Under various operating systems of Unix and Linux, each file (folder is also regarded as a file) has permissions set according to read, write, and run.
ls -l: get
-rw-r--r-- 1 apple users 2254 2006-05-20 13:47 tt.htm
starting from the second character rw- means that the user apple has the right to read and write, but not to run ,
The next r-- means that the user group users has only read permission, but no run permission.
The last r-- means that others (others) only have read permission, but have no write and run rights.
The three permissions of read, write and run can be represented by numbers, that is, r=4, w=2, x=1. So, rw-r--r-- in the above example is represented as 644 in numbers.
Conversely, 777 is rwxrwxrwx, which means that the logged in user (which can be viewed with the command id), his group and others have the highest authority.