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1. Name

watch - execute a program periodically, showing output fullscreen

2. Synopsis

watch [-bdehpvtx] [-n seconds] [--beep] [--differences[=cumulative]] [--errexit] [--exec] [--help] [--interval=seconds] [--no-title] [--precise] [--version] command

3. 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 isrun 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. 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.

 

4. 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.

5. Examples

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)

6. Bugs

Upon terminal resize, the screen will not be correctly repainted until the next scheduled update. All --differenceshighlighting 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).

7. Authors

The original watch was written by Tony Rems <This email address is being protected from spambots. You need JavaScript enabled to view it.> in 1991, with mods and corrections by Francois Pinard. It was reworked and new features added by Mike Coleman <This email address is being protected from spambots. You need JavaScript enabled to view it.> in 1999. The beep, exec, anderror handling features were added by Morty Abzug <This email address is being protected from spambots. You need JavaScript enabled to view it.> in 2008. On a not so dark and stormy morning in March of 2003, Anthony DeRobertis <This email address is being protected from spambots. You need JavaScript enabled to view it.> 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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