Читать книгу Linux Bible - Christopher Negus - Страница 132

For a regular file, a dash appears in front of the nine-bit permissions indicator. Instead of a dash, you might see a d (for a directory), l (for a symbolic link), b (for a block device), c (for a character device), s (for a socket), or p (for a named pipe).

Of the nine-bit permissions, the first three bits apply to the owner's permission, the next three apply to the group assigned to the file, and the last three apply to all others. The r stands for read, the w stands for write, and the x stands for execute permissions. If a dash appears instead of the letter, it means that permission is turned off for that associated read, write, or execute bit.

Because files and directories are different types of elements, read, write, and execute permissions on files and directories mean different things. Table 4.2 explains what you can do with each of them.

TABLE 4.2 Setting Read, Write, and Execute Permissions

Permission	File	Directory
Read	View what's in the file.	See what files and subdirectories it contains.
Write	Change the file's content, rename it, or delete it.	Add files or subdirectories to the directory. Remove files or directories from the directory.
Execute	Run the file as a program.	Change to the directory as the current directory, search through the directory, or execute a program from the directory. Access file metadata (file size, time stamps, and so on) of files in that directory.

As noted earlier, you can see the permission for any file or directory by typing the ls -ld command. The named file or directory appears as those shown in this example:

 $ ls -ld ch3 test -rw-rw-r-- 1 joe sales 4983 Jan 18 22:13 ch3 drwxr-xr-x 2 joe sales 1024 Jan 24 13:47 test

The first line shows that the ch3 file has read and write permission for the owner and the group. All other users have read permission, which means that they can view the file but cannot change its contents or remove it. The second line shows the test directory (indicated by the letter d before the permission bits). The owner has read, write, and execute permissions while the group and other users have only read and execute permissions. As a result, the owner can add, change, or delete files in that directory, and everyone else can only read the contents, change to that directory, and list the contents of the directory. (If you had not used the -d options to ls, you would have listed files in the test directory instead of permissions of that directory.)