procps/watch.1

.TH WATCH "1" "June 2011" "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, the
program is run every 2 seconds.
.B watch
will run until interrupted.
.SH OPTIONS
.TP
\fB\-d\fR, \fB\-\-differences\fR
Highlight the differences between successive updates.
.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.
.TP
\fB\-p\fR, \fB\-\-precise\fR
Make
.BR watch
attempt to run
.I command
every
.I interval
seconds. Try it with
.B ntptime
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\-c\fR, \fB\-\-color\fR
Interpret ANSI color sequences.
.TP
\fB\-x\fR, \fB\-\-exec\fR
.I command
is given to
.B 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
.BR exec (2)
instead.
.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 NOTE
Note that 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 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
or other bootup time-changing mechanisms)
.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.
.PP
Non-printing characters are stripped from program output.  Use "cat -v" as
part of the command pipeline if you want to see them.
.PP
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.
.PP
Combining Characters never count as different in
.I \-\-differences
mode.  Only the base character counts.
.PP
Blank lines directly after a line which ends in the last column do not
display.
.PP
.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 "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
Closign 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 AUTHORS
The original
.B watch
was written by
.UR rembo\@\:unisoft.\:com
Tony Rems
.UE
in 1991, with mods and
corrections by Francois Pinard.  It was reworked and new features added by
.UR mkc\@\:acm.\:org
Mike Coleman
.UE
in 1999. The beep, exec, and error handling features were added by
.UR morty\@\:frakir.\:org
Morty Abzug
.UE
in 2008.  On a not so dark and stormy morning in March of 2003,
.UR asd\@\:suespammers.\:org
Anthony DeRobertis
.UE
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
.UR procps\@\:rrod.\:net
Jarrod Lowe
.UE