System Administration Guide

Changing File Permissions

The chmod command enables you to change the permissions on a file. You must be superuser or the owner of a file or directory to change its permissions.

You can use the chmod command to set permissions in either of two modes:

Absolute Mode - Use numbers to represent file permissions (the method most commonly used to set permissions). When you change permissions by using the absolute mode, represent permissions for each triplet by an octal mode number.
Symbolic Mode - Use combinations of letters and symbols to add or remove permissions.

Table 51-5 lists the octal values for setting file permissions in absolute mode.

Table 51-5 Setting File Permissions in Absolute Mode


Octal Value	File Permissions Set	Permissions Description
`0`	`---`	No permissions
`1`	`--x`	Execute permission only
`2`	`-w-`	Write permission only
`3`	`-wx`	Write and execute permissions
`4`	`r--`	Read permission only
`5`	`r-x`	Read and execute permissions
`6`	`rw-`	Read and write permissions
`7`	`rwx`	Read, write, and execute permissions

Table 51-6 lists the symbols for setting file permissions in symbolic mode. Symbols can specify whose permissions are to be set or changed, the operation to be performed, or the permissions being assigned or changed.

Table 51-6 Setting File Permissions in Symbolic Mode


Symbol	Function	Description
`u`	Who	User (owner)
`g`	Who	Group
`o`	Who	Others
`a`	Who	All
`=`	Operation	Assign
`+`	Operation	Add
`-`	Operation	Remove
`r`	Permission	Read
`w`	Permission	Write
`x`	Permission	Execute
`l`	Permission	Mandatory locking, `setgid` bit is on, group execution bit is off
`s`	Permission	`setuid` or `setgid` bit is on
`S`	Permission	`suid` bit is on, user execution bit is off
`t`	Permission	Sticky bit is on, execution bit for others is on
`T`	Permission	Sticky bit is on, execution bit for others is off

The who operator permission designations in the function column specifies the symbols that change the permissions on the file or directory.

`who`	Specifies whose permissions are changed.
`operator`	Specifies the operation to perform.
`permissions`	Specifies what permissions are changed.

How to Change Permissions in Absolute Mode

If you are not the owner of the file or directory, become superuser.

Only the current owner or superuser can use the chmod command to change file permissions on a file or directory.

Change permissions in absolute mode by using the chmod command.
$ chmod nnn filename
nnn

Specifies the octal values that change permissions on the file or directory. See Table 51-5 for the list of valid octal values.

filename

File or directory.

Verify the permissions of the file have changed.
$ ls -l filename

Example--Changing Permissions in Absolute Mode

The following example sets rwxr-xr-x permissions on myfile.

$ chmod 755 myfile
$ ls -l myfile
-rwxr-xr-x   1 kryten   staff   6144 May 24 10:49 myfile

How to Change Permissions in Symbolic Mode

If you are not the owner of the file or directory, become superuser.

Only the current owner or superuser can use the chmod command to change file permissions on a file or directory.

Change permissions in symbolic mode by using the chmod command.

$ chmod who operator permission filename

who operator permission

who specifies whose permissions are changed, operator specifies the operation to perform, and permission specifies what permissions are changed.

See Table 51-6 for the list of valid symbols.

filename

Is the file or directory.

Verify the permissions of the file have changed.
$ ls -l filename

Examples--Changing Permissions in Symbolic Mode

The following example takes away read permission from others.

$ chmod o-r filea

The following example adds read and execute permissions for user, group, and others.

$ chmod a+rx fileb

The following example assigns read, write, and execute permissions to group.

$ chmod g=rwx filec

`nnn`	Specifies the octal values that change permissions on the file or directory. See Table 51-5 for the list of valid octal values.
`filename`	File or directory.