The Xemacs Editor. Overview

Transcription

1 The Xemacs Editor Editors are used to create and modify simple text files. The public domain editor emacs has achieved great popularity because of its many features and its wide availability. Emacs was developed by the Free Software Foundation and is available for many types of computers. A variant that is available on X-Window systems such as our Solaris/CDE environment is xemacs (usually pronounced ecks-emacs). This version of emacs has some improved window features and most of this chapter applies to it as well as emacs. Both emacs and xemacs work the same way with keyboard commands, and recent versions of xemacs have been able to run without the X-Window System in plain terminal sessions, just like emacs always has. Consequently, we will focus on the xemacs variant in this document; most of what you read here about the basic operation of the editor applies to both variants. The xemacs graphical user interface has a tool bar with buttons for the most common operations and drop-down menus for other actions. xemacs is not a word processor, it is at its heart a plain-text editor. In this regard, it is tempting to compare it to Notepad, since neither provides support for multiple fonts or text styling, but this is a spurious comparison. xemacs is a programmer s editor, providing powerful tools to support writing programs. The three main features of xemacs that will be of interest to us are syntax coloring, automatic code indentation, and compilation and error tracing. As you type in your program, xemacs scans the code and gives special colors to syntactic constructs that it recognizes, such as language key words, comments, character strings and so forth. This makes it easy to see when you have misspelled a keyword or forgotten to close off a string. xemacs also automatically indents your code as you type it; if the indentation looks funny, it is most likely because you have made a syntax error in an earlier line. xemacs provides brace and parenthesis matching, a very valuable tool that helps to avoid many compilation errors. It is also possible to compile your program directly within xemacs. This process is described later, but the upshot is that compilation errors are presented in a split window with the code that caused the error, making correcting syntax problems much easier. These are familiar capabilities if you have worked with an IDE such as JBuilder or SunOne Studio, but the difference is that in xemacs, with the exception of syntax coloring, all of these features are available via remote access to a system, not just via a GUI windowing system. This chapter presents the basics of xemacs needed by computer science students. Thanks go to Richard Stallman, who developed emacs, and the Free Software Foundation which makes emacs available to UNIX users throughout the world. There are also versions available for the Windows environment. Overview Keyboard Commands are distinguished from characters being inserted in the text by the simple fact that commands are restricted to either control characters such as control-v, typed by holding down the Control key while typing the letter v, a character sequence that starts with a special key called the "meta" key, (On the SPARC workstations this is the escape key or diamond key on the keyboard.) or a sequence of commands such as control-x control-s or control-x 2. The Xemacs Editor 1

2 Invoking Xemacs From the command line, the xemacs editor can be invoked with or without a file name. xemacs filename or simply xemacs If no file name is given, then the current editing buffer is called the scratch buffer. Whenever xemacs is invoked in an X Windows environment, it will start up in a new window. The figure below shows a typical xemacs window. Notice the area at the bottom of the window that is a single line high. This last line is called the mini-buffer and is where the user provides the input to some commands and is where inquiries and confirmations are displayed. GUI Commands vs. Keyboard Commands When using the SPARC workstations in the laboratory and the CDE X-Window interface, the user is exposed to the graphical user interface (GUI) of xemacs. The GUI has menus that make it easy to change files, save the file, exit the program, perform editing tasks, etc. The same commands can be given with keyboard commands, and furthermore, the keyboard commands work even when logged in over the network via telnet or ssh. For this reason, (and because the buttons and menu items are fairly self-explanatory) the rest of this document will focus on keyboard commands. In this context emacs and xemacs are identical! While using the menu/button commands is easy and appealing, some of the actions are less flexible and harder to understand than their keyboard equivalents, especially when it comes to the file browsing dialog boxes, so it pays to learn the keyboard commands. If you are a touch typist, this will also allow you to work much more quickly than using the mouse to interact with the graphical user interface. The Xemacs Editor 2

3 The Emacs Tutorial If xemacs is invoked without a file name, the keystrokes "control-h t" will start a tutorial on the use of emacs. Note that control-h means hold down the "control" or "ctrl" key while typing the h. This interactive tutorial is by far the best way to familiarize yourself with the operation of xemacs. The next section discusses control characters. Control and Meta Keys The control key is labeled "ctrl" or control on most keyboards and a control-v is a special character created by holding down the control key while typing a v. We will abbreviate "control-v" as "ctrl-v." Outside of emacs certain control characters have special purposes. For example, ctrl-h is used to backspace while ctrl-s stops screen scrolling and ctrl-q resumes screen scrolling. These other purposes can cause some problems for those using emacs from a personal computer via a modem or the network. In addition to the control characters, emacs designates a "meta" key. Exactly which key you use as the meta modifier is system dependent, but on the SPARC workstations it is the key at each end of the space bar that is labeled with a small diamond. Normally the Esc or escape key will work on all workstations, with one small caveat: if you are using Esc as meta, you do not have to hold it down as you do Ctrl. Just type Esc followed by whatever character you need. Consider the command "meta-v" for moving backwards one screen. On the Suns one holds down the diamond key while typing the v. If the Esc key is used, it must be pressed, then released, then the v key pressed. We will write "meta-" for a command that begins with a meta key. Leaving Emacs Exit emacs with ctrl-x ctrl-c. If you forget to save a file after making editing changes, emacs will ask you if you want to save the file. Cancelling a Command Even experienced users occasionally give a command unintentionally or decide that they really want to do something else. To cancel a command like ctrl-x ctrl-f as it is waiting for you to enter a file name, simply type a ctrl-g. Of course, some commands cannot be cancelled, but usually they can be undone. If a line or region of text is deleted, the command ctrl-y will yank it back. If a single character is deleted, the yank back command won t work - simply retype the character. In many situations, the last command can be undone by ctrl-x _ (underscore). This can be given repeatedly to undo a sequence of commands. Unless you have an unusually long editing session, every action you have taken on the file can be undone all the way back to the original state of the file when you read it from the hard disk. If you have made a mess of the entire file and want to recover the original file, use the fact that emacs automatically creates a backup version with a tilde ~ after the name to fix things. Exit the editor and use the UNIX mv command to replace the modified file with the original version. Buffers The emacs editor can edit several files in a single session as well as run shell commands. Each activity is done in a labeled buffer. A buffer is simply a sub-window where activity such as edit- The Xemacs Editor 3

4 ing can take place. Most buffers are associated with a file, but some temporary buffers are not. When emacs is invoked, it presents two buffers. If no file was specified then the large upper window is called the scratch buffer, while the single line window at the bottom is called the minibuffer. If a file name was given at invocation, then this name is used as the name of the buffer represented by the large window. The command ctrl-x ctrl-b will present a window showing the names of all buffers or pressing the control key and the left mouse button will pop up a menu. New buffers are created whenever certain commands are given. For example, the find file command ctrl-x ctrl-f will create a new buffer with the specified file name as label. It does not destroy the current buffer. The user may return to a previous buffer with the command ctrl-x b followed by the buffer s label. File Commands The basic file commands are to save a file, write a file under a new name, find a file to edit, and insert a file at the cursor. If a command requires an argument such as a file name, the cursor will move to the mini-buffer where the user is prompted for the name. ctrl-x ctrl-f ctrl-x ctrl-v ctrl-x ctrl-s ctrl-x ctrl-w ctrl-x i ctrl-x ctrl-c Find a file and associate a buffer with its name Find an alternate file and replace the contents of the current buffer with the alternate file. (Used when the find file command was given with an incorrect file name.) Save the contents of the current buffer to the file name used to label the buffer. Write the contents of the buffer to a file whose name is prompted for in the mini-buffer. This is the equivalent of save as. Insert a file into the buffer at the cursor. The file name is provided in the mini-buffer. Exit emacs and save buffers. You will be prompted to see if you really want to save any modified buffers. Moving Around When editing a file, it is normal to want to go back to previous lines or go back to the first line of the document or to move forward a screen at a time while proofreading a document. On some computers the arrow keys may be used by emacs to move around the buffer. But even if the arrow keys aren t recognized, the user can always give commands to move around the buffer using the command key combinations. ctrl-f meta-f ctrl-b meta-b ctrl-p ctrl-n ctrl-a ctrl-e Screen Movement ctrl-v move to the right or forward one character move to the next word move to the left or back one character move to the previous word move to the previous line move to the next line move to the beginning of the current line move to the end of the current line move to the next screen The Xemacs Editor 4

5 meta-v Global Movement meta-< meta-> move to the previous screen move to the beginning of the buffer move to the end of the buffer Learning the command key combinations for cursor movement has an added benefit. The ksh command processor (the default command shell for Solaris) recognizes the same cursor movement key combinations, allowing you to search back through commands you have typed, edit them, and reissue them. Searching/Replacing One way to move quickly to a specific place in a buffer is to search for a special string to be found at that point in the buffer. There are three basic searches. Two involve the replacement of the found string. Simple Searching The ctrl-s command will initiate a search forward from the current cursor s location in the document, while ctrl-r initiates a reverse search. We describe only the search forward as the search backward or reverse is similar. When the user types ctrl-s the mini-buffer prompts with I-search: for incremental search. This means as you type each letter of the string that you are seeking, emacs will find the first string in the file after the cursor that matches. Pressing the escape key will terminate the search as will any emacs command. If you have found the first occurrence of a string and perhaps taken some action, and now want to continue to look for additional occurrences, type ctrl-s to get the I-search: prompt. Then type ctrl-s again. Each time you type ctrl-s you will move to the next occurrence of the string. To search backwards rather than forwards, use ctrl-r instead. Replacing Strings Often the user wishes to find a string because the string must be replaced by another string. For example, a programmer may decide that the variable largest needs to be replaced by the variable name smallest throughout the program. The user may do a global replacement or a queried replacement. In the latter case, at each occurrence of the original string, the user is asked if the replacement is to be done or not. Pressing the space bar means perform the replacement, while pressing the delete key means to skip the replacement for this occurrence. Typing a y or n also works. A queried search and replacement is initiated by the sequence meta-% and the user is asked for the original and replacement strings in the mini-buffer. A global search and replacement, without query, is initiated by the command meta-x replace-string. When you type the meta-x command the mini-buffer prompts you for the rest of the command which is the hyphenated word replace-string. When you press return you will be prompted for the original string and then for the replacement string. The command ctrl-x _ will undo this action if you change your mind after the replacement. The Xemacs Editor 5

6 Deleting and Moving Text Normally one deletes the previous character typed with the Backspace key. xemacs typically treats the Delete key as a clone of the Backspace key rather than as a delete forward key. It is possible to delete forward with ctrl-d. Deleting to the end of a line is accomplished with ctrl-k. Any command can be repeated n times, where n is a positive integer, by prefacing the command with the sequence escape n. (Be sure to use a specific number in place of n.) So to delete the rest of the current line and four additional lines, enter the sequence escape 5 ctrl-k. A region of text can be specified and deleted. It can be un-deleted or yanked back at any designated point in the buffer. This feature allows the user to move blocks of text or duplicate blocks of text. To define a region of text, move the cursor to the start of the region and enter ctrlspace to set the mark. The command meta-@ also will set the mark. Then move to the end of the region and give one of the following two commands. ctrl-w meta-w Delete the region and write it into the xemacs kill ring. This is a set of invisible buffers containing recently deleted text. Copy the region into the kill ring without deleting it. To obtain deleted text from the kill ring, move the cursor to the place where the text should be inserted and use ctrl-y to yank back the deleted text. This retrieves the most recent deletion. If you want one of the earlier deletions, continue by typing meta-y which will replace this restoration with the next earlier deletion. This latter command can be used repeatedly. Using this technique, any text that has been deleted during the current editing session can be retrieved and reinserted. Window Operations The normal large window presented by emacs may be split into two windows, each containing the same text, with the command ctrl-x 2." To move the cursor to the other window type ctrl-x o where the o is for other. This is helpful if you have a large file and you want to examine two different sections of it at the same time. To return to a single large window type the command ctrl-x 1" and the window with the cursor will return to full size. The figure below shows an editing session with two windows and different files in each window. The Xemacs Editor 6

7 The most useful aspect of the double window feature is to edit one file while viewing the other in the second window. In fact, text from one window can be copied to the other window since the kill ring is shared by all buffers. When the user forms two windows with ctrl-x 2, the same file will be in each window. Each window is editing the same buffer. The find file command will be used to associate a new file buffer with the window containing the cursor. There is a command that will find a file for the other window. It is ctrl-x 4 f and will associate a new buffer with the other window and move the cursor to that window. When working with two windows, the command meta-ctrl-v will scroll the other window without having to move the cursor to that other window. In summary, ctrl-x 2 form two windows ctrl-x 1 destroy the other window and enlarge the current window ctrl-x o move the cursor to the other window ctrl-x 4f find a file in the other window escape ctrl-v scroll the other window Note. The arrow keys and menu commands can perform most of the actions that we have given in the preceding pages. Menus at the top of the emacs window are quite useful. We give the keystroke versions as they work over a remote connection. The Backup File For safety, xemacs creates a backup copy of any existing file that is loaded into a buffer. The The Xemacs Editor 7

8 backup has the same name as the file but a tilde ( ~ ) is appended to the name. This is a safeguard that can be of tremendous importance when a file is mangled during editing; however, it can waste disk space when large files have small changes made to them. Be aware of this fact and periodically clean up your emacs backup files. The Other Sides of Emacs In addition to editing, xemacs allows the user to create buffers for activities like reading mail, running shell commands, and compiling programs. Other activities will create temporary windows for the user. For example, the command ctrl-x ctrl-b will open a small window that will show the status of all the buffers. The window is dismissed with the ctrl-x 1" command. Users can execute single shell commands with the sequence that starts with escape!" followed by typing the shell command in the mini-buffer. The output of the command will appear in a temporary buffer. Using the ability to copy between windows, the user can incorporate the command s output into the file being edited in the active window. Unfortunately, while the shell command is executing no editing can be done. This inconvenience is solved in the next section. Shell Windows The emacs editor can be a complete working environment where the user can edit, execute shell commands, and do other tasks without leaving emacs. A shell window is an emacs buffer where a UNIX shell runs. Commands issued in the shell window can run in parallel with editing that is being done in other buffers. To create a shell buffer, give the command meta-x shell. The meta-x will initiate a prompt in the mini-buffer where shell is then typed. When in a shell buffer, the user may wish to send special character sequences to the shell like the interrupt ctrl-c or the end-of-file ctrl-d. These commands must be prefaced with a ctrl-c. Therefore, ctrl-c ctrl-d sends ctrl-d to the shell. A useful command is ctrl-c ctrl-y which yanks back the last shell command. The user can then edit it or simply type return to re-execute it. Compiling Programs From Within Emacs In software development, the most common use of the editor is to create program text. Once the text is created, the program must be compiled. If syntax errors occur, then corrections must be made and the program recompiled. This edit-compile sequence is repeated many times in large programs. Inexperienced programmers spend a great deal of time at this task. Emacs provides a convenient way to compile a program from the editor and quickly move to each discovered syntax error to make corrections. The follows steps describe what the user should do to edit, compile, correct errors, compile, etc. Step 1: Edit the program to the point where it is ready for the first compile. Step 2: Give the command meta-x compile (shortcut: meta-c). xemacs will present the default compile command make -k in the minibuffer, which you should change to the appropriate command to compile your program. If you have created a makefile for your program, no changes are necessary at this point. For example, if your program file is sample.cpp and is a C++ program, the appropriate compile command would be g++ -o sample sample.cpp. The Xemacs Editor 8

9 If you forgot to save your file to disk after changing, emacs will ask if you want it saved before compiling. Step 3: A new window will open within xemacs with the output of the compile command showing. If there are any compile errors, give the command ctrl-x " and xemacs will automatically move the cursor to the line in your program where the error occurred. (Note that the character after the control-x is a backwards single quote. This is found on the key with the tilde.) After fixing the error, move to the next error with ctrl-x " and correct it. Continue until all errors have been corrected. (Some later errors may be false and are reported because the compiler software is confused when it handles earlier errors.) Step 4: Go back to step 2. The procedure described here works for both xemacs and emacs, and it works whether you are at the console of a workstation, using Exceed to access the X-Window environment from a PC, or are remotely logged in in across the network using ssh or telnet. If you are using xemacs at the console of a workstation or via Exceed from a PC, you can click on the Compile button in the toolbar to do the same thing. This pops up a dialog box containing the default compilation command (make -k). To change this to the command needed to compile your program, click the Edit command button. This dumps you to the mini-buffer where you can make changes to the command. If you edit another file and want to compile it, you can click the Edit command button again to make the necessary changes. Customizing xemacs The xemacs editor incorporates an interpreter for a special purpose variant of the Lisp programming language called elisp. Just about everything outside of the basic editing functionality of xemacs is written in elisp, including the syntax coloring and indentation features. When xemacs starts up, it looks for a file called.emacs in your home directory and executes any elisp it finds there. (You already have a minimal.emacs file in your home directory, but under normal circumstances any file whose name begins with. is not displayed by ls. To see these hidden files, type ls -a instead.) A great deal of the behavior of xemacs can be customized via this file. It is beyond the scope of this document to describe customization in full, but there are a couple of minor irritants that we will show you how to address here. If you re used to the Windows environment, the fact that xemacs doesn t distinguish between Backspace and Delete is annoying. To fix this, Add the following two lines to your.emacs file: (require 'delbs) (delbs-enable-delete-forward) Many students find the block cursor that xemacs uses by default to be confusing. To switch to the more familiar i-beam cursor, add the line (setq-default bar-cursor t) exactly as shown to your.emacs file. Next time you start xemacs, you should get a more normal looking text insertion point. The Xemacs Editor 9

10 Some Final Notes If you are using the CDE Solaris windowing environment, either at one of the workstations or via Exceed from a PC, it is tempting to just crank up a new xemacs every time you want to edit a different file. As we have shown, this isn t necessary, since a single xemacs process can simultaneously maintain many buffers, each with it s own file. This is a much more efficient use of system resources. If you really want separate windows for different files, use meta-x new-frame to create a new window under the control of your existing xemacs session. It is also tempting to just hit the Exit button on the CDE front panel when you are ready to log out and let it take care of terminating all of your shell windows and other programs. With xemacs, there is a hitch: the xemacs processes will not be terminated, and will sit around using up system resources without doing any work. Over time, this can seriously degrade the performance of the Unix servers. Please be careful to individually terminate all of your xemacs processes (either via the File menu or by typing ctrl-x ctrl-c) before you exit from CDE. Editor preference among programmers is something of a religious issue. The default editor on all Unix systems is called vi, and it is available on our systems. The chief difference between xemacs and vi is that xemacs uses control-key sequences to issue commands to the editor while vi has two modes, insertion mode and command mode. Emacs devotees will tell you that emacs variants have superior capabilities, but recent variants of vi such as vim may have diminished that argument. In the end, it is a matter of personal preference. It is a fact, however, that experienced programmers coming from a background of using either emacs or vi are often horrified at the primitive state of IDE editing components when forced to use them. The Xemacs Editor 10