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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.

list human readable files
include in the list human readable hidden files too: $ file .* *|grep 'ASCII text'|sort -rk2 more reliable command: $ ls|xargs file|grep 'ASCII text'|sort -rk2 and include hidden files: $ ls -a|xargs file|grep 'ASCII text'|sort -rk2

Another Matrix Style Implementation

Display IP : Count of failed login attempts
The lastb command presents you with the history of failed login attempts (stored in /var/log/btmp). The reference file is read/write by root only by default. This can be quite an exhaustive list with lots of bots hammering away at your machine. Sometimes it is more important to see the scale of things, or in this case the volume of failed logins tied to each source IP. The awk statement determines if the 3rd element is an IP address, and if so increments the running count of failed login attempts associated with it. When done it prints the IP and count. The sort statement sorts numerically (-n) by column 3 (-k 3), so you can see the most aggressive sources of login attempts. Note that the ':' character is the 2nd column, and that the -n and -k can be combined to -nk. Please be aware that the btmp file will contain every instance of a failed login unless explicitly rolled over. It should be safe to delete/archive this file after you've processed it.

Grep Recursively Through Single File Extension
Trac 0.12.2-stable

Sort files in folders alphabetically
Creates one letter folders in the current directory and moves files with corresponding initial in the folder.

Prepend a text to a file.
Prepend text to a file. It doen't need temporary files, ed or sed.

Function to change prompt
Bash function to change your default prompt to something simpler and restore it to normal afterwards.

pngcrush all .png files in the directory
Nothing too magical here, just uses pngcrush to losslessly compress all your pngs!

Duplicate several drives concurrently
If you have some drive imaging to do, you can boot into any liveCD and use a commodity machine. The drives will be written in parallel. To improve efficiency, specify a larger block size in dd: $dd if=/dev/sda bs=64k | tee >(dd of=/dev/sdb bs=64k) | dd of=/dev/sdc bs=64k To image more drives , insert them as additional arguments to tee: $dd if=/dev/sda | tee >(dd of=/dev/sdb) >(dd of=/dev/sdc) >(dd of=/dev/sdd) | dd of=/dev/sde


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