8
ADVANCED KEYBOARD TRICKS

I often kiddingly describe Unix as “the operating system for people who like to type.” Of course, the fact that it even has a command line is a testament to that; however, command line users don’t like to type that much. Why else would so many commands have such short names

like cp, ls, mv, and rm? In fact, one of the most cherished goals of the command line is laziness—doing the most work with the fewest number of keystrokes. Another goal is never having to lift your fingers from the keyboard and reach for the mouse. In this chapter, we will look at bash features that make keyboard use faster and more efficient.

The following commands will make an appearance:

clear Clear the terminal screen

history Display or manipulate the history list

Command Line Editing

bash uses a library (a shared collection of routines that different programs can use) called Readline to implement command line editing. We have already seen some of this. We know, for example, that the arrow keys move the cursor, but there are many more features. Think of these as additional tools that we can employ in our work. It’s not important to learn all of them, but many of them are very useful. Pick and choose as desired.

Cursor Movement

Table 8-1 describes the keys used to move the cursor.

Table 8-1: Cursor Movement Commands

Key	Action
CTRL-A	Move cursor to the beginning of the line.
CTRL-E	Move cursor to the end of the line.
CTRL-F	Move cursor forward one character; same as the right arrow key.
CTRL-B	Move cursor backward one character; same as the left arrow key.
ALT-F	Move cursor forward one word.
ALT-B	Move cursor backward one word.
CTRL-L	Clear the screen and move the cursor to the top-left corner. The clear command does the same thing.

Modifying Text

Because it’s possible we might make a mistake while composing commands, we need a way to correct them efficiently. Table 8-2 describes keyboard commands that are used to edit characters on the command line.

Table 8-2: Text Editing Commands

Key	Action
CTRL-D	Delete the character at the cursor location.
CTRL-T	Transpose (exchange) the character at the cursor location with the one preceding it.
ALT-T	Transpose the word at the cursor location with the one preceding it.
ALT-L	Convert the characters from the cursor location to the end of the word to lowercase.
ALT-U	Convert the characters from the cursor location to the end of the word to uppercase.

Cutting and Pasting (Killing and Yanking) Text

The Readline documentation uses the terms killing and yanking to refer to what we would commonly call cutting and pasting (see Table 8-3). Items that are cut are stored in a buffer (a temporary storage area in memory) called the kill-ring.

Table 8-3: Cut-and-Paste Commands

Key	Action
CTRL-K	Kill text from the cursor location to the end of line.
CTRL-U	Kill text from the cursor location to the beginning of the line.
ALT-D	Kill text from the cursor location to the end of the current word.
ALT-BACKSPACE	Kill text from the cursor location to the beginning of the current word. If the cursor is at the beginning of a word, kill the previous word.
CTRL-Y	Yank text from the kill-ring and insert it at the cursor location.

Completion

Another way that the shell can help you is through a mechanism called completion. Completion occurs when you press the TAB key while typing a command. Let’s see how this works. Given a home directory that looks like this:

[me@linuxbox ~]$ ls
Desktop    ls-output.txt  Pictures  Templates      Videos
Documents  Music          Public

try typing the following, but don’t press ENTER:

[me@linuxbox ~]$ ls l

Now press TAB.

[me@linuxbox ~]$ ls ls-output.txt

See how the shell completed the line for you? Let’s try another one. Again, don’t press ENTER.

[me@linuxbox ~]$ ls D

Press TAB.

[me@linuxbox ~]$ ls D

No completion, just nothing. This happened because D matches more than one entry in the directory. For completion to be successful, the “clue” you give it has to be unambiguous. If we go further, as with the following:

[me@linuxbox ~]$ ls Do

and then press TAB:

[me@linuxbox ~]$ ls Documents

the completion is successful.

While this example shows completion of pathnames, which is its most common use, completion will also work on variables (if the beginning of the word is a $), usernames (if the word begins with ~), commands (if the word is the first word on the line), and hostnames (if the beginning of the word is @). Hostname completion works only for hostnames listed in /etc/hosts.

A number of control and meta key sequences are associated with completion, as listed in Table 8-4.

Table 8-4: Completion Commands

Key	Action
ALT-?	Display a list of possible completions. On most systems, you can also do this by pressing the TAB key a second time, which is much easier.
ALT-*	Insert all possible completions. This is useful when you want to use more than one possible match.

There are quite a few more that are rather obscure. A list appears in the bash man page under “READLINE.”

Using History

As we discovered in Chapter 1, bash maintains a history of commands that have been entered. This list of commands is kept in your home directory in a file called bash_history. The history facility is a useful resource for reducing the amount of typing you have to do, especially when combined with command line editing.

Searching History

At any time, we can view the contents of the command history list by doing the following:

[me@linuxbox ~]$ history | less

By default, bash stores the last 500 commands we have entered, though most modern distributions set this value to 1,000. We will see how to adjust this value in Chapter 11. Let’s say we want to find the commands we used to list /usr/bin. This is one way we could do this:

[me@linuxbox ~]$ history | grep /usr/bin

And let’s say that among our results we got a line containing an interesting command like this:

   88  ls -l /usr/bin > ls-output.txt

The 88 is the line number of the command in the history list. We could use this immediately using another type of expansion called history expansion. To use our discovered line, we could do this:

[me@linuxbox ~]$ !88

bash will expand !88 into the contents of the 88th line in the history list. There are other forms of history expansion that we will cover in the next section.

bash also provides the ability to search the history list incrementally. This means we can tell bash to search the history list as we enter characters, with each additional character further refining our search. To start incremental search, press CTRL-R followed by the text you are looking for. When you find it, you can either press ENTER to execute the command or press CTRL-J to copy the line from the history list to the current command line. To find the next occurrence of the text (moving “up” the history list), press CTRL-R again. To quit searching, press either CTRL-G or CTRL-C. Here we see it in action:

[me@linuxbox ~]$

First press CTRL-R.

(reverse-i-search)`':

The prompt changes to indicate that we are performing a reverse incremental search. It is “reverse” because we are searching from “now” to sometime in the past. Next, we start typing our search text. In this example, we’re searching for /usr/bin:

(reverse-i-search)`/usr/bin': ls -l /usr/bin > ls-output.txt

Immediately, the search returns our result. With our result, we can execute the command by pressing ENTER, or we can copy the command to our current command line for further editing by pressing CTRL-J. Let’s copy it. Press CTRL-J.

[me@linuxbox ~]$ ls -l /usr/bin > ls-output.txt

Our shell prompt returns, and our command line is loaded and ready for action!

Table 8-5 lists some of the keystrokes used to manipulate the history list.

Table 8-5: History Commands

Key	Action
CTRL-P	Move to the previous history entry. This is the same action as the up arrow.
CTRL-N	Move to the next history entry. This is the same action as the down arrow.
ALT-<	Move to the beginning (top) of the history list.
ALT->	Move to the end (bottom) of the history list, i.e., the current command line.
CTRL-R	Reverse incremental search. This searches incrementally from the current command line up the history list.
ALT-P	Reverse search, nonincremental. With this key, type in the search string and press ENTER before the search is performed.
ALT-N	Forward search, nonincremental.
CTRL-O	Execute the current item in the history list and advance to the next one. This is handy if you are trying to re-execute a sequence of commands in the history list.

History Expansion

The shell offers a specialized type of expansion for items in the history list by using the ! character. We have already seen how the exclamation point can be followed by a number to insert an entry from the history list. There are a number of other expansion features, as described in Table 8-6.

Table 8-6: History Expansion Commands

Sequence	Action
!!	Repeat the last command. It is probably easier to press the up arrow and ENTER.
!number	Repeat history list item number.
!string	Repeat last history list item starting with string.
!?string	Repeat last history list item containing string.

I caution against using the !string and !?string forms unless you are absolutely sure of the contents of the history list items.

Many more elements are available in the history expansion mechanism, but this subject is already too arcane, and our heads may explode if we continue. The “HISTORY EXPANSION” section of the bash man page goes into all the gory details. Feel free to explore!

Summing Up

In this chapter, we covered some of the keyboard tricks that the shell provides to help hardcore typists reduce their workloads. As time goes by and we become more involved with the command line, we can refer back to this chapter to pick up more of these tricks. For now, consider them optional and potentially helpful.