13.12. Command History and Tab Completion

One solution is to use the command line history. By scrolling with the [Up Arrow] and [Down Arrow] keys, you can find plenty of your previously typed commands.

Try it by taking a look again at sneakers.txt (created in Section 13.9.1 Using Redirection. The first time, however, at the shell prompt, type:

cat sneakrs.txt

Nothing happens, of course, because there is no sneakrs.txt[Enter] again.

We now see the contents of sneakers.txt.

By default, up to 500 commands can be stored in the bash command line history file.

	Tip
	By typing the envHISTFILESIZE=500 shows the number of commands that bash will store.

The command line history is actually kept in a file, called .bash_history in your login directory. We can read it in a number of ways: by using vi, cat, less, more, and others.

Be aware that the file can be long. To read it with the more command, from your home directory type:

more .bash_history

To move forward a screen, press [Space]; to move back a screen, press [b]; to quit, press [q].

Tip

To find a command in your history file without having to keep hitting the arrow keys or page through the history file, use grep, a powerful search utility (see Section 13.11.3 The grep Command. Here is how you can quickly find a previously used command: say you are searching for a command that is similar to cat sneak-something. You have used the command, and you think it might be in your history file. At the shell prompt, type: history | grep sneak

Another time-saving tool is known as command completion. If you type part of a file, command, or pathname and then press the [Tab] key, bash[Tab] again to obtain a list of the /files/unix/010/paths that match what has been typed so far.

For example, if you forget the command updatedb, but remember a portion of the command, you can su to root, then at the shell prompt, type up, press the [Tab] key twice and you will see a list of possible completions, including updatedb and uptime. By typing the partial command upd and pressing [Tab] again, your command is completed for you.