umask

In computing, umask is a command that determines the settings of a mask that controls how file permissions are set for newly created files. It also may refer to a function that sets the mask, or it may refer to the mask itself, which is formally known as the file mode creation mask. The mask is a grouping of bits, each of which helps control how its corresponding permission is set for newly created files. The bits in the mask may be changed by invoking the umask command.

In UNIX, each file has a set of attributes which control who can read or change the file. Those attributes, called file permissions can be changed by users and programs. When a program creates a file, it requests that the file permissions be set to an initial setting. The mask helps determine what that initial file permission setting is. The bits of the mask are applied whenever a file is created. If the mask has a bit set to "1", that means the corresponding initial file permission will always be disabled whenever programs create new files. A bit set to "0" in the mask means that the corresponding permission will be determined by the program and the system when new files are created. In other words, the mask acts as a last-stage filter that strips away permissions as a file is created; each bit that is set to a "1" strips away its corresponding permission for the file as the file is created.

Each program (technically called a process) has its own mask, which is applied whenever that process creates a new file. Each process is able to change the settings of its own mask using a function call. When the process is a shell, the mask is set with the umask command. When a shell, or any other process, launches a new process, the child process inherits the mask from its parent process. The mask does not work retroactively, that is, changes made to the mask only affect new files created after the changes are made. Generally, the mask only affects file permissions during the creation of new files and has no effect when file permissions are changed in existing files, however, in some specific cases it can help determine permissions when file permissions are changed in existing files using the chmod command.

The mask is always stored as a group of bits. It may be displayed in binary, octal, or symbolic notation. Usually, it is represented in octal notation (e.g., 0755) or symbolic notation (e.g., u=wrx,g=rx,o=r) (see symbolic notation below for details). The umask command uses the same octal and symbolic notation as the chmod command.

The umask command is used with Unix-like operating systems and the umask function is defined in the POSIX.1 specification.

History

The mask, the umask command, and the umask function were not part of the original implementation of UNIX. The operating system evolved in a relatively small computer center environment where security was not an issue. It eventually grew to serve hundreds of users from different organizations. At first, developers made creation modes for key files more restrictive, especially for cases of actual security breaches, but this was not a general solution. The mask and the umask command were introduced around 1978 between the sixth edition and the eighth edition of the operating system, so it could allow sites, groups, and individuals to choose their own defaults. The mask has since been implemented in most, if not all, of the contemporary implementations of UNIX-like operating systems.

Shell command

In a shell, the mask is set by using the umask command. The syntax of the command is:

umask [-S ] [maskdefinition]

(the items within the [brackets] are optional)

Displaying the current mask

If the umask command is invoked without any arguments, it will display the current mask. The output will be in either octal or symbolic notation depending on the operating system used, however, invoking umask with the -S argument (i.e., umask -S) will force it to display the current mask in symbolic notation. For example:

$ umask
0022
$ umask -S
u=rwx,g=rx,o=rx

Setting the mask using octal notation

If the umask command is invoked with an octal argument, it will directly set the bits of the mask to that argument:

$ umask 0077
$ umask 
0077
$ umask -S
u=rwx,g=,o=

If fewer than 4 digits are entered, leading zeros are assumed. An error will result if the argument is not a valid octal number (or symbolic argument — see below) or if it has more than 4 digits.

Octal codes used in the command

Octal digit in umask command	Allows (if requested)
0	read, write and execute
1	read and write
2	read and execute
3	read only
4	write and execute
5	write only
6	execute only
7	no permissions

Setting the mask using symbolic notation

If the umask command is invoked with an argument using symbolic notation, it will add to, substract from, or directly set the bits of the mask as directed by the [maskdefinition] argument of the umask command. The sybolic notation is composed of permission-symbols, user-class-letters, and operators.

The syntax of the [maskdefinition] in the umask command looks like this:

umask [user-class-letters] operator permission-symbols[,]...

There are no spaces allowed between the arguments. The only space is between the umask command itself and the arguments.

The permission-symbols indicate which file permission settings are to be allowed or prohibited by the mask when files are subsequently created. There are three basic file permission modes which correspond to the basic permissions. They are r (read), w (write), and x (execute). The more esoteric symbols are X (special execute), s (setuid/guid), t (stiky bit). (See File permissions for more details.)

Symbol	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 allows the application of execute permissions to directories regardless of their current permissions and allows the application of execute permissions to a file which already has at least 1 execute permission bit already set (either user, group or other). It is only really useful when used with '+' d
s	setuid/gid	See File permissions for details.
t	sticky	See File permissions for details.

The permissions of a file are applied to three different classes of users: the user (the file's owner), the group, and others. (See File permissions for details.) Those user classes are represented by one or more of the following letters:

Letter	Class	Description
u	user	the owner of the file
g	group	users who are members of the file's group
o	others	users who are not the owner of the file or members of the group
a	all	all three of the above, it is the same as ugo. (This is the default if no class is specified in the umask command.)

The umask command uses an operator to specify how the permission modes of the mask should be adjusted. The following operators are accepted:

Operator	How the mask works after the command in issued
+	During file creation, the mask will allow the specified file permissions to be enabled for the specified user classes; (permissions that are not specified are unchanged in the mask)
-	During file creation, the mask will prohibit the specified file permissions from being enabled for the specified user classes; (permissions that are not specified are unchanged in the mask)
=	During file creation, the mask will allow the specified file permissions to be enabled for the specified user classes; permissions not specified will be prohibited by the mask during file creation

For example:

umask u+w

This would set the mask so that it would, when files are subsequently created, allow write permission to be enabled for the user (file owner). The rest of the bits in the mask would be unchanged.

Multiple changes can be specified by separating multiple sets of symbolic notation with commas. (Spaces are not allowed within the arguments.) For example:

umask u-x,g=r,o+w

This would set the mask so that it would, when subsequent files are created:

1. prohibit the execute permission from being set for the file's owner (user), while leaving the rest of the owner bits in the mask unchanged;
2. allow the read permission to be enabled for the group, while prohibiting write and execute permission for the group;
3. allow the write permission to be enabled for others, while leaving the rest of the other bits in the mask unchanged.

Command line examples

Here are more examples of using the umask command.

umask command issued	How the mask works after the command is issued
umask a+r	allows read permission to be enabled for all user classes; the rest of the mask bits are unchanged
umask a-x	prohibits enabling execute permission for all user classes; the rest of the mask bits are unchanged
umask a+rw	allows read or write permission to be enabled for all user classes; the rest of the mask bits are unchanged
umask +rwx	allows read, write or execute permission to be enabled for all user classes; (Note: On some UNIX platforms, this will restore the mask to a default.)
umask u=rw,go=	allow read and write permission to be enabled for the owner, while prohibiting execute permission from being enabled for the owner; prohibit enabling any permissions for the group and others
umask u+w,go-w	allow write permission to be enabled for the owner; prohibit write permission from being enabled for the group and others;
umask -S	display the current umask in symbolic notation
umask 777	disallow read, write, and execute permission for all (even owner cannot read it)
umask 000	allow read, write, and execute permission for all (security risk)
umask 664	allow read or write permission to be enabled for the owner and the group, but not write permission; allow read permission to be enabled for others, but not write or execute permission
umask 0755	equivalent to u-rwx (4+2+1),go=w (4+1 & 4+1). (The 0 specifies that special modes may be enabled if allowed by the OS.)

Function call

The mask may be set using a function call. The GNU functions are declared in sys/stat.h. The function is:

mode_t umask (mode_t mask)

The umask function will set the mask of the current process to mask, and return the previous value of the mask.

Here is an example from GNU.org which shows how to read the mask without changing it permanently:

mode_t
read_umask (void) {
    mode_t mask = umask (0);
    umask (mask);
    return mask;
}

However, on GNU/Hurd systems it is better to use the getumask function if reading the mask value is the only requirement, because it is reentrant.

mode_t getumask (void)

Returns the current value of the mask for the current process. (Note: The getumask function is a GNU extension and is only available on GNU/Hurd systems.

Mask effect

The mask is applied whenever a file is created. If the mask has a bit set to "1", that means the corresponding file permission will always be disabled when files are subsequently created. A bit set to "0" in the mask means that the corresponding permission will be determined by the requesting process and the OS when files are subsequently created. In other words, the mask acts as a last-stage filter that strips away permissions as a file is created; each bit that is set to a "1" strips away that corresponding permission for the file.

Truth table

Here is the truth table for the masking logic. Each bit in the requesting process' file permission mode is operated on by the mask using this logic to yield the permission mode that is applied to the file as it is created. (p is a bit in the requested file permission mode of a process that is creating a file; q is a bit in the mask; r is the resulting bit in the created file's permission mode)

p	q	r
T	T	F
T	F	T
F	T	F
F	F	F

How the mask is applied

Programmatically, the mask is applied by the OS by first negating (complementing) the mask, and then performing a logical AND with the requested file mode. In the [probably] first UNIX manual to describe its function, the manual says,

"the actual mode... of the newly-created file is the logical and of the given mode and the complement of the argument. Only the low-order 9 bits of the mask (the protection bits) participate. In other words, the mask shows [indicates] the bits to be turned off when files are created."

— UNIX Eighth Edition Manual, Bell Labs UNIX (manual), AT&T Laboratories

This shows how octal in the umask command appears in the mask and eventually affects a program's request for creating a file with, e.g., full (r, w, x) permissions.

Octal digit in  umask command	Binary in  the mask	Negation  of mask	Logical AND  with "rwx" request
0	000	111	rwx
1	001	110	rw-
2	010	101	r-x
3	011	100	r--
4	100	011	-wx
5	101	010	-w-
6	110	001	--x
7	111	000	---

Logic

The mask is applied using boolean logic known as material nonimplication or abjunction. It is represented in logic notation as:

C: (P&(~Q))

This says that the file's permission mode (C) is a result of a logical AND operation between the negation of the mask (Q), and the process' requested permission mode setting (P).

Exceptions

Note: Many operating systems do not allow a file to be created with execute permissions. In these environments, newly created files will always have execute permission disabled for all users.

The mask is generally only applied to functions that create a new file, however, there are exceptions. For example, when using UNIX and GNU versions of chmod to set the permissions of a file, and symbolic notation is used, and no user is specified, then the mask is applied to the requested permissions before they are applied to the file. For example:

$ umask 0000
$ chmod +rwx filename
$ ls -l filename
-rwxrwxrwx filename
$ umask 0022
$ chmod +rwx filename
$ ls -l filename
-rwxr-xr-x filename

Processes

Each process has its own mask, which is applied whenever the process creates a new file. When a shell, or any other process, spawns a new process, the child process inherits the mask from its parent process. When the process is a shell, the mask is changed by the umask command. As with other processes, any process launched from the shell inherits that shell's mask.

Mount option

In the Linux kernel, the fat, hfs, hpfs, ntfs, and udf file system drivers support a umask mount option, which controls how the disk information is mapped to permissions. This is not the same as the per-process umask described above, although the permissions are calculated in a similar way. Some of these file system drivers also support separate umasks for files and directories, using mount options such as fmask.

See also

• Filesystem permissions
• chmod