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Print just line 4 from a textfile
this method should be the fastest

Compute the average number of KB per file for each dir
Shorter version using --tag

Find Duplicate Files (based on size first, then MD5 hash)
Finds duplicates based on MD5 sum. Compares only files with the same size. Performance improvements on: $find -not -empty -type f -printf "%s\n" | sort -rn | uniq -d | xargs -I{} -n1 find -type f -size {}c -print0 | xargs -0 md5sum | sort | uniq -w32 --all-repeated=separate The new version takes around 3 seconds where the old version took around 17 minutes. The bottle neck in the old command was the second find. It searches for the files with the specified file size. The new version keeps the file path and size from the beginning.

Bitcoin Brainwallet Checksum Calculator
A bitcoin "brainwallet" is a secret passphrase you carry in your brain. The Bitcoin Brainwallet Exponent Calculator is the second of three functions needed to calculate a bitcoin PRIVATE key. Roughly, checksum is the first 8 hex digits of sha256(sha256(0x80+sha256(passphrase))) Note that this is a bash function, which means you have to type its name to invoke it

Batch rename extension of all files in a folder, in the example from .txt to .md
Batch rename extension of all files in a folder, in the example from .txt to .md

Remove specific entries from iptables
Then you can remove the specific entry: iptables -D INPUT 10 Just make sure these are set: IPTABLES_SAVE_ON_STOP="yes" IPTABLES_SAVE_ON_RESTART="yes" Else your changes won't stick when you restart iptables.

nmap port scanning
TCP Connect scanning for localhost and network 192.168.0.0/24

A very quick and slick fork bomb. Handle with care. Don't run on production please...

Find dead symbolic links
-L tells find to follow symbolic links, so -type l will only return links it can't follow (i.e., those that are broken).

Remove a line from a file using sed (useful for updating known SSH server keys when they change)
For example, to remove line 5 from foo, type: vi +5d +wq foo


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