UMASK(1P) POSIX Programmer's Manual UMASK(1P)
PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the corresponding
Linux manual page for details of Linux behavior), or the interface may
not be implemented on Linux.
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 Environ-
ment) 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 envi-
ronment.
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 invok-
ing 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 indi-
cated 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 op-
erand.
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 vari-
ables that are unset or null. (See the Base Definitions vol-
ume of POSIX.1-2008, Section 8.2, Internationalization Vari-
ables 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 oper-
and.
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 correspond-
ing 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 gen-
erally 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 sym-
bolic 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 gen-
erally provided as a shell regular built-in. If it is called in a sub-
shell 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 pro-
vide 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 descrip-
tion in the System Interfaces volume of POSIX.1-2008.
FUTURE DIRECTIONS
None.
SEE ALSO
Chapter 2, Shell Command Language, chmod
The Base Definitions volume of POSIX.1-2008, Chapter 8, Environment
Variables, Section 12.2, Utility Syntax Guidelines
The System Interfaces volume of POSIX.1-2008, umask()
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the source
files to man page format. To report such errors, see https://www.ker-
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 UMASK(1P)
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