Abstract
Compared to Windows® and most other operating systems, files are handled very differently under GNU/Linux. In this section we will cover the most obvious differences. For more information, please read Chapter 9, The Linux File System.
The major differences result directly from the fact that Linux is a multiuser system: every file is the exclusive property of one user and one group. One thing we didn't mention about users and groups is that every one of them possesses a personal directory (called the home directory). The user is the owner of this directory and of all files created in it.
However, this would not be very useful if that were the only notion of file ownership. As the file owner, a user may set permissions on files. These permissions distinguish between three user categories: the owner of the file, every user who is a member of the group associated with the file (also called the owner group) but who is not the owner, and others, which includes every other user who is neither the owner nor a member of the owner's group.
There are three different permissions:
Read permission (r): enables a user to read the contents of a file. For a directory, the user can list its contents (i.e. the files in this directory).
Write permission (w): allows the modification of a file's content. For a directory, the write permission allows a user to add or remove files from this directory, even if he is not the owner of these files.
eXecute permission (x): enables a file to be executed (normally only executable files have this permission set). For a directory, it allows a user to traverse it, which means going into or through that directory. Note that this is different from the read access: you may be able to traverse a directory but still be unable to read its content!
Every permission combination is possible. For example, you can allow only yourself to read the file and forbid access to all other users. As the file owner, you can also change the owner group (if and only if you're a member of the new group).
Let's take the example of a file and a directory. The display below represents entering the ls -l command from the command line:
$ ls -l total 1 -rw-r----- 1 queen users 0 Jul 8 14:11 a_file drwxr-xr-- 2 peter users 1024 Jul 8 14:11 a_directory/ $ |
The results of the ls -l command are (from left to right):
The first ten characters represent the file's type and the permissions associated with it. The first character is the file's type: if it's a regular file, you will see a dash (-). If it's a directory, the leftmost character will be a d. There are other file types, which we'll discuss later on. The next nine characters represent the permissions associated with that file. The nine characters are actually three groups of three permissions. The first group represents the rights associated with the file owner; the next three apply to all users belonging to the owner group; and the last three apply to others. A dash (-) means that the permission is not set.
Next comes the number of links for the file. Later on we'll see that the unique identifier of a file is not its name, but a number (the inode number), and that it's possible for one file on disk to have several names. For a directory, the number of links has a special meaning, which will also be discussed a bit further.
The next piece of information is the name of the file owner and the name of the owner group.
Finally, the size of the file (in bytes) and its last modification time are displayed, with the name of the file or directory itself as the last item on the line.
Let's take a closer look at the permissions associated with each of these files. First of all, we must strip off the first character representing the type, and for the file a_file, we get the following rights: rw-r-----. Here's a breakdown of the permissions.
the first three characters (rw-) are the owner's rights, which in this case is queen. Therefore, queen has the right to read the file (r), to modify its content (w) but not to execute it (-).
the next three characters (r--) apply to any user who is not queen but who is a member of the users group. They will be able to read the file (r), but will not be able to write nor execute it (--).
the last three characters (---) apply to any user who is not queen and is not a member of the users group. Those users don't have any rights on the file at all.
For the a_directory directory, the rights are rwxr-xr--, so:
peter, as the directory owner, can list files contained inside (r), add to or remove files from that directory (w), and may traverse it (x).
Each user who isn't peter, but is a member of the users group, will be able to list files in this directory (r), but not remove or add files (-), and will be able to traverse it (x).
Every other user will only be able to list the contents of this directory (r). Because they don't have wx permissions, they won't be able to write files or enter the directory.
There is one exception to these rules: root. root can change attributes (permissions, owner and group owner) of all files, even if he's not the owner, and could therefore grant ownership of the file to himself! root can read files on which he has no read permissions, traverse directories which he would normally have no access to, and so on. And if root lacks a permission, he only has to add it. root has complete control over the system, which involves a certain amount of trust in the person wielding the root password.
Lastly, it's worth noting the differences between file names in the UNIX® and the Windows® worlds. For one, UNIX® allows for a much greater flexibility and has fewer limitations.
A file name may contain any character, including non-printable ones, except for the ASCII character 0, which denotes the end of a string, and /, which is the directory separator. Moreover, because UNIX® is case sensitive, the files readme and Readme are different, because r and R are considered two different characters on UNIX®-based systems.
As you may have noticed, a file name does not have to include an extension, unless that's the way you prefer to name your files. File extensions don't identify the content of files under GNU/Linux, nor almost any other operating system. So-called “file extensions” are quite convenient though. The period (.) under UNIX® is just one character among others, but it also has one special meaning. Under UNIX®, file names beginning with a period are “hidden files”[1], which also includes directories whose names start with a .
[1] By default, hidden files won't be displayed in a file manager, unless you tell it to. In a terminal, you must type the ls -a command to see all hidden files. Essentially, they hold configuration information. From your home/ directory, take a look at .mozilla or .openoffice to see an example.