Tab completion

Although the command line interpreter does not attempt to execute the command until you have submitted it with enter, there are a few keys that have special purpose. One is the tab key.

If you start typing a command, and then press tab, the system will try to auto-complete the command it thinks you are typing. If there are choices it might show these to you differently according to the system you’re on.

Similarly, if you start to type a filename — this is very common as an argument to a command — you can press tab and the computer will attempt to auto-complete it for you. If there is ambiguity, this will typically autocomplete up to the last unambiguous character. Then press tab again to see the choices.

In general, unless you are making a new file that doesn’t exist yet, you should always use tab completion to enter its filename on the command line. It saves you typing, avoids typos, and confirms that the file is indeed there.

Autocomplete and ambiguity in more detail

Autocomplete works by looking at what you’ve typed already, matching it to the available possibilities, and adding those characters for you.

Sometimes this completes the whole command or filename, because there are no alternatives. Great!

But sometimes it can only add some of the characters until it gets to a choice. For example, suppose you have two files: my-little-pony and my-little-puppy. If you type m then press tab, autocomplete will only give you my-little-p. It can’t go further because your intention — based on what you’ve typed — is ambiguous: is the next letter o or u?

When this happens — when tab completion cannot complete because of this ambiguity — if you press tab again straight away, it will show you all the choices available. Use this information to type enough character or characters so that the ambiguity no longer applies, and then press tab to continue.

Of course, if there are no possibilities it means you’ve typed something that doesn’t match anything, and tab completion cannot help. But it does help because it shows you that nothing match (you keep hitting tab, but no choices are shown). It turns out this is very useful when you’re working on the command line — it’s like being able to look ahead as you’re working.

Note that autocomplete can be very contextual, depending on which program or shell you’re running. So autocompletion can take into account what it knows you are trying to do (from the characters you’ve already typed). For example, if you are trying to change to a directory with the cd command, autocomplete might automatically disregard any filename that is not a valid directory.

The specific behaviour of autocomplete will vary a little depending on what shell (the program that you’re typing your commands into) you are running.