command line

Possibly the most annoying "feature" that could be added to a linux console window is for it to detect newlines and not treat them like the enter key. It can be temporarily disabled, but will undoubtedly re-enable itself after a while.

Use convert for lossy compression, and ghostscript for lossless compression of PDF.

These are just some Exim command line arguments that I use all the time but can never remember the finer details of.

Cancel a message in the queue, informing the sender via bounce message, and also silently delete a message from the queue:

I type pretty fast when I'm on the command line and periodically, for commands that don't really matter, I don't check what I've typed and just press enter. This is especially true for directory listings which I have "ls -la" pretty much hard coded into my fingers. This particular one I mistype as "ls- la" all the time.

On Ubuntu, and other "helpful" platforms, there is a package installed called "command-not-found" which will take about 2-3 seconds to come up with a list of command it thinks you might have wanted to type but you haven't got installed right now.