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:
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:
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
|
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.
