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Which processes are listening on a specific port (e.g. port 80)
swap out "80" for your port of interest. Can use port number or named ports e.g. "http"

write text or append to a file
If you just want to write or append some text to a file without having to run a text editor, run this command. After running the command, start typing away. To exit, type . on a line by itself. Replacing the >> with a single > will let you overwrite your file.

Remove a line from a file using sed (useful for updating known SSH server keys when they change)

open a seperate konsole tab and ssh to each of N servers (konsole 4.2+)
creates a new tab for each of N servers in listofservers.txt and ssh's to said servers then, try the "send to all sessions" feature of konsole to do the same work on all servers at the same time. BIG time saver, but be careful!

Write comments to your history.
A null operation with the name 'comment', allowing comments to be written to HISTFILE. Prepending '#' to a command will *not* write the command to the history file, although it will be available for the current session, thus '#' is not useful for keeping track of comments past the current session.

Send a binary file as an attachment to an email
The uuencode utility will encode your file so that it can be sent as an attachment to an email. It is part of the sharutils package in RHEL/CentOS/Fedora.

Swap the two last arguments of the current command line
Say you just typed a long command like this: $ rsync -navupogz --delete /long/path/to/dir_a /very/long/path/to/dir_b but you really want to sync dir_b to dir_a. Instead of rewriting all the command line, just type followed by , and your command line will read $ rsync -navupogz --delete /very/long/path/to/dir_b /long/path/to/dir_a

find all active IP addresses in a network
There are several other options. This one is plain and simple. Another option is: nmap -sP 192.168.0.0/24

Get the gravatar UTL for a given email address

Delete line number 10 from file
Very useful when the ssh key of a host has changed and ssh refuses to connect to the machine, while giving you the line number that has changed in ~/.ssh/known_hosts.


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