Chmod
In Unix and Unix-like operating systems, is the command and system call which is used to change the access permissions of file system objects. It is also used to change [|special mode] flags. The request is filtered by the umask. The name is an abbreviation of change mode.
History
A 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.
The version of bundled in GNU coreutils was written by David MacKenzie and Jim Meyering.
Command syntax
Throughout this section, ser refers to the owner of the file, as a reminder that the symbolic form of the command uses "u".chmod mode file1 Usually implemented options include:
- Recursive, i.e. include objects in subdirectories.
- verbose, show objects changed.
To view the file mode, the ls| or stat | commands may be used:
$ ls -l findPhoneNumbers.sh
-rwxr-xr-- 1 dgerman staff 823 Dec 16 15:03 findPhoneNumbers.sh
$ stat -c %a findPhoneNumbers.sh
754
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. This 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.
Octal modes
The main parts of the permissions:For example:
The characters to the right of the "d" define permissions for each class:
- the three leftmost characters,, define permissions for the ser class.
- the middle three characters,, define permissions for the Group class.
- 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 have no permission to access the file.
Numerical permissions
| # | Permission | rwx | Binary |
| 7 | read, write and execute | rwx | 111 |
| 6 | read and write | rw- | 110 |
| 5 | read and execute | r-x | 101 |
| 4 | read only | r-- | 100 |
| 3 | write and execute | -wx | 011 |
| 2 | write only | -w- | 010 |
| 1 | execute only | --x | 001 |
| 0 | none | --- | 000 |
For example, would allow:
- "read", "write", and "execute" for the ser class, as the binary value of 7 is 111.
- "read" and "execute" for the Group class, as the binary value of 5 is 101.
- Only "read" for the Others class, as the binary value of 4 is 100.
Numeric example
$ 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, and bits are not specified, this is equivalent to:
$ chmod 0664 sharedFile
Symbolic modes
The 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:$ chmod 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:
| 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 |
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. 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, which would normally happen if you just used, whereas with you can do instead | |
| setuid/gid | details in [|Special modes section] | |
| sticky | details in Special modes section |
Multiple changes can be specified by separating multiple symbolic modes with commas. 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.Symbolic examples
- Add write permission to the Group's 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 for all classes, 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 ser and the Group to read and execute only on, 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
Special modes
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 using octal modes, but some do not. On these systems, only the symbolic modes can be used.
Command line examples
| Command | Explanation |
chmod a+r publicComments.txt | adds read permission for all classes |
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 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 myCV.txt | sets read, write, and execute permissions for ser, and sets read permission for Group and Others |
chmod 1755 findReslts.sh | sets sticky bit, 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+rwX,g-rwx,o-rx personalStuff | Recursively adds read, write, and special execution 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 removes execute permission for all classes and adds special execution permission for all classes |
System call
The POSIX standard defines the following function prototype:int chmod;
The mode parameter is a bitfield composed of various flags:
| Flag | Octal value | Purpose |
| S_ISUID | 04000 | Set user ID on execution |
| S_ISGID | 02000 | Set group ID on execution |
| S_ISVTX | 01000 | Sticky bit |
| S_IRUSR, S_IREAD | 00400 | Read by ser |
| S_IWUSR, S_IWRITE | 00200 | Write by ser |
| S_IXUSR, S_IEXEC | 00100 | Execute/search by ser |
| S_IRGRP | 00040 | Read by group |
| S_IWGRP | 00020 | Write by group |
| S_IXGRP | 00010 | Execute/search by group |
| S_IROTH | 00004 | Read by others |
| S_IWOTH | 00002 | Write by others |
| S_IXOTH | 00001 | Execute/search by others |