linux An in depth understanding of umask's new file permission Settings

  • 2021-01-06 00:50:24
  • OfStack


If your umask is set to 022, what is the default file permission you create?

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

User Group Description

-rwxrw-r � -1 root root 1213 Feb 2 09:39 abc
The first character represents file (-), directory (d), link (l) The remaining characters are in groups of 3 (rwx), read (r), write (w), execute (x). Group 1 rwx: The permissions of the file owner are read, write, and execute rw- : Users in the same group as the file owner can read, write, but not execute Group 3 r-- : Other users who are not in the same group as the file owner have permissions to read and not write and execute It can also be expressed numerically: r=4, w=2, x=1, because rwx represents a 3-bit binary system, which is exactly the number to calculate.

Description of Digital Permission

777 is a 3-digit octadecimal number, corresponding to 111111111, which means that all three groups can be read, written, and executed. We can use it like this:

chmod 755 abc //chmod  Change the file abc The permissions for the file owner are readable, writable and executable, and the same group and other group users are readable and executable 

umask instructions

umask is the permission mask, which represents the default permissions not to be used. It calculates the default permissions for the user to create new objects based on the maximum default value of 666 and the folder 777.

For example, in question 1, the default permission to create a file is 666-022=644, which is -rw -r --r

What is the umask used for

The default umask value is 022(umask), and the default permissions are 644(6-0,6-2,6-2). The default permissions are 755(7-0,7-2,7-2). You can use ls-l to verify this.

[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 you can see from above, umask of root is 022(the first 0 represents the special permission bit, which is not considered here), the default permissions for the files created are 644, and the directory created is 755.

Before we look at the use of umask, we need to explain the basic file permissions

  r w x
文件  可以查看文件内容  可以修改文件  可以把文件启动为1个运行的程序
目录  可以ls查看目录中的文件名  可以在目录中创建或者删除文件(只有w权限没法创建,需要x配合)  可以使用cd 进入这个目录ls-l显示目录内文件的元数据的信息


Related articles: