[adrian_b]

That option has nothing to do with limited disk space.

Eliminating the duplicates makes it much easier to search the history for a command that was used long ago instead of having to skip over hundreds of duplicates of some non-interesting command, such as "ls".

It also makes it much more likely that complex commands used a long time are still preserved. No matter how large you make your history file, it is much more likely that it will become filled with simple commands that you do not need to recall from history, instead of keeping the complex commands that you hate to retype.

Moreover, while having a command history is useful to avoid retyping some commands, some may be less willing to preserve a history from which it is easy to discover which have been their exact actions while using the computer, though for this it is preferable to also disable the saving of the history file.

[TRiG_Ireland]

    $ cat ~/.inputrc 
    "\e[A": history-search-backward
    "\e[B": history-search-forward

This means that I can type the first couple of letters of a command, and then use the up and down arrow keys to cycle through history to search the rest of it. It makes life simpler for me, and confuses me only on the occasions when I've used `git stash`, so `git status` doesn't appear when I expect it to.

[post-it]

fish shell does this by default, and also autocompletes commands from your history. I highly recommend it.

[JoshTriplett]

Ctrl-O is incredibly useful. Go back to an earlier command, press Ctrl-O repeatedly, and you re-run a series of commands. That doesn't work if some of them were deleted as duplicates.