WATCH

Section: User Commands (1)
Updated: 2020-12-06
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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, command is run every 2 seconds and watch will run until interrupted.

OPTIONS

-d, --differences[=permanent]: Highlight the differences between successive updates. If the optional permanent argument is specified then watch will show all changes since the 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. Both '.' and ',' work for any locales. The WATCH_INTERVAL environment can be used to persistently set a non-default interval (following the same rules and formatting).
-p, --precise: Make watch attempt to run command every --interval seconds. Try it with ntptime (if present) 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 and style sequences.
-x, --exec: Pass command to exec(2) instead of sh -c which reduces the need to use extra quoting to get the desired effect.
-w, --no-linewrap: Turn off line wrapping. Long lines will be truncated instead of wrapped to the next line.
-h, --help: Display help text and exit.
-v, --version: Display version information and exit.

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.

ENVIRONMENT

The behaviour of watch is affected by the following environment variables.

WATCH_INTERVAL: Update interval, follows the same rules as the --interval command line option.

NOTES

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.

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).

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 (if present) or other bootup time-changing mechanisms)