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Red Hat Linux: Text EditorsWhat Are Editors and Why Do I Need One? A text editor is one of the most essential tools provided with the Linux (or
virtually any) operating system. With an editor, you can create and modify text
files that have a wide variety of applications:
These are but a few of the many different types of text files that you will
use when working with Linux. Basically, editors enable you to insert, delete,
move, and search text ranging from individual characters to thousands of lines.
Two of the most popular editors for the Linux system are emacs and vi. These
editors are both full-screen text editors: Put simply, they use every row and
column of your terminal screen to display the textual contents of a file. Both
of these editors feature a rich set of commands. The essential commands for
manipulating text can be learned reasonably quickly; the more sophisticated
commands may take a little longer to master. However, you will likely appreciate
this investment as you see how much time these powerful tools can save you. Choosing one editor over another can be a matter of taste. Both emacs and vi
are efficient and can handle virtually any size of file. The emacs editor is
better suited to complex editing tasks and comes with an online help facility,
but, for simple editing jobs, either editor is equally good. It really just
comes down to whichever one you feel more comfortable using. Although there are a variety of text editors for Linux that have different
interfaces, they all basically do the same things. Any useful text editor should
support the following features at a minimum. Inserting and Deleting Text The most intrinsic function of a text editor is to enable you to enter and
erase characters as you see fit. This also implies that you have complete
control over the movement of the cursor and its placement in the text. Reading and Writing Files Because you will want to save the text files that you create for future use
and reuse, an editor can write your text to an external file. Whenever you need
to make changes to your file, an editor can read the file from disk. A nice
feature is that text editors are designed to accommodate ASCII formatted files,
so an editor (such as emacs) can read any file written by another editor (such
as vi), and vice versa. Searching Text Personally scanning line after line of a large file for instances of a
particular word is either a great way to improve your powers of concentration or
an exercise in self-torture. That is why text editors provide sophisticated
search capabilities. These include the use of regular expressions as well as
fixed strings. Remember that regular expressions include metacharacters (such as
., ?, and *) that replace and expand unknown text patterns. Editors also support search-and-replace functions that enable you to change
multiple instances of a string pattern with a single command. Copying and Moving Text Because there is no guarantee that the way text is initially typed into a
file is the way it should forever remain, editors provide you with the means to
copy, cut, and move (or paste) blocks of text. These blocks can range in size
from several pages to a single character. The distinction between copying and
cutting text is that cutting deletes the selected block of text after it has
been copied to a buffer, whereas copying does not.
Editing Buffers What is a buffer, you ask? Buffers are places in the memory of the editing
program where text can reside as you make changes to a file. For example, the
first time you edit a file, the text you have entered actually exists in a
buffer that is written to an external file when you do a save. Buffers can also
be used at other times in editing, particularly when it is necessary to
temporarily move a block of text to memory as you make changes (in other words,
cutting and pasting). Many editors enable you to manage multiple buffers
simultaneously. These editors have many commands that will not be fully detailed in this
chapter. Before engaging in any long and arduous editing task, consult the man
page for the editor you are using. There may be an easier way of doing whatever
it is that you want to do. As you gain experience with an editor, you will
discover convenient shortcuts and functions to perform your most tedious editing
chores. The vi Editor The vi editor is installed with virtually every UNIX system in existence.
Because of this, vi is considered by many to be the default text editor of the
UNIX system (upon which Linux is based). vi has two modes of operation and terse
commands, both of which make it a somewhat more difficult editor to learn than
emacs. However, it is a useful editor to learn if emacs has not been installed
on your Linux system. You invoke vi from the command line by typing vi The screen will clear and a column of tildes (~) will appear in the leftmost
column. You are now editing an empty, unnamed file. Whatever text you place in
this file will exist in a buffer until you write the contents of the buffer to
some named file. The tilde is vi's way of telling you that the line where the
tilde appears is empty of text. vi can also be started with a file or a list of files to edit: Typically, you will probably edit only one file per vi session. If you are
editing a list of files, vi will edit each one in the sequence that they appear
on the command line. Alternatively, vi can be invoked from the command line as where n represents the line number where vi will place its cursor in
filename. This is useful for programmers debugging large source code files who
need to quickly jump to a known line containing an error. Another example is useful in illustrating the vi editor. If you still have a
vi session on your screen, exit it by pushing Esc, and then typing :q!. To start
a new vi session, enter at the command line. At the bottom of the screen in the left corner, you will see The messages displayed on this status line tell you what vi is doing or has
just done. In this case, vi is telling you that it has opened an empty buffer
whose contents will be saved (whenever you do a save) to the file asong. At this moment, you are in the command mode of vi. This is the major
conceptual leap required in working with this editor. When editing text, you
must remember if you are in command mode or text mode. In command mode, any
character sequences that you enter are interpreted as vi commands. In text mode,
every character typed is placed in the buffer and displayed as text on-screen.
Four commands are echoed at the bottom of the screen on the status line:
Each of these types of status-line commands must be entered by pressing
Return. This is not true for other types of vi commands, such as the ones that
do insertions.
Inserting Text So, knowing that you are in command mode, let's insert some text. Basically,
there are two commands for entering text on the current line: the letters i and
a. These letters in lowercase insert (i) text to the left of the cursor or
append text to the right of the cursor. As with many vi commands, the uppercase
versions of these letters have similar effects with subtle differences:
uppercase I and A insert and append at the beginning and end of the current
line, respectively. After you type either of these letters, you will be placed in input mode. Any
text entered after this point will be displayed on-screen. Type an i and then type the following: Down I walk<Enter> To exit from input mode, press Esc. Notice that you did not see the letter i
displayed before you entered the text, meaning that the i was correctly
interpreted as a command. Also, it is important to note that it was not
necessary to press Enter after pressing i for input mode. Quitting vi Now that you have some text for your file, let's quit the editor to see the
results. The commands used for saving the file and exiting vi are slightly
different from the i and d commands used in editing text: you must precede the
command with a colon (:). In this case, you want to do a save and exit, which are actually combined in
one command. Type a :. At the bottom left of your screen, you will notice that a
colon has appeared. vi has recognized that you are about to enter an ex command,
and it will echo the remaining characters of the command after the colon. Type
wq and press Return. vi quickly informs you that it has written the file to disk
and tells you how many lines it contains. If the file is small and you have a
fast system, this message may appear and be erased so quickly you won't catch
it. Don't worry—the file has been saved if you issued the command properly. vi
exits and you find yourself back at the shell prompt. Another way to save and
exit is to type ZZ. The difference between this method and using wq is that ZZ
will write the file only if it has been modified since the last save. You can quit vi by typing :q if no changes have been made to the file you
opened. This will not work if the file has been modified. If you are sure that
you don't want to save what you have done, enter :q!. This command forces vi to
quit, regardless of any edits. To make sure that vi saved the file asong correctly, use the cat command to
quickly view the file's contents: $ cat asong Everything is exactly as you typed it in the file, so no surprises here. Moving the Cursor Moving the cursor around in vi essentially involves the following four keys:
These keys can perform their operations only when vi is in command mode. For
convenience, most implementations of vi map these keys to their directional
counterparts on the keyboard arrow keys. vi enables you to move through a file in bigger "leaps" as well. Following
are some commands for scrolling more than one line at a time:
The size of these movements largely depends on the terminal settings. It is also possible to move the cursor to a specific line in a file. If you
want to move to the tenth line, type 10G or :10 in command mode. G by itself
will move the cursor to the end of the file. The cursor will not move if the
number given is not applicable (for example, typing :10 in a eight-line file
will have no effect). vi will also enable you to move the cursor a word at a time. A word is
defined as any sequence of non-whitespace characters. To move to the beginning
of the next word or punctuation mark on the current line, type w. Type b to move
the cursor to the beginning of the current or previous word or punctuation mark.
Deleting Text vi has commands for deleting characters, lines, and words. Deletion means
that the selected text is removed from the screen but is copied into an unnamed
text buffer from which it can be retrieved. To delete a word, use the dw command. If you want to delete the word to the
right of the cursor, type dw. If you are in the middle of a word, it will delete
from the cursor position to the end. You can also delete several words at a
time. For example, the command 4dw will delete the next four words on the
current line. Lines can be deleted individually or by specifying a range of lines to
delete. To delete the current line, type dd. The command 4dd deletes four lines
(the current line and three below it). dG will delete all lines from the current
one to the end of the file. On the current line, you can delete in either direction: d^ will delete
backward to the beginning of the line; d$ (or D) will delete forward to the end
of the line. To delete individual characters, x deletes the character underneath the
cursor, and X deletes the character to the left of the cursor. Both of these
commands will accept a number modifier: For example, 4x deletes the current
character and the four characters to the right. Unwanted changes such as deletions can be immediately undone by the u
command. This "rolls back" the last edit made.
Copying and Moving Text Moving sections of text around in a file basically requires three steps:
Yanking text means to copy it into either a named or unnamed buffer. The
unnamed buffer is a temporary storage space in memory that is continually
overwritten by successive yanks. vi has 26 named buffers that correspond to each
letter of the alphabet. To yank the current line into the unnamed buffer, the command is yy or Y.
These commands can be modified by a number indicating how many lines beneath the
cursor are to be yanked. For example, the command in your file asong (with the cursor on the top line) yanks the following text
into the temporary buffer: Down I walk This text could also be yanked into the named buffer a by the following
command: The yank command to overwrite the contents of the named buffer a. If you had
typed a capital A instead of a lowercase a, the three lines would have been
appended to the end of the a buffer. This overwrite-versus-append concept works
the same for all of the named buffers. If you move the cursor to the end of the file using the :$ command, you can
then paste the contents of the unnamed buffer to the end of the file. This is
done using the p command, which pastes the contents of a buffer to the right of
the cursor (P pastes to the left of the cursor). The paste command can also
specify a named buffer in the same way as the yank command: Yanks can also be performed on words using the command yw. This command can
also use named buffers and accepts numeric modifiers. Searching and Replacing Text Text searches in vi can be performed in either direction: forward or
backward. Searches are always started from the current cursor location and
continue from the top or bottom of the file depending on which direction you
use. In other words, searches "wrap around" the file. You can use your file asong to illustrate searches. To search forward through
asong for the word "bay," you would type and press Return. Notice that this is a status-line command. The command /bay
is echoed on the status line and the cursor is moved to the first occurrence it
finds in the forward direction of the string "bay." Interested in finding
another instance of "bay"? Enter a / character. This command continues the
search for "bay" in the forward direction and places the cursor at the next
instance of "bay." Each time you enter the / key, vi will try to find an
instance of the previous string pattern. When it reaches the end of the file, vi
will loop back and continue its search at the start of the file. You can also search backward for strings in vi by using the ? command. It
works in exactly the same manner as the / command, but in the opposite
direction. Try it out by typing in asong, instructing vi to search back for instances of "I." This search can
be repeated by typing ?, as you may have suspected. You can continue a search by
pressing n, which always continues a search in the same direction as the
previous search. However, typing N will use the same search string but in the
opposite direction. As I mentioned earlier, searches can be made very powerful through the use of
regular expressions. The search command is supplied in the same fashion as
described before (/ or ?), but square brackets are added to instruct vi to do a
regular expression expansion of the enclosed characters. For example, search
forward through asong from the first line for all strings containing the
substring "er". Type vi's first matching string arrives at "Where." If you type n, vi will move
the cursor to "where," and so on. You can also specify collections of characters
or ranges of characters to match. Try typing the following: This command used in asong will find the strings "by" and "my," as well as
any word with these strings inside them (such as "bay"). This works because the
range of characters given are treated as an enumerated range of ASCII values.
Thus, you could also include a range of numbers (for example, 0-9). Now try the
following command: This will locate the strings "My" and "my." In vi, searches without regular expressions will find only exact matches of
the supplied pattern (including the case of the letters in the pattern).
Clearly, regular expressions can be used to enhance many types of searches in
which you may not know exactly how a pattern appears in a file. One of the more common applications of a search is to replace instances of
one word (or pattern) with another. This is done with an ex command that starts
with a colon. To search the entire asong file for the string "Down" and replace
it with the string "Up," type The s indicates that this is a search operation, the % means that the entire
file is to be searched, "Down" is the pattern to be found, "Up" is the new
pattern, and the g tells vi that the search should continue until there are no
more pattern matches. Without the g, vi would perform the replacement on only
the first match it finds. This command also works with regular expressions
appearing in the search pattern and the replacement pattern. Setting Preferences vi is configurable, which means that you can set options to control your
editing environment. These options are initialized with default values that you
can modify in vi at any time. vi is configured using the set command. The set
command must be preceded by a colon and entered by pressing Return. For example,
to display line numbers in the editor, you would issue The following table describes a few of the more common set commands.
set commands that do not take a value can be switched off by inserting a "no"
as a prefix to the set parameter. For example, the command switches line numbering off. The command shows only the options that you have changed. The settings that you use in a vi session are (unfortunately) lost each time
you exit vi. If you do not like the idea of resetting these options each time
you use vi, there is an easier way to perform this initialization. Use the vi
initialization file called .exrc. vi searches for this file in your home
directory each time it is invoked. If it can't find this file, it uses the
defaults set within the vi program. As you will see in the following example,
the .exrc file can also be used to define vi macros. A sample .exrc file would look something like this: set number Note that the colon is not required before a set command in a .exrc file. Summary of Commands The following is a summary of the more essential commands described in this
chapter. You should consult the vi man page for more details on the many other
vi commands.
The emacs Editor emacs has become the editor of choice for many users because of its online
help facility and its extensive collection of editing commands. For programmers,
emacs is especially attractive because it can be configured to format source
code for a variety of languages such as C, C++, and Lisp. emacs is somewhat
easier to learn than vi, but it also features a much larger set of commands. Starting emacs emacs is invoked from the command line by entering To start emacs with a file to be edited, enter If you start emacs with a file, the screen will display the contents starting
from the first line. Note the two lines at the bottom of the screen. The first
of these lines, known as the mode line, displays the name of the file being
edited and which part of the file that you are looking at (for example, TOP,
20%, BOT). The last line on the screen is the echo line, which emacs uses to
display system messages and as a prompt for more input. Control and Meta Keys You are quite free at this point to start entering text into the edit buffer
at the cursor location. However, you're probably wondering, "How do I move the
cursor around?" Before I fill you in on this little detail, there are two keys
that you should know about: the Control key (which I will refer to as C) and the
Meta key (denoted by M). The Control key is used in most of the commands for
emacs, but some use the Meta key instead. Commands in emacs consist of
combinations of the Control or Meta key followed by some other character. It is
necessary to hold down the Control key when pressing the next character, whereas
the Meta key can be pressed and released before you enter the next character.
For the PC, the Meta key is usually the Alt key.
Moving the Cursor Now that you know about the Control key, we can talk about the
cursor-movement commands. The basic ones that you need to remember are:
Most implementations of emacs conveniently map the first four movement
commands to the arrow keys on the keyboard. Let's edit a new file called asong2.
(If you are in the middle of a previous file, exit the editor by typing Ctrl-X,
Ctrl-C.) Start up a new copy of emacs by entering the following command from the
shell: emacs asong2<Enter> Now enter the following text into the buffer: This is a file for edit Now use the C-b command to move back through this horrendous piece of poetry.
Notice how the cursor jumps up to the end of each line after the reaching the
beginning of the previous line. This works the same way in the opposite
direction using the C-f command. Another useful way of moving around is by scrolling through a file one screen
at a time. The command C-v moves the cursor forward one screen at a time. The
command M-v moves the cursor in the opposite direction. Like vi, emacs treats a sequence of non-whitespace characters as a word. You
can move the cursor forward one word at a time with the M-f command. The M-b
command moves back one word. Quitting emacs At this time, you can stop editing to save the contents of the buffer to your
file asong2. To do this, issue the command sequence C-x C-s. As you enter this
command, notice how the command is displayed on the echo line as you type it. To
quit emacs and return to the shell, enter the command C-x C-c. If you have made
changes that haven't been saved using C-x C-s, emacs will ask for confirmation
before quitting. You can delete text in several ways. The Backspace (or Delete) key is used to
erase the character immediately preceding the cursor. The command C-d deletes
the character underneath the cursor, and C-k deletes or "kills" all characters
from the cursor to the end of the line. Words can be deleted also: M-d deletes
the word the cursor is currently located over and M-Del (the Delete key) deletes
the word previous to the current word. If you ever find that you have committed an edit that you didn't really want,
just type C-x u to undo the previous editing changes. You can repeat the undo
command as many times as you want, rolling over all the changes you made. This
is an advantage over vi, which can only undo the last change.
Working with Multiple Files emacs enables you to edit several files in one session, each contained within
its own buffer. To copy an external file into a new buffer, use the C-x C-f
command. After entering this command, you will see the following prompt on the
echo line: Find file: ~/ emacs is smart when it looks for files. It supports filename completion,
which means that you can simply type a few characters of a filename and emacs
will attempt to match a file (or files) to what you have typed so far. To do
this, type in the letters ".log" and press the Tab key. emacs expands this to
~/.login (or any other filename that matches). If two or more files match the
pattern supplied, pressing the Tab key will cycle through them. After you have loaded a new file into emacs, you can switch between buffers
by using the C-x b command followed by the name of the buffer that you want. The
buffer's name is that of the file that was loaded into it. The C-x b command
also uses filename completion, so you can use the Tab key to cycle through your
edit buffers after supplying a few relevant characters. When you have finished editing a buffer, instead of saving the contents using
the C-x C-s command, you may decide that you do not really want to keep the
edits you have made. You can "kill" the current buffer by entering the command
C-x k. emacs will prompt you for the name of the buffer to kill, but you can
kill the current buffer by simply pressing Return. emacs will ask for
confirmation, to which you can respond by typing yes (if you're sure) and press
Return.
Copying and Moving Text In order to copy and move blocks of text in emacs, you must define the region
of text by marking the beginning and end points of the text block. This is done
by moving the cursor to where you want the block to begin and marking it using
the C-Space command (in this case, Space means literally the spacebar). The end
of the block is defined by wherever you place the cursor after that. To make a
copy of the block, enter the command M-w. The text within the block is copied to
emacs's internal clipboard, from which it can be pasted at another location
using the C-y command. Alternatively, you can cut the block into the clipboard
using C-w instead of M-w. Cutting, of course, deletes the text from its current
location. Let's try out some of these techniques on your buffer asong2. Use the M-<
command to jump to the beginning of the buffer. Enter a C-Space to mark the
start of the block and then use C-n to move down a line. Cut the block to the
clipboard using C-w, move the cursor to the end of the buffer using M->, and
paste it using C-y. The result should look like this: It's really quite swell Searching and Replacing Text You can search forward and backward through text using the C-s and C-r
commands, respectively. These commands, like many in emacs, use command
completion. This is the same concept as filename completion: you supply a few
characters and emacs tries to fill in the rest. In this case, however, emacs
moves the cursor to each instance it finds of the string supplied. As you enter more characters, emacs narrows its search further. When you have
found a correct match, press Return or use any of the cursor-movement commands
to halt the search. As with vi, searching in either direction wraps around the beginning or end
of the file, depending on in which direction you are searching. However, when
emacs reaches the top or bottom of the file, it will tell you that the search
failed. You can keep searching by pressing C-s or C-r accordingly and emacs will
continue using the current string. To illustrate how searching in emacs works, let's search backward through
your file asong2. Enter C-r and type an s. emacs moves the cursor to the "s" in
"works". Now type a w. emacs now tries to find a pattern that matches the string
sw. The cursor ends up on the "w" in "swell". You can edit the search string
using the Backspace or Delete key. Delete the w and type a p. What happens? Search-and-replaces are done by entering the query-replace command. This is
qualified by the M-x command, which tells emacs that the text to follow is a
full command and not a key combination. After you have entered the query-replace
command, you will be prompted for the string to be found. Enter the string and
press Return. emacs will then prompt you for the replacement string. Once you
have entered the replacement string, emacs will search for every instance of the
first string and, if it finds one, asks you if it should be replaced with the
second string.
Using Modes with Buffers emacs is versatile enough to handle many different types of editing chores.
It enables you to associate modes to buffers so that you can have text
formatting specific to your editing application. If you enter the command C-x m,
emacs enters mail mode, which formats a buffer with To: and Subject: fields as
well as a space for the body of the mail message. emacs can even send the mail
message for you (by entering C-c C-c) after you have finished editing it. emacs also supports modes for many different programming languages such as C.
When a file with the extension .c (C source code) or .h (C header file) is
loaded into emacs, the buffer is automatically set to C mode. This mode has
knowledge of how C programs are formatted, and pressing the Tab key will indent
a line correctly based on its place in the program (a for loop within another
for loop, as an example). Online Help in emacs One of the best features of the emacs editor is that if you ever get stuck,
or are just plain overwhelmed by it all, help is just a few keystrokes away—and
lots of it! If you need a short emacs tutorial, just enter C-h t. If you would
like to find out what function a particular key supports, type C-h k and then
press the key. The help option has many different topics. Use C-h i to load the
information documentation reader and read about all the types of help available.
A Summary of Commands emacs, like the vi editor, has such a rich command set that we can cover only
a portion of it in this chapter. The following table is a summary of the
strictly essential commands that you will need for basic editing in emacs. The
emacs man page should be consulted for a more comprehensive description of the
full emacs command set.
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