About default permissions and umask values

When a user creates a file/directory, the default permission is determined by the value of umask, and the default permission is a relatively secure permission.

In /etc/profile/ we can see:

if [ $UID -gt 199 ] && [ "`id -gn`" = "`id -un`" ]; then
    umask 002
else
    umask 022


Note: id - gn: the group name corresponding to gid
      id - un: the username corresponding to the uid

That is, when uid > 199 and the user name is the same as its corresponding group name, the umask is 002, otherwise it is 022

It is worth noting that the default permission of the administrator's home directory in /etc/login.defs: 077

The umask value is different, which will cause the corresponding default permissions to be different:

	File default permissions	Directory default permissions
umask=0002	664（rw-rw-r--）	775（rwxrwxr-x）
umask=0022	644（rw-r--r--）	755(rwxr-xr-x)

 

 

 

 

Calculation of umask:

File: default permission = default maximum permission - umask (when umask has odd bits, the odd number of unmask in the result corresponds to permission bits + 1)

directory: default permissions = default max permissions - umask

example	Default maximum permissions	umask 值	mDefault permissions
Create file (umask is all even digits)	666	022	644
create file (odd digits of umask exist)	666	123	666-123=543 5+1-->6 4 unchanged 3+1-->4 The result is 644
Create a directory	777	022	755