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Which processes are listening on a specific port (e.g. port 80)
swap out "80" for your port of interest. Can use port number or named ports e.g. "http"

Synchronize date and time with a server over ssh
If you are stuck behind a firewall and want to synchronize time with another server but you do not want to port forward NTP (which uses UDP) then this command is handy. It gets the time from a server and sets the local time. It is not that accurate but I can live with a second or so drift.

HDD Performance Write Test
Test your XFS filesystem and Raptor hard drives for write performance.

find an unused unprivileged TCP port
perl-less way

Shows space used by each directory of the root filesystem excluding mountpoints/external filesystems (and sort the output)
Excludes other mountpoints with acavagni's "mountpoint" idea, but with -exec instead of piping to an xargs subshell. Then, calling "du" only once with -exec's "+" option. The first "\! -exec" acts as a test so only those who match are passed to the second "-exec" for du.

Calculating number of Connection to MySQL

Random colours at random locations

Keep track of diff progress
When running a long `diff -r` over folders, this simulates a "verbose" mode where you can see where diff is in the tree. Replace $file with the first part of the path being compared.

diff the outputs of two programs
I've been looking for this for a long time. Does anybody know how to do this in dash (POSIX shell)? An alternative version might be: $ exiftool img_1.jpg | diff -

Big (four-byte) $RANDOM
Sometimes, in a shell script, you need a random number bigger than the range of $RANDOM. This will print a random number made of four hex values extracted from /dev/urandom.


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