In Unix-like operating systems,
chmod is the command 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.
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.
$ ls -l findPhoneNumbers.sh -rwxr-xr-- 1 dgerman staff 823 Dec 16 15:03 findPhoneNumbers.sh $ stat -c %a findPhoneNumbers.sh 754
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 main parts of the chmod permissions:
To the right of the "d":
the left three characters rwx define permissions of the OWNER.
the middle three characters rwx define permissions of the GROUP.
the right three characters --- define permissions of EVERYONE ELSE. In this example EVERYONE ELSE has no permissions.
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.
|7||read, write and execute||rwx|
|6||read and write||rw-|
|5||read and execute||r-x|
|3||write and execute||-wx|
In order to permit all users who are 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, 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 ...
The references (or classes) are used to distinguish the users to whom the permissions apply. If no references are specified it defaults to "all" but modifies only the permissions allowed by the umask. The references are represented by one or more of the following letters:
|g||group||users who are members of the file's group|
|o||others||users who are neither the file's owner nor members of the file's group|
|a||all||all three of the above, 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:
|+||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:
|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 one execute permission bit already set (either owner, 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|
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.
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 owner 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
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.
|chmod a+r publicComments.txt||adds read permission for all classes (i.e. owner, group and others)|
|chmod +r publicComments.txt||adds read permission for all classes depending on umask|
|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 owner, 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 owner and group|
|chmod 664 global.txt||sets read and write permissions for owner and group, and provides read to others.|
|chmod 0744 myCV.txt||sets read, write, and execute permissions for owner, and sets read permission for group and others (the 0 specifies no special modes)|
|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 (this suggests that the script be retained in memory)|
|chmod 4755 setCtrls.sh||sets UID, sets read, write, and execute permissions for owner, and sets read and execute permissions for group and others|
|chmod 2755 setCtrls.sh||sets GID, sets read, write, and execute permissions for owner, and sets read and execute permissions for group and others|
|chmod -R u+rwX,g-rwx,o-rx personalStuff||Recursively (i.e. on all files and directories in personalStuff) adds read, write, and special execution permissions for owner, 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|
int chmod(const char *path, mode_t mode);
The mode parameter is a bitfield composed of various flags:
|S_ISUID||04000||Set user ID on execution|
|S_ISGID||02000||Set group ID on execution|
|S_IRUSR, S_IREAD||00400||Read by owner|
|S_IWUSR, S_IWRITE||00200||Write by owner|
|S_IXUSR, S_IEXEC||00100||Execute/search by owner|
|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|
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
umask, restricts mode (permissions) at file or directory creation on Unix-like systems
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