procps/watch.1

.TH WATCH 1 "2020-10-19" "procps-ng" "User Commands"
.SH NAME
watch \- execute a program periodically, showing output fullscreen
.SH SYNOPSIS
.B watch
[\fIoptions\fR] \fIcommand\fR
.SH DESCRIPTION
.B watch
runs
.I command
repeatedly, displaying its output and errors (the first screenfull).  This
allows you to watch the program output change over time.  By default,
\fIcommand\fR is run every 2 seconds and \fBwatch\fR will run until interrupted.
.SH OPTIONS
.TP
.BR \-d ", " \-\-differences [ =permanent ]
Highlight the differences between successive updates. If the optional
\fB=permanent\fR argument is specified then
.B watch
will show all changes since the first iteration.
.TP
\fB\-n\fR, \fB\-\-interval\fR \fIseconds\fR
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).
.TP
\fB\-p\fR, \fB\-\-precise\fR
Make
.BR watch
attempt to run
.I command
every
.I interval
seconds. Try it with
.B ntptime
(if present) and notice how the fractional seconds stays (nearly) the same, as opposed to
normal mode where they continuously increase.
.TP
\fB\-t\fR, \fB\-\-no\-title\fR
Turn off the header showing the interval, command, and current time at the
top of the display, as well as the following blank line.
.TP
\fB\-b\fR, \fB\-\-beep\fR
Beep if command has a non-zero exit.
.TP
\fB\-e\fR, \fB\-\-errexit\fR
Freeze updates on command error, and exit after a key press.
.TP
\fB\-g\fR, \fB\-\-chgexit\fR
Exit when the output of
.I command
changes.
.TP
\fB\-c\fR, \fB\-\-color\fR
Interpret ANSI color and style sequences.
.TP
\fB\-x\fR, \fB\-\-exec\fR
Pass
.I command
to
.BR exec (2)
instead of
.B sh \-c
which reduces the need to use extra quoting to get the desired effect.
.TP
\fB\-w\fR, \fB\-\-no\-linewrap\fR
Turn off line wrapping. Long lines will be truncated instead of wrapped to the next line.
.TP
\fB\-h\fR, \fB\-\-help\fR
Display help text and exit.
.TP
\fB\-v\fR, \fB\-\-version\fR
Display version information and exit.
.SH "EXIT STATUS"
.PP
.RS
.PD 0
.TP
.B 0
Success.
.TP
.B 1
Various failures.
.TP
.B 2
Forking the process to watch failed.
.TP
.B 3
Replacing child process stdout with write side pipe failed.
.TP
.B 4
Command execution failed.
.TP
.B 5
Closing child process write pipe failed.
.TP
.B 7
IPC pipe creation failed.
.TP
.B 8
Getting child process return value with
.BR waitpid (2)
failed, or command exited up on error.
.TP
.B other
The watch will propagate command exit status as child exit status.
.SH ENVIRONMENT
The behaviour of
.B watch
is affected by the following environment variables.

.TP
.B WATCH_INTERVAL
Update interval, follows the same rules as the
.B \-\-interval
command line option.
.SH NOTES
POSIX option processing is used (i.e., option processing stops at
the first non\-option argument).  This means that flags after
.I command
don't get interpreted by
.BR watch
itself.
.SH BUGS
Upon terminal resize, the screen will not be correctly repainted until the
next scheduled update.  All
.B \-\-differences
highlighting is lost on that update as well.

Non-printing characters are stripped from program output.  Use \fBcat -v\fR 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
.I \-\-differences
mode.  Only the base character counts.

Blank lines directly after a line which ends in the last column do not
display.

.I \-\-precise
mode doesn't yet have advanced temporal distortion technology to compensate
for a
.I command
that takes more than
.I interval
seconds to execute.
.B watch
also can get into a state where it rapid-fires as many executions of
.I command
as it can to catch up from a previous executions running longer than
.I interval
(for example,
.B netstat
taking ages on a DNS lookup).
.SH EXAMPLES
.PP
To watch for mail, you might do
.IP
watch \-n 60 from
.PP
To watch the contents of a directory change, you could use
.IP
watch \-d ls \-l
.PP
If you're only interested in files owned by user joe, you might use
.IP
watch \-d 'ls \-l | fgrep joe'
.PP
To see the effects of quoting, try these out
.IP
watch echo $$
.br
watch echo '$$'
.br
watch echo "'"'$$'"'"
.PP
To see the effect of precision time keeping, try adding
.I \-p
to
.IP
watch \-n 10 sleep 1
.PP
You can watch for your administrator to install the latest kernel with
.IP
watch uname \-r
.PP
(Note that
.I \-p
isn't guaranteed to work across reboots, especially in the face of
.B ntpdate
(if present) or other bootup time-changing mechanisms)