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watch - execute a program periodically, showing output fullscreen watch
] [-n seconds
] command watch
repeatedly, displaying its output and errors (the first screenfull). This allows you to watch the program output change over time. By default, the program isrun every 2 seconds; use -n
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
option, you can make watch
attempt to run command
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. The --cumulative option 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 command 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 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
option, which passes the command to exec(2)
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.
To watchfor 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)
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).
The original watch
> got sick of his watches that should update every minute eventually updating many seconds after the minute started, and added microsecond precision.
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