Introducing UNIX and Linux

Getting started

Overview
Using UNIX
      Usernames
      Logging in
Logging out
Commands
      Typing in commands
      Commands and options
Communication with other users
      Email
      Other communication facilities
Files
      The editor Vi
            Vi commands (command mode)
            Vi commands (colon mode)
      Other editors
Input and output
      Scripts
      Here-documents
      Pipes
      Making copies of input and output
      Pagers
Emergencies
Getting help
Summary
Exercises

The editor Vi

Vi is the 'standard' screen editor on UNIX. Pronounced 'vee-eye', it stands for 'VIsual display editor'. Whilst being the editor of choice for many systems programmers, it has a reputation, arguably undeserved, for being difficult to use. There is a tradeoff here - if you use a simple, graphical editor, then you will be able to create files quickly, and easily make simple changes to them. However, complex editing cannot often be done quickly with simple editors. There is a learning curve for using Vi, but once mastered the benefits later on are substantial. Furthermore, the skills needed to use Vi overlap substantially with those needed for UNIX shell programming, and include topics covered later on in this book, such as Regular Expressions and the stream editor Sed.

The command vi invokes the Vi screen editor, which has facilities to enter text into a file and to change text already there. In this subsection we discuss only a small proportion of Vi's facilities, enough to allow you to create and edit files for the rest of this book. To edit the file myfile (say), type vi followed by the name of the file:

$ vi myfile

If you have already created a file called myfile then choose a filename you do not yet have.

Your screen (or window) will be cleared, and the cursor will appear in the top-left corner of the screen. Along the left-hand side of the screen will be a column of ~ (tilde) indicating that those lines on the screen are not (yet) being used by Vi. Additionally, a message may appear on the bottom line of the screen.

Now, type the letter a (append text) and type in several lines of text - the Return key will terminate each line - followed by ESC. Then press h, j, k, l, and see the cursor moving around the screen one square at a time:

Cursor movement in Vi

You won't be allowed to move the cursor to any location that does not contain text. Position the cursor near the centre of your text and press a again. Type in more text - it will appear after the cursor position. As before, ESC will terminate the input. If you're not sure whether or not you have typed ESC, then type it again - if you type too many, the extra ones will be ignored and will do no harm. Move the cursor to the centre again, type i (insert text), and repeat what you did for a - you will see that text is inserted, but this time before the cursor position.

To delete text, you can remove one character at a time by placing the cursor on it and typing x. To remove a whole line, place the cursor on the line and type dd. To remove part of the line from the cursor to the end of the line, type D. Try these commands now.

When you have finished making all the changes you desire, type ZZ (being careful to use capitals) and the contents of the file will be stored on disk and you will be returned to the shell.

There are three modes that Vi may be in. There is command mode, during which you can move the cursor around the screen, and generally move from one part of the file to another, deleting and altering text at the point of your cursor. When you enter Vi, you always start in command-mode. Secondly, there is colon-mode. This is necessary when you wish to perform more complicated operations on your file that cannot (easily) be done with simple keystrokes. Third, there is input mode during which you can enter text.

To enter colon-mode, you must be in command-mode, so make sure you are not entering text (type ESC if necessary). Then type a colon (:) (not followed by Return). The cursor will immediately move to the bottom line of your screen and will be preceded by a colon. At this stage, there are a few colon-mode commands that you must know. If you make a mistake while typing, then the command u (undo), either in colon-mode or in command-mode, will correct it. If you accidentally type a colon, you can return to command-mode by just pressing Return. In the following discussion we assume that you are in command-mode unless otherwise stated.

Try using Vi to create in file myfile just two lines:

hello there

After you have done this, and left Vi by typing ZZ, use the command ls to check which files you now have. You should find:

$ ls myfile

Now edit the file myfile again and remove the two lines by typing dd twice to delete each one in turn. Choose another document, such as a book or a newspaper, and copy a couple of paragraphs into the file. Make sure that you enter them correctly, using the Vi commands we have just discussed to make any corrections. If the file fills more than one screen you can 'scroll' backwards and forwards through the file by typing ctrl-U and ctrl-D respectively.

Some other cursor-moving commands are useful:

^ (caret) moves the cursor to the start of the current line,
$ (dollar) moves the cursor to the end of the current line.

The file will contain words, which just as in English are sequences of letters and/or digits:

w moves forward through the file to the start of the next word,
e moves forward through the file to the next end of a word,
b moves backwards to the start of a word.

If you know that there's a word, or sequence of characters, in the file that you wish to find, then Vi will search for that string:

typing / followed by the string you are looking for, followed by Return, will look forwards in the file for the string, and
typing ?, rather than /, searches for a previous occurrence of a string.

So, in order to search for the next occurrence of hello, you should type

/hello

followed by Return. If a line is too long, you can split it into two by positioning the cursor where you wish it to be split, and using i or a to insert a Newline character. If you have two lines you wish to join to a single one, place the cursor on the first one and type J (join).

There are also colon-mode commands for moving about the file. For each colon-mode command you must press Return at the end of the command:

:0 (digit zero) moves the cursor to the start of the file,
:$ moves the cursor to the beginning of the last line of the file,
:n moves the cursor to the beginning of line n.

Practice the other commands listed above and those in Tables of command mode and colon mode commands, and get used to them. You will have to create many files, and it is worthwhile getting used to the editor at this stage. Some sites will support a command called vilearn (not POSIX) which is a user-friendly program that teaches you how to use Vi, and if it is available you may find it very helpful.

At this point, mention must be made of an editor called ex. Strictly speaking, ex and Vi are the same animal - think of Vi permanently in colon-mode so that after each colon-mode command the cursor is prompted on the bottom line by a colon, and you have ex. The command visual to ex will turn it into Vi. If you are in Vi and in command-mode the command Q (quit) will put you permanently in colon-mode, namely in ex.