Navigating the File System with Linux Commands

Use the cd command without any arguments to change the current directory to your home directory. Actually, the lone cd command changes the current directory to the directory listed in the HOME environment variable; that environment variable contains your home directory by default.

Notice that the tilde character (~) also refers to the directory the HOME environment variable specifies. Thus, the command cd ~ changes the current directory to whatever directory the HOME environment variable specifies.

You can use a shortcut to refer to any user's home directory. Prefix a user's login name with a tilde (~) to refer to that user's home directory. Therefore, ~naba refers to the home directory of the user naba, and ~root refers to the home directory of the root user. If your system has a user with the login name emily, you can type cd ~emily to change to Emily's home directory.

The directory names . and .. have special meanings. A single period (.) indicates the current directory, whereas two periods (..) indicate the parent directory. If the current directory is /usr/share/doc, for example, you can change the current directory to /usr by typing the cd .. command (if you are a DOS/Windows user, note that there is a space between cd and the two periods). Essentially, this command takes you up one level in the file-system hierarchy.

By default, the PS1 environment variable is defined in Red Hat Linux as follows:

echo $PS1
[\u@\h \W]\$

With this setting for PS1, the prompt looks like this:

[username@hostname dirname]$

In this example, username is the login name of the user; hostname is the system's name (Excluding the domain name), and dirname is the last part of the current working directory. Thus, if the current directory is /usr/src/linux, the dirname is linux. Thus, when a usernamed joe on a system with hostname mycompany.com changes the directory to /usr/share/doc, the shell prompt appears as follows:

[joe@mycompany doc]$

For the root user, the $ sign changes to a number sign (#) and the prompt looks like this:

[root@mycompany doc]#

Table 7-5 shows the character sequences you can use in the PS1 environment variable to customize your prompt.

The detailed directory listing displayed by the ls -l command shows considerable information about each directory entry, which can be a file or another directory. A typical line in the detailed directory listing has the following appearance:

drwxr-xr-x   6 root     root     4096 Oct 26 21:18 gdm

Looking at a line from the right column to the left, you see that the rightmost column shows the name of the directory entry. The date and time before the name show when the last modifications to that file were made. Before the date and time is the size of the file, in bytes.

The file's group and owner appear to the left of the column that shows the file size. The next number to the left indicates the number of links to the file. (A link is like a shortcut in Windows.) Finally, the leftmost column shows the file's permission settings, which determine who can read, write, or execute the file.

The first letter of the leftmost column has a special meaning, as the following list shows:

• - means an ordinary file

• b means a block special file

• c means a character special file

• d means a directory

• l means a symbolic link to another file

• p means a first-in first-out (FIFO) special file (also known as a pipe file)

• s means a local socket

  After that first letter, the leftmost column shows a sequence of nine characters, which appears as rwxrwxrwx when each letter is present. Each letter indicates a specific permission; a hyphen (-) in place of a letter indicates no permission for a specific operation on the file. Think of these nine letters as three groups of three letters (rwx), interpreted as follows:

• The leftmost group of rwx controls the read, write, and execute permission of the file's owner. In other words, if you see rwx in this position, the file's owner can read (r), write (w), and execute (x) the file. A hyphen in the place of a letter indicates no permission. Thus, the string rw- means that the owner has read and write permission but not execute permission. Typically, executable programs (including shell programs) have execute permission. However, for directories, execute permission is equivalent to use permission-a user must have execute permission on a directory to open and read the contents of the directory.

• The middle three rwx letters control the read, write, and execute permission of any user belonging to that file's group.

• The rightmost group of rwx letters controls the read, write, and execute permission of all other users (collectively referred to as the world).

  Thus, a file with the permission setting rwx--- is accessible only to the file's owner, whereas the permission setting rwxr-r- makes the file readable by the group and the world.