off the header showing the
interval, command, and current time at the top of the display, as well
as the following blank line. The -b or --beep option causes the com‐
mand to beep if it has a non-zero exit.
watch will normally run until interrupted. If you want watch to exit on
an error from the program running use the -e or --errexit options,
which will cause watch to exit if the return value from the program is
non-zero.
NOTE
Note that command is given to "sh -c" which means that you may need to
use extra quoting to get the desired effect. You can disable this with
the -x or --exec option, which passes the command to exec(2) instead.
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.
--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).
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 sec‐
onds after the minute started, and added microsecond precision.
2009 May 11 WATCH(1)
