Modes (Unix)
Modes are the file system permissions given to "user", "group" and "others" classes to access files under Unix. They are shown when listing files in long format, or, if access control lists are in use, using getfacl. Modes can be changed with chmod (for traditional Unix permissions) or with setfacl (for access control lists).
Format
For traditional Unix permissions, the symbolic mode is composed of three components, which are combined to form a single string of text:
$ chmod [references][operator][modes] file1 ...
The references (or classes) are used to distinguish the users to whom the permissions apply. If no references are specified it defaults to "all". Except that when no references are explicitly specified, the final outcome is masked by the umask value. The references are represented by one or more of the following letters:
| Reference | Class | Description |
|---|---|---|
| u | user | the owner of the file |
| g | group | users who are members of the file's group |
| o | others | users who are not the owner of the file or members of the group |
| a | all | all three of the above, is the same as ugo |
The chmod program uses an operator to specify how the modes of a file should be adjusted. The following operators are accepted:
| Operator | Description |
|---|---|
| + | adds the specified modes to the specified classes |
| - | removes the specified modes from the specified classes |
| = | the modes specified are to be made the exact modes for the specified classes |
The modes indicate which permissions are to be granted or taken away from the specified classes. There are three basic modes which correspond to the basic permissions:
| Mode | Name | Description |
|---|---|---|
| r | read | read a file or list a directory's contents |
| w | write | write to a file or directory |
| x | execute | execute a file or recurse a directory tree |
| X | special execute | which is not a permission in itself but rather can be used instead of x. It applies execute permissions to directories regardless of their current permissions and applies execute permissions to a file which already has at least 1 execute permission bit already set (either user, group or other). It is only really useful when used with '+' and usually in combination with the -R option for giving group or other access to a big directory tree without setting execute permission on normal files (such as text files), which would normally happen if you just used "chmod -R a+rx .", whereas with 'X' you can do "chmod -R a+rX ." instead |
| s | setuid/gid | details in Special modes section |
| t | sticky | details in Special modes section |
The combination of these three components produces a string that is understood by the chmod command. Multiple changes can be specified by separating multiple symbolic modes with commas.
String mode examples
For example, the following command would be used to add the read and write permissions to the user and group classes of a file or directory named sample:
$ chmod ug+rw sample
$ ls -ld sample
drw-rw---- 2 unixguy unixguy 96 Dec 8 12:53 sample
This command removes all permissions, allowing no one to read, write, or execute the file named sample.
$ chmod a-rwx sample
$ ls -l sample
---------- 2 unixguy unixguy 96 Dec 8 12:53 sample
The following command changes the permissions for the user and the group to read and execute only (no write permission) on sample.
$ # Sample file permissions before command
$ ls -ld sample
drw-rw---- 2 unixguy unixguy 96 Dec 8 12:53 sample
$ chmod ug=rx sample
$ ls -ld sample
dr-xr-x--- 2 unixguy unixguy 96 Dec 8 12:53 sample