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Add directory to $PATH if it's not already there
Sometimes in a script you want to make sure that a directory is in the path, and add it in if it's not already there. In this example, $dir contains the new directory you want to add to the path if it's not already present. There are multiple ways to do this, but this one is a nice clean shell-internal approach. I based it on http://stackoverflow.com/a/1397020. You can also do it using tr to separate the path into lines and grep -x to look for exact matches, like this: $ if ! $(echo "$PATH" | tr ":" "\n" | grep -qx "$dir") ; then PATH=$PATH:$dir ; fi which I got from http://stackoverflow.com/a/5048977. Or replace the "echo | tr" part with a shell parameter expansion, like $ if ! $(echo "${PATH//:/$'\n'}" | grep -qx "$dir") ; then PATH=$PATH:$dir ; fi which I got from http://www.commandlinefu.com/commands/view/3209/. There are also other more regex-y ways to do it, but I find the ones listed here easiest to follow. Note some of this is specific to the bash shell.

Print permanent subtitles on a video
It prints myvideo.srt subtitle files in myvideo.avi, saving it in myvideo_subtitled.avi

Testing hard disk reading speed

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"

Check syntax of all Perl modules or scripts underneath the current directory
Finds all *.p[ml]-files and runs a perl -c on them, checking whether Perl thinks they are syntactically correct

List the number and type of active network connections

runs a X session within your X session
http://www.debian-administration.org/article/Running_multiple_X11_sessions

disable history for current shell session
this will cause any commands that you have executed in the current shell session to not be written in your bash_history file upon logout

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"

Export a subset of a database
Limits the number of rows per table to X


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