Chmod

In Unix-like operating systems, chmod is the system and system call which may change the access permissions to file system objects (files and directories). It may also alter special mode flags. The request is filtered by the umask. The name is an abbreviation of change mode.

A chmod command first appeared in AT&T Unix version 1.

As systems grew in number and types of users, access control lists were added to many file systems in addition to these most basic modes to increase flexibility.

chmod [options] mode[,mode] file1 [file2 ...]

Usual implemented options include:

If a symbolic link is specified, the target object is affected. File modes directly associated with symbolic links themselves are typically never used.

To view the file mode, the ls or stat commands may be used:

The r, w, and x specify the read, write, and execute access, respectively. The first character of the ls display denotes the object type; a hyphen represents a plain file. This script can be read, written to, and executed by the owner, read and executed by other members of the staff group and can also be read by others.

The chmod numerical format accepts up to four octal digits. The three rightmost digits refer to permissions for the file owner, the group, and other users. The optional leading digit (when 4 digits are given) specifies the special setuid, setgid, and sticky flags.

Numerical permissions

In order to permit all users who are members of the programmers group to update a file

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

The chmod command also accepts a finer-grained symbolic notation, 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:

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