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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"

Summarize size of all files of given type in all subdirectories (in bytes)
This deals nicely with filenames containing special characters and can deal with more files than can fit on a commandline. It also avoids spawning du.

Print a list of all hardlinks in the working directory, recursively
libpurple likes to hardlink files repeatedly. To ignore libpurple, use sed: | sed '/\.\/\.purple/d'

convert a pdf to jpeg
Converts a .pdf to .jpg . should work with jpeg | tiff | png | gif | jp2 | pict | bmp | qtif | psd | sgi | tga

Send e-mail if host is 'dead' or not reachable
Cleaner with a mailto assignment in crontab (if the command fails you get an email): MAILTO=admin@example.com 10,30,50 * * * * ping -q -c1 -w3 192.168.0.14 >/dev/null

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.

LIST FILENAMES OF FILES CREATED TODAY IN CURRENT DIRECTORY
Then pipe to 'xargs ls' for a familiar listing, possibly using find's -print0 and xarg's -0 options.

Generate a list of items from a couple of items lists A and B, getting (B - A ) set
ItemsListtoAvoid (A) could be a list of files with a special characteristic to exclude. It can be a result of previous processing list, ex. a list of files containing a special string. AlItemsList.txt (B) Is a complete list of items including some or all items in A. $Difference is saved in ItemsDifference.txt

which process has a port open

Find the process you are looking for minus the grepped one
preferred way to query ps for a specific process name (not supported with all flavors of ps, but will work on just about any linux afaik)


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