In-depth understanding of umask in new linux file permission settings

Preface

The origin is a question 1: If your umask is set to 022, what are the default permissions for the files you create?

This reminds me of another question I was asked 2: What permissions does 777 represent?

User Group Description

-rwxrw-r--1 root root 1213 Feb 2 09:39 abc

• The first character represents a file (-), a directory (d), or a link (l).
• The remaining characters are grouped in groups of 3 (rwx), read (r), write (w), execute (x)
• The first group rwx: The file owner's permissions are read, write, and execute
• The second group rw-: users in the same group as the file owner have read and write permissions but cannot execute
• The third group r--: The permissions for other users who are not in the same group as the file owner are read but not write and execute
• It can also be expressed digitally as: r=4, w=2, x=1, because rwx represents three binary numbers, these numbers can be calculated exactly.

Digital Rights Description

So back to question 2, 777 is a three-digit octal number, corresponding to 111111111, which means that all three groups are readable, writable, and executable. We can use it like this:

chmod 755 abc //chmod changes the permissions of file abc to be readable, writable and executable by the file owner, and readable and executable by users in the same group and other groups

umask Description

umask is a permission mask, which represents the default permissions. It is used to calculate the default permissions for new objects created by the user based on the maximum default values ​​of 666 for files and 777 for folders.

For example, in question 1, the default permissions for creating a file are 666-022=644, which is -rw-r--r--

What is umask used for?

The default umask value is 022 (you can use the umask command to view it). At this time, the default permissions of the files you create are 644 (6-0, 6-2, 6-2), and the default permissions of the directories you create are 755 (7-0, 7-2, 7-2). You can verify it with ls -l. Now you should know the purpose of umask, which is to control the default permissions.

[root@bogon test]# id
uid=0(root) gid=0(root) groups=0(root) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
[root@bogon test]# umask
[root@bogon test]# touch a.txt
[root@bogon test]# ls -l
total 0
-rw-r--r--. 1 root root 0 Jul 3 00:40 a.txt
[root@bogon test]# mkdir b
[root@bogon test]# ls -l
total 0
-rw-r--r--. 1 root root 0 Jul 3 00:40 a.txt
drwxr-xr-x. 2 root root 6 Jul 3 00:41 b

As can be seen above, the umask of root is 022 (the first 0 represents the special permission bit, which is not considered here), the default permissions of created files are 644, and the default permissions of created directories are 755.

Before understanding the use of umask, you need to explain the basic permissions of the file.

Summarize

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