| chmod | |
| Author: | AT&T Bell Laboratories |
| Developer: | Various open-source and commercial developers |
| Programming Language: | Plan 9: C |
| Operating System: | Unix, Unix-like, Plan 9, Inferno, IBM i |
| Platform: | Cross-platform |
| Genre: | Command |
| License: | coreutils GPLv3 |
In Unix and Unix-like operating systems, is the command and system call used to change the access permissions and the special mode flags (the setuid, setgid, and sticky flags) of file system objects (files and directories). Collectively these were originally called its modes,[1] and the name was chosen as an abbreviation of change mode.[2]
A command first appeared in AT&T Unix version 1, along with the system call.
As systems grew in number and types of users, access-control lists[3] were added to many file systems in addition to these most basic modes to increase flexibility.
The version of bundled in GNU coreutils was written by David MacKenzie and Jim Meyering.[4] The command is available as a separate package for Microsoft Windows as part of the UnxUtils collection of native Win32 ports of common GNU Unix-like utilities.[5] The command has also been ported to the IBM i operating system.[6]
Throughout this section, ser refers to the owner of the file, as a reminder that the symbolic form of the command uses "u".
chmod [options] mode[,mode] file1 [file2 ...][7]
Usually implemented options include:
If a symbolic link is specified, the target object is affected. File modes directly associated with symbolic links themselves are typically not used.
To view the file mode, the or commands may be used:
The,, and specify the read, write, and execute access (the first character of the display denotes the object type; a hyphen represents a plain file). The script can be read, written to, and executed by the user ; read and executed by members of the group; and only read by any other users.
The main parts of the permissions:
For example:
Each group of three characters define permissions for each class:
The numerical format accepts up to four 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,, and 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.
| Sum | rwx | Permission | ||
|---|---|---|---|---|
| read, write and execute | ||||
| read and write | ||||
| read and execute | ||||
| read only | ||||
| write and execute | ||||
| write only | ||||
| execute only | ||||
| none |
For example, would allow:
A numerical code permits execution if and only if it is odd (i.e.,,, or). A numerical code permits "read" if and only if it is greater than or equal to (i.e.,,, or). A numerical code permits "write" if and only if it is,,, or .
Change permissions to permit members of the group to update a file:
Since the, and bits are not specified, this is equivalent to:
The command also accepts a finer-grained symbolic notation,[8] 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:
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:
| Reference | Class | Description | |
|---|---|---|---|
| user | file owner | ||
| group | members of the file's group | ||
| others | users who are neither the file's owner nor members of the file's group | ||
| all | all three of the above, same as | ||
| (empty) | default | same as "all", except that bits in the umask will be unchanged |
The 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 removed from the specified classes. There are three basic modes which correspond to the basic permissions:
| Mode | Name | Description | |
|---|---|---|---|
| read | read a file or list a directory's contents | ||
| write | write to a file or directory | ||
| execute | execute a file or recurse a directory tree | ||
| special execute | which is not a permission in itself but rather can be used instead of . It applies execute permissions to directories regardless of their current permissions and applies execute permissions to a file which already has at least one execute permission bit already set (either User, Group or Others). It is only really useful when used with and usually in combination with the flag for giving Group or Others 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, whereas with you can do instead | ||
| setuid/gid | |||
| sticky |
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.[9]
$ ls -ld shared_dir # show access modes before chmoddrwxr-xr-x 2 jsmitt northregion 96 Apr 8 12:53 shared_dir$ chmod g+w shared_dir$ ls -ld shared_dir # show access modes after chmoddrwxrwxr-x 2 jsmitt northregion 96 Apr 8 12:53 shared_dir
$ ls -l ourBestReferenceFile-rw-rw-r-- 2 tmiller northregion 96 Apr 8 12:53 ourBestReferenceFile$ chmod a-w ourBestReferenceFile$ ls -l ourBestReferenceFile-r--r--r-- 2 tmiller northregion 96 Apr 8 12:53 ourBestReferenceFile
$ ls -ld referenceLibdrwxr----- 2 ebowman northregion 96 Apr 8 12:53 referenceLib$ chmod ug=rx referenceLib$ ls -ld referenceLibdr-xr-x--- 2 ebowman northregion 96 Apr 8 12:53 referenceLib
$ chmod ug+rw sample$ ls -ld sampledrw-rw---- 2 rsanchez budget 96 Dec 8 12:53 sample
$ chmod a-rwx sample$ ls -l sample---------- 2 rswven planning 96 Dec 8 12:53 sample
$ # Sample file permissions before command$ ls -ld sampledrw-rw---- 2 oschultz warehousing 96 Dec 8 12:53 NY_DBs$ chmod ug=rx sample$ ls -ld sampledr-xr-x--- 2 oschultz warehousing 96 Dec 8 12:53 NJ_DBs
See also: File-system permissions.
The command is also capable of changing the additional permissions or special modes of a file or directory. The symbolic modes use '' to represent the setuid and setgid modes, and '' 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 numerically, particularly in octal, but some do not. On these systems, only the symbolic modes can be used.
| Command | Explanation | |
|---|---|---|
chmod a+r publicComments.txt | Adds read permission for all classes (i.e. ser, Group and Others) | |
chmod a-x publicComments.txt | Removes execute permission for all classes | |
chmod a+rx viewer.sh | Adds read and execute permissions for all classes | |
chmod u=rw,g=r,o= internalPlan.txt'' | Sets read and write permission for ser, sets read for Group, and denies access for Others | |
chmod -R u+w,go-w docs | Adds write permission to the directory docs and all its contents (i.e. Recursively) for owner, and removes write permission for group and others | |
chmod ug=rw groupAgreements.txt | Sets read and write permissions for ser and Group | |
chmod 664 global.txt | Sets read and write permissions for ser and Group, and provides read to Others. | |
chmod 744 Show_myCV.sh | sets read, write, and execute permissions for ser, and sets read permission for Group and Others | |
chmod 1755 findReslts.sh | Sets sticky bit (this suggests that the script be retained in memory), sets read, write, and execute permissions for owner, and sets read and execute permissions for group and others | |
chmod 4755 setCtrls.sh | Sets UID, sets read, write, and execute permissions for ser, and sets read and execute permissions for Group and Others | |
chmod 2755 setCtrls.sh | sets GID, Sets read, write, and execute permissions for ser, and sets read and execute permissions for Group and Others | |
chmod -R u+rw,g-,o-rx privateStuff | Recursively (i.e. on all files and directories in privateStuff) adds read, write permissions for ser, removes read, write, and execution permissions for Group, and removes read and execution permissions for Others | |
chmod -R a-x+X publicDocs | Recursively (i.e. on all files and directories in publicDocs) removes execute permission for all classes and adds special execution permission for all classes |
[[chattr]], the command used to change the attributes of a file or directory on Linux systems[[chown]], the command used to change the owner of a file or directory on Unix-like systems[[chgrp]], the command used to change the group of a file or directory on Unix-like systems[[cacls]], a command used on Windows NT and its derivatives to modify the access control lists associated with a file or directory[[attrib]][[umask]], restricts mode (permissions) at file or directory creation on Unix-like systems