194 lines
4.6 KiB
Groff
194 lines
4.6 KiB
Groff
.TH WATCH 1 "2020-06-04" "procps-ng" "User Commands"
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.SH NAME
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watch \- execute a program periodically, showing output fullscreen
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.SH SYNOPSIS
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.B watch
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[\fIoptions\fR] \fIcommand\fR
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.SH DESCRIPTION
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.B watch
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runs
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.I command
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repeatedly, displaying its output and errors (the first screenfull). This
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allows you to watch the program output change over time. By default,
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\fIcommand\fR is run every 2 seconds and \fBwatch\fR will run until interrupted.
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.SH OPTIONS
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.TP
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.BR \-d ", " \-\-differences [ =permanent ]
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Highlight the differences between successive updates. If the optional
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\fB=permanent\fR argument is specified then
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.B watch
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will show all changes since the first iteration.
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.TP
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\fB\-n\fR, \fB\-\-interval\fR \fIseconds\fR
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Specify update interval. The command will not allow quicker than 0.1 second
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interval, in which the smaller values are converted. Both '.' and ',' work
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for any locales. The WATCH_INTERVAL environment can be used to persistently
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set a non-default interval (following the same rules and formatting).
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.TP
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\fB\-p\fR, \fB\-\-precise\fR
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Make
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.BR watch
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attempt to run
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.I command
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every
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.I interval
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seconds. Try it with
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.B ntptime
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and notice how the fractional seconds stays (nearly) the same, as opposed to
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normal mode where they continuously increase.
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.TP
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\fB\-t\fR, \fB\-\-no\-title\fR
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Turn off the header showing the interval, command, and current time at the
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top of the display, as well as the following blank line.
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.TP
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\fB\-b\fR, \fB\-\-beep\fR
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Beep if command has a non-zero exit.
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.TP
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\fB\-e\fR, \fB\-\-errexit\fR
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Freeze updates on command error, and exit after a key press.
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.TP
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\fB\-g\fR, \fB\-\-chgexit\fR
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Exit when the output of
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.I command
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changes.
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.TP
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\fB\-c\fR, \fB\-\-color\fR
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Interpret ANSI color and style sequences.
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.TP
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\fB\-x\fR, \fB\-\-exec\fR
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Pass
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.I command
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to
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.BR exec (2)
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instead of
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.B sh \-c
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which reduces the need to use extra quoting to get the desired effect.
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.TP
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\fB\-h\fR, \fB\-\-help\fR
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Display help text and exit.
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.TP
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\fB\-v\fR, \fB\-\-version\fR
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Display version information and exit.
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.SH "EXIT STATUS"
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.PP
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.RS
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.PD 0
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.TP
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.B 0
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Success.
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.TP
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.B 1
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Various failures.
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.TP
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.B 2
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Forking the process to watch failed.
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.TP
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.B 3
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Replacing child process stdout with write side pipe failed.
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.TP
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.B 4
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Command execution failed.
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.TP
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.B 5
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Closing child process write pipe failed.
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.TP
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.B 7
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IPC pipe creation failed.
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.TP
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.B 8
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Getting child process return value with
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.BR waitpid (2)
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failed, or command exited up on error.
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.TP
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.B other
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The watch will propagate command exit status as child exit status.
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.SH ENVIRONMENT
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The behaviour of
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.B watch
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is affected by the following environment variables.
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.TP
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.B WATCH_INTERVAL
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Update interval, follows the same rules as the
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.B \-\-interval
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command line option.
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.SH NOTES
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POSIX option processing is used (i.e., option processing stops at
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the first non\-option argument). This means that flags after
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.I command
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don't get interpreted by
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.BR watch
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itself.
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.SH BUGS
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Upon terminal resize, the screen will not be correctly repainted until the
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next scheduled update. All
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.B \-\-differences
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highlighting is lost on that update as well.
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Non-printing characters are stripped from program output. Use \fBcat -v\fR as
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part of the command pipeline if you want to see them.
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Combining Characters that are supposed to display on the character at the
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last column on the screen may display one column early, or they may not
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display at all.
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Combining Characters never count as different in
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.I \-\-differences
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mode. Only the base character counts.
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Blank lines directly after a line which ends in the last column do not
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display.
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.I \-\-precise
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mode doesn't yet have advanced temporal distortion technology to compensate
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for a
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.I command
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that takes more than
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.I interval
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seconds to execute.
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.B watch
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also can get into a state where it rapid-fires as many executions of
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.I command
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as it can to catch up from a previous executions running longer than
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.I interval
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(for example,
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.B netstat
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taking ages on a DNS lookup).
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.SH EXAMPLES
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.PP
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To watch for mail, you might do
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.IP
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watch \-n 60 from
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.PP
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To watch the contents of a directory change, you could use
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.IP
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watch \-d ls \-l
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.PP
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If you're only interested in files owned by user joe, you might use
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.IP
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watch \-d 'ls \-l | fgrep joe'
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.PP
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To see the effects of quoting, try these out
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.IP
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watch echo $$
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.br
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watch echo '$$'
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.br
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watch echo "'"'$$'"'"
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.PP
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To see the effect of precision time keeping, try adding
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.I \-p
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to
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.IP
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watch \-n 10 sleep 1
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.PP
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You can watch for your administrator to install the latest kernel with
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.IP
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watch uname \-r
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.PP
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(Note that
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.I \-p
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isn't guaranteed to work across reboots, especially in the face of
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.B ntpdate
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or other bootup time-changing mechanisms)
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