Section: User Commands (1)
Updated: June 2011
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.
- -d, --differences [permanent]
- Highlight the differences between successive updates. Option will read optional argument that changes highlight to be permanent, 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 converted.:
- -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 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.:
- -c, --color
- Interpret ANSI color 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.:
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
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).
- Various failures.:
- Forking the process to watch failed.:
- Replacing child process stdout with write side pipe failed.:
- Command execution failed.:
- Closign child process write pipe failed.:
- IPC pipe creation failed.:
- Getting child process return value with waitpid?(2) failed, or command exited up on error.:
- The watch will propagate command exit status as child exit status.:
The original watch was written by Tony Rems in 1991, with mods and corrections by Francois Pinard. It was reworked and new features added by Mike Coleman in 1999. The beep, exec, and error handling features were added by Morty Abzug in 2008. On a not so dark and stormy morning in March of 2003, Anthony DeRobertis 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