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## Additional Install Features

This section describes some more specifics of installers.

### Installer File Manipulation

The previous section described a basic installer. You may wish to perform some more elaborate processing during the installation. For example, you may want to install a package only when certain conditions are met, such as having a particular library installed.

In addition to the Install object, a File object is also available during an installation script. It provides some functions which can be used to examine and modify files on disk. You can use these to move, copy or delete files before or after the files are installed. For example, you might want to make a backup of some files first.

The following code will make a copy of the file "/bin/grep" and put it in the directory "/main".

    var binFolder=getFolder("file:///","bin");
    var grep=getFolder(binFolder,"grep");

    var mainFolder=getFolder("file:///","main");

    File.copy(grep,mainFolder);

The first line will retrieve a reference to the /bin directory. The text 'file:///' is a special string which means the root of the filesystem. From there, we get the file 'grep' which is contained inside the 'bin' directory. If this file does not exist, an error will occur during the installation. Next, we get the 'main' folder, again from the file system root. Finally, we call the File.copy function which copies the source file to the destination.

Functions also exist to move, rename and execute files. Thus, you can move files that might conflict with your package out of the way.

### Handling Errors

You will likely want to handle errors gracefully. This will occur if a file or directory cannot be found, there is insufficient disk space or for a number of other reasons.

You can use the getLastError function to determine whether an error occured. If it returns SUCCESS, no error occured. Otherwise, the number will be an error code which indicates the type of error that occured. You can call this function at any point during the installation script to determine whether an error occured during the last operation.

If an error occurs, you will likely want to abort the installation. You may also want to display an error message to the user. For example, you might put the following as the last section of your script:

    if (getLastError() == SUCCESS){
      performInstall();
    }
    else {
      cancelInstall();
    }

Error codes that could be returned by getLastError are listed in the Mozilla source file nsInstall.h. During installation, a log file is created that contains the operations that are performed. It will also show any errors that occured. The log file can be found in the file 'install.log' in the Mozilla installation directory. A block of text will be added to this file for each installation that occurs.

The logComment function can be used to write a string of text to the log file. It takes one argument, the text to write.

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