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PROGRAM:

NAME

umask — get or set the file mode creation mask

SYNOPSIS

umask [−S] [mask]

DESCRIPTION

The umask utility shall set the file mode creation mask of the current shell execution
environment (see Section 2.12, Shell Execution Environment) to the value specified by the
mask operand. This mask shall affect the initial value of the file permission bits of
subsequently created files. If umask is called in a subshell or separate utility execution
environment, such as one of the following:

(umask 002)
nohup umask ...
find . −exec umask ... \;

it shall not affect the file mode creation mask of the caller's environment.

If the mask operand is not specified, the umask utility shall write to standard output the
value of the file mode creation mask of the invoking process.

OPTIONS

The umask utility shall conform to the Base Definitions volume of POSIX.1‐2008, Section
12.2, Utility Syntax Guidelines.

The following option shall be supported:

−S Produce symbolic output.

The default output style is unspecified, but shall be recognized on a subsequent
invocation of umask on the same system as a mask operand to restore the previous file mode
creation mask.

OPERANDS

The following operand shall be supported:

mask A string specifying the new file mode creation mask. The string is treated in
the same way as the mode operand described in the EXTENDED DESCRIPTION section
for chmod.

For a symbolic_mode value, the new value of the file mode creation mask shall be
the logical complement of the file permission bits portion of the file mode
specified by the symbolic_mode string.

In a symbolic_mode value, the permissions op characters '+' and '−' shall be
interpreted relative to the current file mode creation mask; '+' shall cause the
bits for the indicated permissions to be cleared in the mask; '−' shall cause
the bits for the indicated permissions to be set in the mask.

The interpretation of mode values that specify file mode bits other than the
file permission bits is unspecified.

In the octal integer form of mode, the specified bits are set in the file mode
creation mask.

The file mode creation mask shall be set to the resulting numeric value.

The default output of a prior invocation of umask on the same system with no
operand also shall be recognized as a mask operand.

STDIN

Not used.

INPUT FILES

None.

ENVIRONMENT VARIABLES

The following environment variables shall affect the execution of umask:

LANG Provide a default value for the internationalization variables that are unset or
null. (See the Base Definitions volume of POSIX.1‐2008, Section 8.2,
Internationalization Variables for the precedence of internationalization
variables used to determine the values of locale categories.)

LC_ALL If set to a non-empty string value, override the values of all the other
internationalization variables.

LC_CTYPE Determine the locale for the interpretation of sequences of bytes of text data
as characters (for example, single-byte as opposed to multi-byte characters in
arguments).

LC_MESSAGES
Determine the locale that should be used to affect the format and contents of
diagnostic messages written to standard error.

NLSPATH Determine the location of message catalogs for the processing of LC_MESSAGES.

ASYNCHRONOUS EVENTS

Default.

STDOUT

When the mask operand is not specified, the umask utility shall write a message to
standard output that can later be used as a umask mask operand.

If −S is specified, the message shall be in the following format:

"u=%s,g=%s,o=%s\n", <owner permissions>, <group permissions>,
<other permissions>

where the three values shall be combinations of letters from the set {r, w, x}; the
presence of a letter shall indicate that the corresponding bit is clear in the file mode
creation mask.

If a mask operand is specified, there shall be no output written to standard output.

STDERR

The standard error shall be used only for diagnostic messages.

OUTPUT FILES

None.

EXTENDED DESCRIPTION

None.

EXIT STATUS

The following exit values shall be returned:

0 The file mode creation mask was successfully changed, or no mask operand was
supplied.

>0 An error occurred.

CONSEQUENCES OF ERRORS

Default.

The following sections are informative.

APPLICATION USAGE

Since umask affects the current shell execution environment, it is generally provided as a
shell regular built-in.

In contrast to the negative permission logic provided by the file mode creation mask and
the octal number form of the mask argument, the symbolic form of the mask argument
specifies those permissions that are left alone.

EXAMPLES

Either of the commands:

umask a=rx,ug+w

umask 002

sets the mode mask so that subsequently created files have their S_IWOTH bit cleared.

After setting the mode mask with either of the above commands, the umask command can be
used to write out the current value of the mode mask:

$ umask
0002

(The output format is unspecified, but historical implementations use the octal integer
mode format.)

$ umask −S
u=rwx,g=rwx,o=rx

Either of these outputs can be used as the mask operand to a subsequent invocation of the
umask utility.

Assuming the mode mask is set as above, the command:

umask g−w

sets the mode mask so that subsequently created files have their S_IWGRP and S_IWOTH bits
cleared.

The command:

umask −− −w

sets the mode mask so that subsequently created files have all their write bits cleared.
Note that mask operands −r, −w, −x or anything beginning with a <hyphen>, must be preceded
by "−−" to keep it from being interpreted as an option.

RATIONALE

Since umask affects the current shell execution environment, it is generally provided as a
shell regular built-in. If it is called in a subshell or separate utility execution
environment, such as one of the following:

(umask 002)
nohup umask ...
find . −exec umask ... \;

it does not affect the file mode creation mask of the environment of the caller.

The description of the historical utility was modified to allow it to use the symbolic
modes of chmod. The −s option used in early proposals was changed to −S because −s could
be confused with a symbolic_mode form of mask referring to the S_ISUID and S_ISGID bits.

The default output style is unspecified to permit implementors to provide migration to the
new symbolic style at the time most appropriate to their users. A −o flag to force octal
mode output was omitted because the octal mode may not be sufficient to specify all of the
information that may be present in the file mode creation mask when more secure file
access permission checks are implemented.

It has been suggested that trusted systems developers might appreciate ameliorating the
requirement that the mode mask ``affects'' the file access permissions, since it seems
access control lists might replace the mode mask to some degree. The wording has been
changed to say that it affects the file permission bits, and it leaves the details of the
behavior of how they affect the file access permissions to the description in the System
Interfaces volume of POSIX.1‐2008.

FUTURE DIRECTIONS

None.

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