In Unix and Unix-like operating systems, chmod is the command and system call used to change the access permissions of file system objects (files and directories) sometimes known as modes. It is also used to change special mode flags such as setuid and setgid flags and a 'sticky' bit. The request is filtered by the umask. The name is an abbreviation of change mode.[1] They are shown when listing files in long format.

chmod
Original author(s)AT&T Bell Laboratories
Developer(s)Various open-source and commercial developers
Initial release3 November 1971; 49 years ago (1971-11-03)
Operating systemUnix, Unix-like, Plan 9, Inferno, IBM i
PlatformCross-platform
TypeCommand
Licensecoreutils: GPLv3

In the stat structure, file type and permissions (the mode) are stored together in a st_mode bit field, which has a size of at least 12 bits (3 bits to specify the type among the seven possible types of files; 9 bits for permissions). The layout for permissions is defined by POSIX to be at the least-significant 9 bits, but the rest is undefined.[2]

HistoryEdit

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

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 chmod bundled in GNU coreutils was written by David MacKenzie and Jim Meyering.[4]

The chmod command has also been ported to the IBM i operating system.[5]

PreliminariesEdit

Unix-like systems implement three specific permissions that apply to each class:

  • The read permission is denoted by r and has numerical value 4.
    • When set for a directory, the read permission may grant the right to read the names of files in the directory (depending on the implementation) . No access to information about the files is permitted.
    • When set for a file, the read permission grants the right to read the file and it's attributes.
  • The execute permission is denoted by x and has value 1.
    • When set for a directory, the execute permission grants the ability to search : it grants the ability to access file contents and meta-information if its name is known, but not list contents of files inside the directory, unless read is set also.
    • When set for a file, the execute permission grants the ability to execute the file. This permission must be set for executable programs and scripts invoked from a command prompt.
  • The write permission is denoted by w and numerical value 2.
    • When set for a directory, the write permission grants the ability to modify entries in the directory, which includes creating, deleting, and renaming files. The execute must also set.
    • When set for a file, the write permission grants the right to modify the file. Programs which "modify" the file by actually creating a new file and upon saving, delete the old version and rename the new one will fail without write permission on the directory.


When a permission is not set, the corresponding access is denied.

Permissions on Unix-like systems are not inherited. Files created within a directory do not necessarily have the same permissions as that directory.

Permissions on Unix-like systems are managed in three distinct scopes or classes. These scopes are known as user (denoted by u), group (denoted by g), and others (denoted by o). Files and directories are owned by a user which defines the file's user class. Files and directories have a group attribute and any user who is a member of that group is granted rights as defined by the group flags. Users that are not the owner nor a member of the file or directory's group are granted rights based on the othere flags. The owner of the file will have the permissions given to the user class regardless of the rights granted to the group class or others class which may be more restrictive than the rights granted to the members of the group or others.

Command syntaxEdit

Throughout this section, user 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 ...][6]

Usually implemented options include:

  • -R Recursive, i.e. include objects in subdirectories.
  • -v verbose, show objects changed (unchanged objects are not shown).

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 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 r, w, and x specify the read, write, and execute access. 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 user dgerman; read and executed by members of the staff group; and only read by any other users.

Octal modesEdit

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.

Numerical permissionsEdit

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.

# Sum rwx Permission
7 4(r) + 2(w) + 1(x) rwx read, write and execute
6 4(r) + 2(w) rw- read and write
5 4(r)        + 1(x) r-x read and execute
4 4(r) r-- read only
3        2(w) + 1(x) -wx write and execute
2        2(w) -w- write only
1               1(x) --x execute only
0 0 --- none

For example, 754 would allow:

  • "read" (4), "write" (2), and "execute" (1) for the User class i.e. 7 (4+2+1).
  • "read" (4) and "execute" (1) for the Group class i.e. 5 (4+1).
  • Only "read" (4) for the Others class.

A numerical code permits execution if and only if it is odd (i.e. 1, 3, 5, or 7). Whereas a numerical code permits "read" if and only if it is greater than or equal to 4 , this means 4, 5, 6, or 7).

In order to determine if "write" is permitted, subtract 1 from the numerical value if it is odd (and subtract nothing if it is even); then "write" is permitted if and only if this resulting number is 2 or 6. For example:

  • If the value was 6 (respectively, 2) then (subtract nothing and) conclude that writing is permitted.
  • If the numerical value is 7 (respectively, 3) then subtract 1 to get 6 (respectively, 2) and conclude that writing is permitted.
  • If the value was 4 (respectively, 0) then (subtract nothing and) conclude that writing is not permitted.
  • If the value was 5 (respectively, 1) then subtract 1 to get 4 (respectively, 0) and conclude that once again, writing is not permitted.

Numeric exampleEdit

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

Symbolic modesEdit

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:

Reference Class Description
u user file owner
g group 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:

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
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 User, Group or Others). It is only really useful when used with + and usually in combination with the -R 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 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.[8]

Symbolic examplesEdit

  • 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 jsmitt  northregion 96 Apr 8 12:53 shared_dir
$ chmod  g+w shared_dir
$ ls -ld shared_dir  # show access modes after chmod
drwxrwxr-x   2 nfinny  eastregion 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 tmiller  northregion 96 Apr 8 12:53 ourBestReferenceFile
$ chmod a-w ourBestReferenceFile
$ ls -l ourBestReferenceFile
-r--r--r--   2 ebowman  northregion 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 ebowman  northregion 96 Apr 8 12:53 referenceLib
$ chmod ug=rx referenceLib
$ ls -ld referenceLib
dr-xr-x---   2 dhinkle  northregion 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 rsanchez  budget       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 oschultz  warehousing       96 Dec  8 12:53 NY_DBs
$ chmod ug=rx sample
$ ls -ld sample
dr-xr-x---   2 aolensky  warehousing       96 Dec  8 12:53 NJ_DBs

Special modesEdit

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.

Command line examplesEdit

Command Explanation
chmod a+r publicComments.txt adds read permission for all classes (i.e. user, 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 user, 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 user and Group
chmod 664 global.txt sets read and write permissions for user and Group, and provides read to Others.
chmod 744 myCV.txt sets read, write, and execute permissions for user, 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 (this suggests that the script be retained in memory)
chmod 4755 setCtrls.sh sets UID, sets read, write, and execute permissions for user, and sets read and execute permissions for Group and Others
chmod 2755 setCtrls.sh sets GID, sets read, write, and execute permissions for user, 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 user, 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

System callEdit

The POSIX standard defines the following function prototype:[9]

int chmod(const char *path, mode_t mode);

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 user
S_IWUSR, S_IWRITE 00200 Write by user
S_IXUSR, S_IEXEC 00100 Execute/search by user
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

See alsoEdit

ReferencesEdit

  1. ^ "Tutorial for chmod". catcode.com.
  2. ^ "<sys/stat.h>". The Open Group Base Specifications Issue 6. The Open Group. 21 July 2019.
  3. ^ "AIX 5.3 System management". IBM knowledge Center. IBM. Retrieved 30 August 2015.
  4. ^ "chmod(1): change file mode bits - Linux man page". linux.die.net.
  5. ^ IBM. "IBM System i Version 7.2 Programming Qshell" (PDF). Retrieved 5 September 2020.
  6. ^ "chmod Man Page with examples and calculator - Linux - SS64.com". ss64.com.
  7. ^ "AIX 5.5 Commands Reference". IBM Knowledge Center. IBM. Retrieved 30 August 2015.
  8. ^ "Permissions masking with umask, chmod, 777 octal permissions". teaching.idallen.com.
  9. ^ "chmod function". The Open Group Base Specifications Issue 7, 2013 Edition. The Open Group. Retrieved 30 August 2015.

External linksEdit