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What are file permissions (e.g., 640, 755, 775)?

Every file and directory in UNIX/Linux has read, write, and execute permissions. Read permission means that the file can be read but not modified or deleted. Write permission means that the file can be created, modified, or deleted. Execute permission means that the file can be executed. The execute permission is similar to a .exe file in Windows or DOS. For directories, execute permission means you can open the directory.

Permissions
There are 3 sets of permissions for every file or directory -- owner, group, and world. For each set, there are separate read, write, and execute permissions. The owner permissions are for the owner of the file or directory. The group permissions are for everyone in the group. The global permissions are for anyone. To see the current permissions, owner, and group for a file or directory, type the following commands:

"cd"
"ls -l"
This will display the contents of your home directory in long format. The following is an example of what you might have seen with the above 2 commands:

drwxr--r-- 5 you public 4096 Feb 7 14:33 Desktop
drwxr--r-- 2 you public 4096 Apr 4 10:55 linuxpractice
-rwxr-xr-x 1 you public 223 Apr 6 11:37 myfirstscript
drwxr-xr-x 4 you public 4096 Apr 9 12:03 public_html

The third column (you) tells the owner of the file or directory, and the fourth column (public) is the name of the group for the file or directory.The permissions are listed in the first column. The first letter is whether the item is a directory or a file. If the first letter is a "d", then the item is a directory as in the first item listed above, Desktop. Notice, for the file myfirstscript, the first letter is "-". The next three letters are the permissions for the owner of the file, the next three letters apply to everyone in the group, and the last three letters are for everyone else. The read, write, and execute permissions are referred to as r,w, and x respectively.

Thus, for the directory Desktop above, the owner you has read, write, and execute permissions to the directory Desktop, everyone in the group public has read permissions, and everyone else has read permissions. The only one who can modify or delete any file in this directory is the owner you (or the superuser- "root").

"chmod" is a standard UNIX/Linux command that allows you to change the permissions of a file or directory. There are two arguments for chmod: the permissions and the file/directory name. The permission argument for chmod is based on numbers.

1 stands for execute.
2 stands for write.
4 stands for read.

To set more than one permission on a file or directory, you just add up the permissions. For example, 7 means read, write, and execute permissions. chmod takes the permissions as the first argument in the order user, group, global. Thus, the command chmod 777 hello will change the permissions of the file hello to read, write and execute by user, group, and everyone else.
Note: To change the permissions of a file/directory, you must be the owner of that file/directory. However, root can change permissions on any file or directory.

One last topic that we should touch on here is the ownership commands. If you are ROOT, you can change the ownership of files using the "chown username filename" command. As with most commands, there are many options. You can learn about those options by typing "man chown"

Example:

"chmod 777 Desktop" will allow EVERYBODY full access to read, write and execute.

"chmod 644 Desktop" will all the owner (you) read and write access, the group read access, and everybody else (world) read access too.

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