On a Linux system users permissions problem

Provides an overview of the LINUX file permissions for objects divided into three classes (mutually exclusive): 1.user (file owner) 2.group (file owner group, but does not include user) 3.other (other users, that is, other than user and group) LINUX with a 3-bit binary number corresponding to the three kinds of permissions file (1 indicates the permissions, 0 for none): 1-bit read r1004 2nd write 3 execution w0102 x0011 view permission # ls-l first column, a total of 10 bit (drwxrwxrwx) that represents the permissions of the file: 1) first d represents is a directory, if you display the "-", the description is not a directory 2) 2-4-3 on behalf of the user's permission) 5-7 on behalf of the group's permissions 4) 8-10 represents the permissions for the other 9-bit after: r for read (read), its value is the 4w representative may write (write), its value is 2x represent executable (execute), its value is 1-representative does not have the appropriate permissions, the value is 0 to modify file permissions # chmod [ugoa] [+-=] [rwx] filename 1) representative on behalf of a user u userg groupo representatives who represent all otera, including u, g, and o2) action + represents add permissions-represents the delete permission = represents a unique permissions permissions rwx 3) or digital representations, but very troublesome to their calculation, for example, rw = 6 common permissions-rw--(600), only the owner can read and write permissions-rw-r--r--(644), only the owner can read and write permissions, groups, and other people only read permissions-rwx--(700), only the owner has read, write, execute permission-rwxr-xr-x (755) only the owner has read, write, and execute permissions, groups, and other people only read and execute permissions-rwx — x — x (711) only the owner has read, write, and execute permissions, groups and others only execution permission-rw-rw-rw-(666) everyone has read-write permissions-rwxrwxrwx (777) everyone has read-write and execute permissions, maximum permissions.