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Command History and Tab Completion

It doesn't take long before the thought of typing the same command over and over becomes unappealing, at best. In Linux, since you can string together commands at the shell prompt, one minor typing error can ruin lines of a command.

One solution is to use the command line history. By scrolling with the up 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 the section called Using Redirection. The first time, however, at the shell prompt, type:

cat sneakrs.txt

Nothing happens, of course, because there is no sneakrs.txt file. No problem. We'll just use the up-arrow key to bring back the command, then use the left-arrow key to get to the point where we missed the "e." Insert the letter and press [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.


The History Environment Variable


By typing the env command at a shell prompt, we can see the environment variable that controls the size of the command line history. The line which reads, HISTFILESIZE=500 shows the number of commands that bash will store.

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

Be aware that the file can be long.

Let's read it with more:

more .bash_history

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


Locating a Previously Used Command


Want 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 the section called The grep Command. Here's how you can quickly find a previously used command: Let's say you're searching for the command that was something like cat sneak-something. You've used the command and think it might be in your history file. At the shell prompt, then, 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 will present you with either the remaining portion of the file/path, or a beep. If you get a beep, just press [Tab] again to obtain a list of the files/paths that match what's 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'll see a list of possible completions, including updatedb and uptime. By adding the letter "d" to up and pressing [Tab] again, your command is completed for you.

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