CONCEPT: UNIX supports access control. Every file and directory has associated with it ownership, and access permissions. Furthermore, one is able to specify those to whom the permissions apply.

Permissions are defined as read, write, and execute. The read, write, and execute permissions are referred to as r, w, and x, respectively.

Those to whom the permissions apply are the user who owns the file, those who are in the same group as the owner, and all others. The user, group, and other permissions are referred to as u, g, and o, respectively.

A short note on groups: UNIX allows users to be placed in groups, so that the control of access is made simpler for administrators.

The meaning of file and directory permissions

Read permission For a file, having read permission allows you to view the contents of the file. For a directory, having read permission allows you to list the directory's contents. Write permission For a file, write permission allows you to modify the contents of the file. For a directory, write permission allows you to alter the contents of the directory, i.e., to add or delete files. Execute permission For a file, execute permission allows you to run the file, if it is an executable program, or script. Note that file execute permission is irrelevant for nonexecutable files. For a directory, execute permission allows you to cd to the directory, and make it your current working directory.

Viewing permissions

To see the permissions on a file, use the ls command, with the -l option.

EXAMPLE: Execute the command ls -l /etc/passwd to view the information on the system password database. The output should look similar to this:

-rw-r--r-- 1 root sys 41002 Apr 17 12:05 /etc/passwd

The first 10 characters describe the access permissions. The first dash indicates the type of file (d for directory, s for special file, - for a regular file). The next three characters ("rw-) describe the permissions of the owner of the file: read and write, but no execute. The next three characters ("r--) describe the permissions for those in the same group as the owner: read, no write, no execute. The next three characters describe the permissions for all others: read, no write, no execute.

Setting permissions

UNIX allows you to set the permissions on files that you own. The command to change the file permission mode is chmod. Chmod requires you to specify the new permissions you want, and specify the file or directory you want the changes applied to.

To set file permissions, you may use to the "rwxnotation to specify the type of permissions, and the "ugonotation to specify those the permissions apply to.

To define the kind of change you want to make to the permissions, use the plus sign (+) to add a permission, the minus sign (-) to remove a permission, and the equal sign (=) to set a permission directly.

EXAMPLE: Type the command chmod g=rw- ~/.shrc to change the file permissions on the file .shrc, in your home directory. Specifically, you are specifying group read access and write access, with no execute access.

EXERCISE: Change the permissions on the .shrc file in your home directory so that group and others have read permission only.

To change your location in the filesystem heirarchy, use the cd (change directory) command, followed by an argument defining where you want to go. The argument can be either an absolute path to the destination, or a relative path.

EXAMPLE: Type the command cd /tmp to go to the /tmp directory. You can type pwd to confirm that you're actually there.

If you type the cd command without an argument, the shell will place you in your home directory.

EXERCISE: Type the command pwd and note the result. Then type cd .. to the shell. Type pwd again to see where you ended up.

The ls command allows you to see the contents of a directory, and to view basic information (like size, ownership, and access permissions) about files and directories. The ls command has numerous options, so see the manual page on ls (type man ls ) for a complete listing. The ls command also accepts one or more arguments. The arguments can be directories, or files.

EXAMPLE: Type the command ls -lr /etc/i* to the UNIX shell.

In the example, the "land "roptions of the ls command are invoked together. Some commands permit you to group options in that way, and some commands require the options to be named separately, e.g., ls -l -r . The l option calls for a long output, and the r option causes ls to operate recursively, moving down directory trees.

The last part of the example, /etc/i*, directs the ls command to list files and directories in the /etc directory, that begin with the letter i. The wildcard character, *, matches any character(s).

EXERCISE: Type the command ls -m /etc/i*g to the shell. How did the shell respond, and why?

EXPLANATION: The shell responded by printing all the entries in the /etc directory that start with the letter i, and end with the letter g. The -m option causes the output to be streamed into a single line. See the manual page for ls to get a complete description of the ls command's options.

EXERCISE: Find the permissions on your home directory.

CONCEPT: Text files are intended for direct viewing, and other files are intended for computer interpretation.

The UNIX file command allows you to determine whether an unknown file is in text format, suitable for direct viewing.

EXERCISE: Type the command file /bin/sh to see what kind of file the shell is.

EXPLANATION: The shell is a shared executable, indicating that the file contains binary instructions to be executed by the computer.

The cat command
The cat command concatenates files and sends them to the screen. You can specify one or more files as arguments. Cat makes no attempt to format the text in any way, and long output may scroll off the screen before you can read it.

EXAMPLE: Send the contents of your .profile file to the screen by typing cat ~/.profile to the shell. The tilde character (~) is UNIX shorthand for your home directory.

The more command
The more command displays a text file, one screenful at a time. You can scroll forward a line at a time by pressing the return key, or a screenful at a time by pressing the spacebar. You can quit at any time by pressing the q key.

EXAMPLE: Type more /etc/rc to the shell. Scroll down by pressing return, and by pressing the spacebar. Stop the more command from displaying the rest of the file by typing q.

The head and tail commands
The head command allows you to see the top part of a file. You may specify the number of lines you want, or to ten lines.

EXAMPLE: Type head -15 /etc/rc to see the first fifteen lines of the /etc/rc file.

The tail command works like head, except that it shows the last lines of of file.

The UNIX command to copy a file or directory is cp. The basic cp command syntax is cp source destination .

EXAMPLE: The command cp ~/.profile ~/pcopy makes a copy of your .profile file, and stores it in a file called "pcopyin your home directory.

EXERCISE: Describe the permissions necessary to successfully execute the command in the previous example.

EXPLANATION: To copy the .profile file, one must have read permission on the file. To create the new file called pcopy, one must have write permission in the directory where the file will be created.

The UNIX mv command moves files and directories. You can move a file to a different location in the filesystem, or change the name by moving the file within the current location.

EXAMPLE: The command mv ~/pcopy ~/qcopy takes the pcopy file you created in the cp exercise, and renames it "qcopy.

The rm command is used for removing files and directories. The syntax of the rm command is rm filename . You may include many filenames on the command line.

The UNIX mkdir command is used to make directories. The basic syntax is mkdir directoryname . If you do not specify the place where you want the directory created (by giving a path as part of the directory name), the shell assumes that you want the new directory placed within the current working directory.

EXAMPLE: Create a directory called foo within your home directory by typing mkdir ~/foo

EXERCISE: Create a directory called bar, within the directory called foo, within your home directory.

The UNIX rmdir command removes a directory from the filesystem tree. The rmdir command does not work unless the directory to be removed is completely empty.

The rm command, used with the -r option can also be used to remove directories. The rm -r command will first remove the contents of the directory, and then remove the directory itself.

EXERCISE: Describe how to remove the "foodirectory you created, using both rmdir, and rm with the -r option.

EXPLANATION: You could enter the commands rmdir ~/foo/bar; rmdir ~/foo to accomplish the task with the rmdir command. Note that you have to rmdir the bar subdirectory bfore you can rmdir the foo directory. Alternately, you could remove the foo directory with the command rm -r ~/foo