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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 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 invoking process' file mode creation mask.
OPTIONS
The umask utility shall conform to the Base Definitions volume of IEEE Std 1003.1-2001,
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 IEEE Std 1003.1-2001, 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 implementation-defined 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 IEEE Std 1003.1-2001.
FUTURE DIRECTIONS
None.
SEE ALSO
Shell Command Language , chmod , the System Interfaces volume of IEEE Std 1003.1-2001,
umask()
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System
Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by
the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and The Open Group
Standard, the original IEEE and The Open Group Standard is the referee document. The
original Standard can be obtained online at http://www.opengroup.org/unix/online.html .