chmod

The main parts of the chmod permissions:

For example: drwxrwx---

The characters to the right of the "d" define permissions for each class:

• the three leftmost characters, rwx, define permissions for the User class (i.e. the file owner).
• the middle three characters, rwx, define permissions for the Group class (i.e. the group owning the file)
• the last three characters, ---, define permissions for the Others class. In this example, users who are not the owner of the file and who are not members of the Group (and, thus, are in the Others class) have no permission to access the file.

The chmod numerical format accepts up to four octal digits. The three rightmost digits define permissions for the file user, the group, and others. The optional leading digit, when 4 digits are given, specifies the special setuid, setgid, and sticky flags. Each digit of the three rightmost digits represents a binary value, which controls the "read", "write" and "execute" permissions respectively. A value of 1 means a class is allowed that action, while a 0 means it is disallowed.

For example, 754 would allow:

• "read" (4), "write" (2), and "execute" (1) for the User class, as the binary value of 7 (4+2+1) is 111.
• "read" (4) and "execute" (1) for the Group class, as the binary value of 5 (4+1) is 101.
• Only "read" (4) for the Others class, as the binary value of 4 (4) is 100.

Change permissions to permit members of the programmers group to update a file:

$ ls -l sharedFile
-rw-r--r--  1 jsmith programmers 57 Jul  3 10:13  sharedFile
$ chmod 664 sharedFile
$ ls -l sharedFile
-rw-rw-r--  1 jsmith programmers 57 Jul  3 10:13  sharedFile

Since the setuid, setgid and sticky bits are not specified, this is equivalent to:

$ chmod 0664 sharedFile

The chmod command also accepts a finer-grained symbolic notation,[7] which allows modifying specific modes while leaving other modes untouched. The symbolic mode is composed of three components, which are combined to form a single string of text:

$ chmod [references][operator][modes] file ...

Classes of users are used to distinguish to whom the permissions apply. If no classes are specified "all" is implied. The classes are represented by one or more of the following letters:

The chmod program uses an operator to specify how the modes of a file should be adjusted. The following operators are accepted:

The modes indicate which permissions are to be granted or removed from the specified classes. There are three basic modes which correspond to the basic permissions:

Multiple changes can be specified by separating multiple symbolic modes with commas (without spaces). If a user is not specified, chmod will check the umask and the effect will be as if "a" was specified except bits that are set in the umask are not affected.[8]

• Add write permission (w) to the Group's (g) access modes of a directory, allowing users in the same group to add files:

$ ls -ld shared_dir # show access modes before chmod
drwxr-xr-x   2 teamleader  usguys 96 Apr 8 12:53 shared_dir
$ chmod  g+w shared_dir
$ ls -ld shared_dir  # show access modes after chmod
drwxrwxr-x   2 teamleader  usguys 96 Apr 8 12:53 shared_dir

• Remove write permissions (w) for all classes (a), preventing anyone from writing to the file:

$ ls -l ourBestReferenceFile
-rw-rw-r--   2 teamleader  usguys 96 Apr 8 12:53 ourBestReferenceFile
$ chmod a-w ourBestReferenceFile
$ ls -l ourBestReferenceFile
-r--r--r--   2 teamleader  usguys 96 Apr 8 12:53 ourBestReferenceFile

• Set the permissions for the user and the Group (ug) to read and execute (rx) only (no write permission) on referenceLib, preventing anyone to add files.

$ ls -ld referenceLib
drwxr-----   2 teamleader  usguys 96 Apr 8 12:53 referenceLib
$ chmod ug=rx referenceLib
$ ls -ld referenceLib
dr-xr-x---   2 teamleader  usguys 96 Apr 8 12:53 referenceLib

• 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

• Remove 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

• Change 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

The chmod command is also capable of changing the additional permissions or special modes of a file or directory. The symbolic modes use 's' to represent the setuid and setgid modes, and 't' to represent the sticky mode. The modes are only applied to the appropriate classes, regardless of whether or not other classes are specified.

Most operating systems support the specification of special modes using octal modes, but some do not. On these systems, only the symbolic modes can be used.