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bash shell expansion
The expansion {,} in bash will repeat the given string once for each item seperated by commas. The given command will result in the following being run: cp /really/long/path/and/file/name /really/long/path/and/file/name-`date -I` These can be embedded as needed, ex: rm file{1,2,3{1,2,3}} would delete the files file1, file2, file31, file32, file32, and no other files.

Randomize lines in a file
Works in sort (GNU coreutils) 7.4, don't know when it was implemented but sometime the last 6 years.

Host cpu performance
Measure the cpu performance: In-case if the cpu is thermal throttling then you can find it using this command. Check the first line of the output. Example: Doing md5 for 3s on 16 size blocks: 11406892 md5's in 2.98s ? #(When cpu is not throttling) Doing md5 for 3s on 16 size blocks: 110692 md5's in 2.98s ?? #(When cpu is thermal throttling) Practical use case: Once we had cooling outage in data center which caused thermal throttling in some of the worker nodes. We used this tool to prove that some servers are not performing well because of the cpu thermal throttling.

Rsync a directory excluding pesky .svn dirs

list block devices
Shows all block devices in a tree with descruptions of what they are.

Destroy all disks on system simultaneously
This command will use the fdisk utility to find all block devices on your system, and overwrite them with data from the /dev/urandom non-blocking random number generator. CAUTION: This will irrevocably erase EVERY SINGLE physical block storage device visible to the fdisk utility, including plugged USB devices, RAID sets, LVM, etc.

IP addresses connected to port 80
IP addresses and number of connections connected to port 80.

Synchronize date and time with a server over ssh
If you are stuck behind a firewall and want to synchronize time with another server but you do not want to port forward NTP (which uses UDP) then this command is handy. It gets the time from a server and sets the local time. It is not that accurate but I can live with a second or so drift.

Insert a line at the top of a text file without sed or awk or bash loops
Yet another way to add a line at the top a of text file with the help of the tac command (reverse cat).


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