Linux File Ownership and Permissions

Oracle Database Tips by Donald Burleson

A characteristic of most operating systems is file ownership and permissions. File permissions help to keep the operating system safe from accidental or intentional mishaps. To view a file's ownership and permissions, use the ls ‑l command as shown in Figure 3.5.

The first character (pointer a) in the listing indicates whether the file is a regular file (with a dash) or a directory (with a letter d). The next three groups of characters (pointers b,c,d) indicate the read (r), write (w), and execute (x) permissions of the file's owner, group and all other users respectively. The other items in the listing (pointers e through i) are self-explanatory.

As seen in Figure 3.5, the file jan05.txt is set so that the owner has read and write permissions, while all other users have read only permission. The file my_schedule.txt has been set so that for example Mary can read or write the file, but no one else can read or write to it. The file rc can be read or executed by anyone, but only its owner can write over it.

The file a user creates has default permissions according to the way that it was created. In most cases, the permissions will be set so that the owner can read and write but all others will have read permission only. File permissions and ownership can be changed after a file is created.

Changing a File's Ownership

Only the root user can use the command chownto change a file's owner and group. The syntax is easy: chown user:group /path/filename. The root user can also add the switch ?R to make the chown command recursive for all files in a given directory, including files in subdirectories.

A user other than root cannot change a file's ownership, but the user can change the group of a file he owns to any group of which he is a member by using the chgrpcommand, as in: chgrp group /path/filename.

Changing a File's Permissions or ?Mode?

Use the chmod command to change a file's permissions (also known as its mode). There are two syntaxes available for use with chmod: symbolic and absolute. The following table lists a few examples of each. Note that when the symbolic syntax is used, no spaces are permitted after the commas when listing the permissions assigned to user, group and others.

SYMBOLIC SYNTAX	ABSOLUTE SYNTAX	DESCRIPTION
chmod ugo=rwx filename	chmod 777 filename	Allows user, group, others to read, write and execute.
chmod u=rw,g=r,o=r filename	chmod 644 filename	Allows user to read and write, group and others to read.
chmod u=rwx,g-rwx,o-rwx file	chmod 700 file	Allows user to read, write and execute, group and others are denied any access to the file.
chmod u+x filename		Adds execute permission to file owner without effecting other permissions.

Table 3.5: Symbolic Syntax

The symbolic syntax can be used to add or remove permissions without effecting other permissions as they are.

The absolute syntax redefines all permissions at once according to bits. The following table gives an example of how the 744 bits are computed to determine read, write, execute for user, read for group and read for others:

MODE	User	Group	Others
Read	400 *	40 *	4 *
Write	200 *	20	2
Execute	100 *	10	1
Sum	700 + 40 + 4 = 744

Table 3.4: How 744 bits are computed

The chmod command can be combined with the switch ?R and a wildcard to make it recursive (for example chmod u=rw,g=r,o=r ?R *), so that it changes the mode of all files in that directory and all subdirectories as well.

To get more information about chmod, type the command man chmod which will bring up the manual for chmod, and type q to exit the manual.

Changing the Default Permissions for Newly Created Files

Just as chmod is used to change the mode of files that exist, the umaskcommand is used to set up a mask which forces newly created files during the current session to take on a specific set of permissions. To view the current mask, run the umask 's command. The symbolic and absolute syntaxes are available for use with the umask command.

The symbolic syntax is easy to use. For example, the command umask u=rwx,g=,o= forces files created from this point forward to allow the file owner (user) to read, write and execute; whereas members of the group, or others cannot read, write or execute. The absolute syntax requires a bit of math to make it work. For example the command umask u=rwx,g=,o= can also be accomplished with the command umask 077. The numbers 077 are derived by subtracting the value that would be obtained with the chmod command from the maximum value possible, 777. So, to change the mask to allow read and write to user, and read to group and others, you would run the command umask 133 and then check it with umask -S.

To set a default mask for future logins, the umaskcommand can be added to the user's ~/.bash_profile.

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