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umask-设置文件权限掩码.md

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umask command

In Linux and other Unix-like operating systems, when new files are created, a set of default permissions will be used. Specifically, the way in which new file permissions are restricted can be defined through the use of a permission mask called umask. The umask command is used to set a default permission mask when creating files, or to display the current mask value.

Syntax

umask [-S] [mask]

Parameters

  • -S: Represents the mask in symbolic mode.
  • mask: Specifies an effective mask value. If none is specified, the current umask value is returned.

Permissions

Use the ll command to view file and directory information. Regarding its permission information:

drwxr-xr-x 12 linuxize users 4.0K Apr  8 20:51 dirname
|[-][-][-]    [------] [---]
| |  |  |        |       |       
| |  |  |        |       +-----------> Group
| |  |  +-------------------> Owner
| |  +-----------------------> Others Permissions
| +--------------------------> Group Permissions
+----------------------------> Owner Permissions
  • The first symbol - represents a regular file, while d represents a directory.
  • The next three characters rw- represent the user u permission attributes.
  • The next three characters r-- represent the user group g permission attributes.
  • The last three characters r-- represent the other o permissions.

Permissions can be represented using characters, and can also be represented in octal numbers:

  • r: Read attribute, with a value of 4.
  • w: Write attribute, with a value of 2.
  • x: Execute attribute, with a value of 1.

Regarding operators for manipulating permissions:

  • +: Adds specified file permissions for the specified user class.
  • -: Removes specified file permissions for the specified user class.
  • =: Assigns specified file permissions for the specified user class.

Work

Essentially, each digit of umask is subtracted from the operating system’s default value to obtain the defined default value. This is not true subtraction, it technically involves taking the bitwise complement of the mask and then applying this value using a logical AND operation to the default permissions. For example, if the umask value is 0022:

  • On a Linux system, the default file creation permission is 666, and the default folder creation permission is 777.
  • So, 666 - 022 = 644, which means the permission of the newly created file is 644.
  • For folders, 777 - 022 = 755, meaning the permission of the newly created folder is 755.

Also, the leading zero is a special permission number that can be ignored. For the current purpose, 0002 is the same as 002.

Examples

View the current umask value in the system.

umask
# 0022

Represent the current system permission mask in symbolic form.

umask -S
# u=rwx,g=rx,o=rx # effectively expressing which permissions have been revoked for the owner, group, and other.

Set the system umask value to 022.

umask 022

Daily Question

https://github.com/WindrunnerMax/EveryDay

References

https://www.computerhope.com/unix/uumask.htm
https://linuxize.com/post/umask-command-in-linux/
https://www.runoob.com/linux/linux-comm-umask.html