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Get/set the process mask.

A mask is a set of bits, each of which restricts how its corresponding permission is set for newly created files. On POSIX platforms, each file has a set of attributes that control who can read, write, or execute that file. Upon creating a file, file permissions must be set to an initial setting. The process mask restricts those permission settings.

If the mask contains a bit set to 1, the corresponding initial file permission is disabled. If the mask contains a bit set to 0, the corresponding permission is left to be determined by the requesting process and the system. The process mask is thus a filter that removes permissions as a file is created; i.e., each bit set to a 1 removes its corresponding permission.

In octal representation, a mask is a four digit number comprised as follows (using 0077 as an example):

  • 0: special permissions (setuid, setgid, sticky bit)
  • 0: (u)ser/owner permissions
  • 7: (g)roup permissions
  • 7: (o)thers/non-group permissions

Octal codes correspond to the following permissions:

  • 0: read, write, execute
  • 1: read, write
  • 2: read, execute
  • 3: read
  • 4: write, execute
  • 5: write
  • 6: execute
  • 7: no permissions

If provided fewer than four digits, the mask is left-padded with zeros. Note, however, that only the last three digits (i.e., the file permissions digits) of the mask are actually used when the mask is applied (i.e., mask & 0777).

Permissions can be represented using the following symbolic form:



  • u: user permissions
  • g: group permissions
  • o: other/non-group permissions
  • r: read
  • w: write
  • x: execute

When setting permissions using symbolic notation, one may use a mask expression of the form:


where <classes> may be a combination of

  • u: user
  • g: group
  • o: other/non-group
  • a: all

<symbols> may be a combination of

  • r: read
  • w: write
  • x: execute
  • X: special execute
  • s: setuid/gid on execution
  • t: sticky

and <operator> may be one of

  • +: enable
  • -: disable
  • =: enable specified and disable unspecified permissions

For example,

  • u-w: disable user write permissions
  • u+w: enable user write permissions
  • u=w: enable user write permissions and disable user read and execute

To specify multiple changes, one can specify a comma-separated list of mask expressions. For example,


would enable user read, write, and execute permissions, disable group execute permissions, enable other read permissions, and disable other write and execute permissions.

The a class indicates "all", which is the same as specifying ugo. This is the default class if a class is omitted when specifying permissions. For example, +x is equivalent to a+x which is equivalent to ugo+x which is equivalent to u+x,g+x,o+x and enables execution for all classes.


npm install @stdlib/process-umask


var umask = require( '@stdlib/process-umask' );

umask( [mask,] [options] )

Returns the process mask.

var mask = umask();
// returns <number>

To set the process mask, provide a mask argument. When provided a mask, the function returns the previous mask value.

var mask = umask();
// returns <number>

var prev = umask( 0 );
// returns <number>

var bool = ( prev === mask );
// returns true

The mask argument may be either an integer value or a string representing the mask using symbolic notation.

var mask = umask( 'u=rwx,g=rw,o=rw' );

The function accepts the following options:

  • symbolic: boolean indicating whether to return the mask in symbolic notation. Default: false.

To return the process mask in symbolic notation, set the symbolic option to true.

var opts = {
    'symbolic': true

// Get the mask:
var mask = umask( opts );
// e.g., returns 'u=rwx,g=rw,o=rw'

// Set the mask:
mask = umask( 0, opts );
// e.g., returns 'u=rwx,g=rw,o=rw'


  • To set the process mask using an octal string (e.g., 0777), use parseInt to convert the string to an integer value.

    umask( parseInt( '0777', 8 ) );
  • See umask(2).


var lpad = require( '@stdlib/string-left-pad' );
var umask = require( '@stdlib/process-umask' );

// Print the process mask as an integer:
var mask = umask();
console.log( mask.toString() );

// Print the process mask as an octal string:
console.log( lpad( mask.toString(), 4, '0' ) );

// Print the process mask using symbolic notation:
var opts = {
    'symbolic': true
console.log( umask( opts ) );

See Also


This package is part of stdlib, a standard library for JavaScript and Node.js, with an emphasis on numerical and scientific computing. The library provides a collection of robust, high performance libraries for mathematics, statistics, streams, utilities, and more.

For more information on the project, filing bug reports and feature requests, and guidance on how to develop stdlib, see the main project repository.






Copyright © 2016-2024. The Stdlib Authors.

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