Classes of Accounts
The files and directories in the home directory of your Mathematical Sciences account can be accessed on computers running the Linux operating system. Linux is a type of UNIX and uses UNIX file and directory permissions. For purposes of permissions, UNIX divides accounts into three classes:
| user | Your account |
| group | Any permissions group that your account belongs to |
| other | Any account that is not yours and that does not belong to a permissions group that your account belongs to |
Types of Permissions
There are three basic types of permissions which can be assigned to each of these three classes of accounts:
| read |
| write |
| execute |
File Permissions
These three types of permissions mean slightly different things for files than for directories. For files, these permissions grant these rights:
| read | Allowed to read the contents of the file |
| write | Allowed to modify or delete the file |
| execute | Allowed to run the file as a process, if possible |
Directory Permissions
For directories, the permissions grant these rights:
| read | Allowed to list the contents of the directory |
| write | Allowed to create, modify or delete files in the directory |
| execute | Allowed to access a file in the directory if you know the name of the file |
Viewing File Permissions
The ls command is used to list files and the contents of directories. The -l parameter displays permissions. For example, to see the permissions of a file named foo in the directory /usr/bin/bar, you would execute:
ls -l /usr/bin/bar/foo
And the command would return something like this:
-rwxr-xr-- 1 jsmith guest 3072 Feb 11 09:25 /usr/bin/foo
In the example, jsmith is the account that owns foo, and guest is the name of the group that owns /usr/bin/foo. The -rwxr-xr-- at the left indicates the permissions. The first character, the -, indicates that /usr/bin/foo is a file, not a directory. The rwx shows the permissions for the user class of accounts--in this case, jsmith. The r indicates read permission; the w, write permission; and the x, execute permission. The next three characters, r-x, show permissions for the group class of accounts, which is guest in this example. Finally, the last three characters, r--, display permissions for the other class--any account that is not jsmith and is not in the guest group.
Viewing Directory Contents
If you want to see the contents of a directory, you also use ls. Suppose that /usr/bin/bar is a directory. Then the command
ls -l /usr/bin/bar
may return something like this:
drwxr-x--x 5 jsmith guest 4096 Jan 23 2008 foodir -rw-r----- 1 jsmith guest 48128 Sep 14 2004 WhatToDo.doc -rw-rw-r-- 1 jsmith guest 464 Jul 6 2005 WinCA.txt
This shows us the contents of the directory bar. The d at the left of the entry for foodir indicates that foodir is a directory.
Viewing Directory Permissions
If you want to see the permissions of the /usr/bin/bar directory itself, not its contents, then you need to use the -d command-line argument for ls. So, you'd execute this command:
ls -ld /usr/bin/bar
and you'd see something like this:
dr-xrwxr-x 3 jsmith guest 4096 Jan 23 2008 /usr/bin/bar
The Current Working Directory
The current working directory is the directory that, by default, a UNIX command will use when it is executed. For example, if you do not specify a file or directory when you run the ls command, then ls will assume that you want to see the contents of your current working directory. So,
ls
will return a list of the files and directories in your current working directory. To see the absolute path of your current working directory, use the pwd command.
UNIX provides a shorthand for your current working directory. A single period, ., indicates the current working directory. Two periods, .., indicate the directory immediately above your current working directory.
Hidden Files and Directories
In UNIX, if a file or directory name begins with a period, ., then by default, ls will not display the file or directory in a directory listing. To see all the files in a directory, including hidden files, use the a command-line argument. The command
ls -a
will show all files and directories in a directory, including hidden files. The command
ls -al
will display all files and directories, and also show their permissions.
Home Directories
Each Linux account is associated with a home directory. When you login to your Linux account, by default, your current working directory will be your home directory. UNIX provides a short-hand symbol for your home directory, the tilde character, ~. So, to see a list of files in your home directory, you can execute
ls ~
For Further Information
To learn more about Linux file and directory permissions, search on the Web or use the Linux man command to research the chmod and umask commands.
