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7. Files

The basic unit of stored data in Unix is the file. To edit a file, you must tell Emacs to read the file into a buffer. This is called visiting the file. You can now edit the buffer and to save the changes you must write the buffer back to the file.

In addition to visiting and saving files, Emacs can delete, copy, rename, and append to files, and operate on file directories.

7.1 File Names  How to type and edit file name arguments.
7.2 Visiting Files  Visiting a file prepares Emacs to edit the file.
7.3 Saving Files  How to save Emacs files.


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7.1 File Names

Most of the Emacs commands that operate on a file require you to specify a file name. For example, you might specify the file name initially when you enter Emacs :

 
xemacs myfile RET

After you hit RET, you will enter XEmacs with "myfile" read into the current buffer. If you do not specify the filename when entering Emacs, you can use the Open... option from the File menu. You will be prompted for a filename in the echo area:

 
Find file: /usr/workspace/

Type in a file name which you want to open after the "/" and hit RET. The specified file will be read into the current buffer. The "/usr/workspace" might be the default directory. When Emacs prompts you for a file, it uses the default-directory unless you specify a directory. You can see what the default directory of the current buffer is by using the Describe Variable option from the Help menu. When Emacs prompts you for the variable name to describe, type default-directory. If you wish to open a file in some other directory, use DEL or the BackSpace key to go back and type the path name of the new directory.

You can create a new directory by typing M-x make-directory. This command will prompt you for a directory name:

 
Create directory: /usr/workspace/

After you type a directory name and press RET, a new directory with the specified name will be created. If you do not wish to create a new directory, then simply press C-g to quit the command. Similarly, you can also remove a directory by using the command remove-directory. The command M-x pwd will print the current buffer's default directory. For more information on file names, See section `File Names' in XEmacs User's Manual.


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7.2 Visiting Files

To edit a file in Emacs you need to visit it. Visiting a file means copying its contents (or reading them) into the current buffer. Emacs will create a new buffer for each file that you visit. The buffer will be named after the file that you open. If you open a file `/usr/workspace/myfile.texinfo', the buffer will be called "myfile.texinfo". If a buffer with this name already exists, a unique name will be constructed by appending `<2>', `<3>', etc. If this is the second buffer with the same name, a "<2>" will be appended, "<3>" for a third buffer and so on. The name of the buffer which is being displayed in the window will be shown both at the top and bottom of the frame. Once you are in XEmacs, you can use the following commands:

C-x C-f
This command will visit a file (find-file). It will prompt you for a file name to visit. The Open... option from the File menu does the same thing:

 
Find file: /usr/workspace/

Type in a filename and press RET. You will see a new buffer on the screen with its name in the mode-line. If the filename you specify already exists in Emacs, the buffer containing that file will be selected. You will get an error message if the filename does not exist. If you still press RET, a new buffer with the given filename will be displayed on the screen.

C-x C-v
This command (find-alternate-file), will visit a different file instead of the one visited last. It is similar to C-c C-f except that it kills the current buffer (after offering to save it).

C-x 5 C-f
This command will visit a file in another frame (find-file-other-frame) without changing the current window or frame. The Open in New Frame... from the File menu will do the same thing. It will prompt you for a file name in the echo area. After you type the file name and press RET, the specified file will be read into a new buffer and displayed on a new frame.


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7.3 Saving Files

The changes that you make after visiting a file will not be saved unless you save the buffer. When you save the buffer, Emacs writes the current contents of the buffer into the visited file. Some commands to save buffers are:

C-x C-s
This command will permanently save the current buffer in its visited file (save-buffer). You will see the following message in the echo area if you save a file called "myfile.texinfo" :

 
Wrote /usr/workspace/myfile.texinfo

Try using this command twice. You will get the above message the first time you use this command, the second time you will get the following message:

 
(No changes need to be saved)

This message indicates that you haven't made any changes since the last time you saved the file.

C-x s
This command will save all the buffers in their visited files (save-some-buffers). It will prompt you for typing yes or no:

 
Save file /usr/workspace/myfile.texinfo? (y or n)

You will get the above message for all the buffers. Type "y" if you want to save the buffer.

C-x C-w
This command will prompt you for a file name and save the current buffer in that file. (write-file). You will see the following message in the echo area:

 
Write file: /usr/workspace/

After you type in a file name, press RET. The buffer will be saved in a new file. You can make copies of a particular file using this command.

You can also undo all the changes made since the file was visited or saved by reading the text from the file again (called reverting). For more information on this option, See section `Reverting' in XEmacs User's Manual.

When you save a file in Emacs, it destroys its old contents. However, if you set the variable make-backup-files to non-nil i.e. `t', Emacs will create a backup file. Select the Describe variable option from the Help menu and look at the documentation for this variable. Its default value should be `t'. However, if its not then use M-x set-variable to set it to `t' (see section 8.1 Other Customizations). The backup file will contain the contents from the last time you visited the file. Emacs also provides options for creating numbered backups. For more information on backups, See section `Backup' in XEmacs User's Manual.

Emacs also saves all the files from time to time so that in case of a system crash you don't lose lot of your work. You will see the message `Auto-saving...' displayed in the echo area when the buffer is being saved automatically. The auto saved files are named by putting the character `#' in front and back. For example a file called "myfile.texinfo" would be named as `#myfile.texinfo#'. For information on controlling auto-saving and recovering data from auto-saving, See section `Auto Save Files' in XEmacs User's Manual.

Emacs provides protection from simultaneous editing which occurs if two users are visiting the same file and trying to save their changes. It will put a lock on a file which is being visited and modified. If any other user tries to modify that file, it will inform the user about the lock and provide some options. For more information on protection against simultaneous editing, See section `Interlocking' in XEmacs User's Manual.


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