Linux Commands Examples

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watch

execute a program periodically, showing output fullscreen

Synopsis

watch [options] command


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examples

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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 or other bootup time-changing mechanisms)


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bash watch command with colors preserved

I think it may not be possible with the 'watch' command. Here is a longer way of doing it:

while true; do clear; date;echo;ls -al --color; sleep 2; done

You could put this in a script, for example:

echo "while true; do clear; date;echo;\$*;sleep 2; done" > watch2
chmod +x watch2
./watch2 ls -al --color


To clarify, here's why I think it's not possible with the 'watch' command. See what happens if you use cat -v:

watch "ls -al --color|cat -v"

It shows you the color control characters...which I think is not what you want.

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Store the output of date and watch command to a file

watch is meant for output to a display. If you simply want to run a command every X seconds then you should just use a delay loop for that.

while true ; do somecommand ; sleep 2 ; done

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source

Is there a linux command that lets me color-code the result of another command?

I think you can color the man pages, however (after running several Google searches, I don't think what you're looking for exists.

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source

Linux watch command: "--differences" not working

Why not redirect output to a file using

>>

And then do tailf on that file ?

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How do I use the watch and jobs commands together in Bash?

The watch command is documented as follows:

SYNOPSIS
   watch  [-dhvt]  [-n  <seconds>] [--differences[=cumulative]] [--help]
          [--interval=<sec-onds>] [--no-title] [--version] <command>
[...]
NOTE
   Note that command is given to "sh -c" which means that you may need to
   use extra quoting to get the desired effect.

The part about giving the command to sh -c means the jobs command you are running via watch is running in a different shell session than the one that spawned the job, so it cannot be seen that other shell. The problem is fundamentally that jobs is a shell built-in and must be run in the shell that spawned the jobs you want to see.

The closest you can get is to use a while loop in the shell that spawned the job:

$ while true; do jobs; sleep 10; done

You could define a function in your shell startup script to make that easier to use:

myjobwatch() { while true; do jobs; sleep 5; done; }

Then you just have to type myjobwatch.

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, the program is run every 2 seconds. By default, 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.

-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 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

Closign 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.

note

Note that 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).


authors

The original watch was written by Tony Rems (rembo[:at:]unisoft[:dot:]com) in 1991, with mods and corrections by Francois Pinard. It was reworked and new features added by Mike Coleman (mkc[:at:]acm[:dot:]org) in 1999. The beep, exec, and error handling features were added by Morty Abzug (morty[:at:]frakir[:dot:]org) in 2008. On a not so dark and stormy morning in March of 2003, Anthony DeRobertis (asd[:at:]suespammers[:dot:]org) 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 Jarrod Lowe (procps[:at:]rrod[:dot:]net)

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