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You are here: manpages

WATCH

Section: User Commands (1)
Updated: 2016-06-03
Index Return to Main Contents

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. 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. Both '.' and ',' work for any locales.
-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 and style 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.

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.

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).