watch (Unix)
watch is a command line program and belongs to the Linux packages procps and procps-ng . It starts another Unix command or a command sequence at regular short intervals and transfers the result to the standard output , i.e. it normally displays it in the terminal so that changes can be observed.
Watch is defined as a C program with a source code of approx. 700 lines.
Working method
By default, watch starts the specified command or the specified command sequence every 2 seconds. While watch is working, it uses the entire terminal for its display, i.e. the terminal content is apparently deleted. Watch is usually terminated with the key combination [Ctrl] + [C], after which the terminal displays its previous content again.
If you want a different repetition rhythm, you can specify this in seconds with the call parameter -nor --intervaland a specification immediately following it. The shortest possible repetition interval is a tenth of a second. It should be noted that with fractions of a second the spelling depends on the localization of the system, that is, in an English / American shell you write, for example watch -n 0.4 …, in a shell with German localization, on the other hand, watch -n 0,4 …or shorter watch -n,4 ….
watch calls for the execution of the commands that have been passed to it /bin/shor /bin/dash, therefore watch does not know the aliases that are usually /bin/bashvalid in the shell session from which watch was started. To make the effect of an alias available for watch, you have to write what the alias stands for in a separate shell script file, which has the same name as the alias. This shell script must then be saved in a directory in the search path, for example in /usr/local/bin.
service
Calling syntax
watch [Optionen] [']Kommando [Kommandooptionen][']
Call arguments
The Unix parameters and GNU options in the following table have the same function; it is up to you whether you want to use the short or long form.
| Unix parameters | GNU option | effect |
|---|---|---|
-d
|
--differences | Highlights changes since the last pass. |
-h
|
--help | Displays the quick help with all possible call parameters. |
-n Sek.
|
--interval | Specifies the waiting time between program calls in seconds. |
-t
|
--no-title | Makes watch not display a header |
-v
|
--version | Displays the version of watch. |
Application examples
The watch command is useful for observing changes, for example when you repeatedly call the command ls -lto track the change in the size of a file or directory, or when watch, as in the following example, is called repeatedly psto keep certain processes running can be observed.
Show all running instances of a program
$ watch "ps -ef | grep -e '\<apache' -e '^UID\>' | grep -Ev '\<(grep|watch)\>'"
This command requests a list of the processes every two seconds , but only keeps those lines in which there is a word beginning with "apache" or a line beginning with the word "UID", then removes all lines in which the word " grep ”or“ watch ”occurs, and finally displays the remaining lines selected in this way on the screen.
The output can look like this:
Alle 2,0s: ps -ef | grep -e '\<apache' -e '^UID\>' | grep -Ev '\<(grep|watch)\>' Sat May 26 07:03:45 2018
UID PID PPID C STIME TTY TIME CMD root 1555 1 0 Mai09 ? 00:01:07 /usr/sbin/apache2 -k start www-data 10927 1555 0 Mai25 ? 00:00:00 /usr/sbin/apache2 -k start www-data 10928 1555 0 Mai25 ? 00:00:32 /usr/sbin/apache2 -k start www-data 10929 1555 0 Mai25 ? 00:00:32 /usr/sbin/apache2 -k start
Web links
- watch (1) manual page in English
- Procps - The / proc file system utilities (English)
- procps-ng / procps (English speaking)
Individual evidence
- ↑ procps-ng / procps. Accessed June 5, 2018 .