Ubuntu Manpage: This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

Provided by: manpages-posix_2017a-2_all

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 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.1‐2017,  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.1‐2017, 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-minus>, 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.1‐2017.

FUTURE DIRECTIONS

       None.

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
       for  Information  Technology  --  Portable  Operating  System  Interface  (POSIX),  The  Open  Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .

       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.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                                   2017                                         UMASK(1POSIX)