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Do a search-and-replace in a file after making a backup

Make a file not writable / immutable by root
http://linuxhelp.blogspot.com/2005/11/make-your-files-immutable-which-even.html

Replace all in last command

Print random emoji in terminal
This will print a random emoji within the range of 1F600 - 1F64F, which includes all the face emoji. Obviously, this will only show something meaningful if your terminal can display emoji, but it may be useful in scripts. This likely requires recent versions of bash

Search recursively to find a word or phrase in certain file types, such as C code
I have a bash alias for this command line and find it useful for searching C code for error messages. The -H tells grep to print the filename. you can omit the -i to match the case exactly or keep the -i for case-insensitive matching. This find command find all .c and .h files

Compare two directories
Output of this command is the difference of recursive file lists in two directories (very quick!). To view differences in content of files too, use the command submitted by mariusbutuc (very slow!): $ diff -rq path_to_dir1 path_to_dir2

find sparse files
Prints the path/filename and sparseness of any sparse files (files that use less actual space than their total size because the filesystem treats large blocks of 00 bytes efficiently).

beep when a server goes offline
pings a server once per second, and beeps when the server is unreachable. Basically the opposite of: $ ping -a server-or-ip.com which would beep when a server IS reachable. You could also substitute beep with any command, which makes this a powerful alternative to ping -a: $ while true; do [ "$(ping -c1W1w1 server-or-ip.com 2>/dev/null | awk '/received/ {print $4}')" = 1 ] && date || echo 'server is down!'; sleep 1; done which would output the date and time every sec until the ping failed, in which case it would echo. Notes: Requires beep package. May need to run as root (beep uses the system speaker) Tested on Ubuntu which doesn't have beep out of the box... $ sudo apt-get install beep

BASH: Print shell variable into AWK
Alternatively: export MyVAR=84; awk '{ print ENVIRON["MyVAR"] }'

Get information about memory modules
To take information about the characteristics of the installed memory modules.


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