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Display your ${PATH}, one directory per line
This works in bash and zsh. You may also want to alias it, if you need to look at it often... $ alias lpath="echo \$PATH | tr : \\\\n" "\$PATH" to make sure to look at your current $PATH

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"

underscore to camelcase

Calculate days on which Friday the 13th occurs (inspired from the work of the user justsomeguy)
Friday is the 5th day of the week, monday is the 1st. Output may be affected by locale.

Remove multiple spaces
The command removes all the spaces whithin a file and leaves only one space.

Get duration of an audio file in seconds.
This is an alternative to #9131. ffmpeg didn't work on my .au files, though it did on the .wav ones. Also useful if you don't have ffmpeg but do have sox. Handily, sox already reports in seconds (decimal).

ASCII webcam live stream video using mplayer
Mplayer starts a webcam capture using ASCII art. Only mplayer required

find an unused unprivileged TCP port
perl-less way

List all groups and the user names that were in each group
"cut" the user names from /etc/passwd and then running a loop over them.

Retrieve the size of a file on a server
Downloads the entire file, but http servers don't always provide the optional 'Content-Length:' header, and ftp/gopher/dict/etc servers don't provide a filesize header at all.


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