Watch – A Useful Linux Command You May Have Never Heard Of

http://beerpla.net/2007/08/04/watch-a-useful-linux-command-you-may-have-never-heard-of/

How many times did I want to watch a directory waiting for a file to appear in it? Constant ls, for example, quickly got boring. A quick Perl script that would reload ls every 5 seconds… yeah it works but it takes a while to type up, and often enough I'm too lazy for that. And then I found 'watch' – a utility that comes with most *nix distros. Look at this beauty [man watch]:

NAME
watch – execute a program periodically, showing output fullscreen

SYNOPSIS

watch [-dhvt] [-n ] [--differences[=cumulative]] [--help]
[--interval=] [--no-title] [--version]

DESCRIPTION
watch runs command repeatedly, displaying its output (the first screen
full). This allows you to watch the program output change over time.
By default, the program is run every 2 seconds; use -n or –interval to
specify a different interval.

The -d or –differences flag will highlight the differences between
successive updates. The –cumulative option makes highlighting
"sticky", presenting a running display of all positions that have ever
changed. The -t or –no-title option turns off the header showing the
interval, command, and current time at the top of the display, as well
as the following blank line.

watch will run until interrupted.

NOTE
Note that command is given to "sh -c" which means that you may need to
use extra quoting to get the desired effect.

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.

EXAMPLES
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 "'"'$$'"'"

You can watch for your administrator to install the latest kernel with

watch uname -r

(Just kidding.)