* Fast and easy way of setting a reminder for yourself, while working in the command line: ( sleep 10 && echo -e "Tea is ready\a" ) & 10 is a number of secons to wait until fire up the alarm, "-e" argument to echo forces it to see the "\a" sequence, which is an alarm bell.
* execute commands over and over again - run fortune (program) every 20 seconds, run program 'w' every 2 seconds (default), keep track of filename.

 watch -n 20 fortune
 watch w
 watch ls -l filename 

* This command finds all the files and removes them. The slash is to escape the semicolon. We don't want the semicolon to be taken as a command seperator. (it's a command delimitor for the parameter -exec).

find -name "*" -exec rm {} \;

* This command finds and removes all files and executes ls.

find -name "*" -exec rm {} \;; ls

* Find only regular files (not directories, sockets, etc).

find -type f -print

* Find patterns 'foo' in all files starting from the current directory(.).

find . -type f -exec grep foo {} \;

* List full pathnames of files in directory dirname.

find dirname

* Finds the file test.txt.

find . | grep test.txt

* Use su - instead of su, The "-" gives you the target user's environment instead of your own. This is most evident when you forget the dash and the shell can't find commands in /sbin and /usr/sbin.
* Run command as root. su - -c <command>

* set -o vi (bash will respond to vi input).
* Placing a command in `` (left slanted apostrophes) it will simply put the output of that command on the command line. For example, renice -18 `pidof xmms` will give all xmms processes a priority of -18. (and set -o for more options).
* To unalias a alias, alias aliasname=.

diff two directories

diff <dir1> <dir2>

print the first column, delimited by space

nawk -Fc '{print $1}' test.txt

print the second column, delimited by :

nawk -F: '{print $2}' test.txt

Making Patch -- to represent the addition of a new file, diff it against /dev/null.

diff -Naur test.old test.cpp > test.patch

Apply Patch

patch test.cpp test.patch

Undo Patch

patch -R test.cpp test.patch