Linux Users, Groups, and Permissions

Files
-----
/etc/passwd
/etc/shadow
/etc/group
/etc/shells
/etc/skel


/etc/passwd
-----------
username
encrypted password
uid
gid
GECOS
home directory
shell

/etc/shadow
-----------
username
encrypted password
day of last password change (since Jan 1, 1970)
days before password may be changed
days after which password must be changed
days before expiration of password to warn the user
days after password expires that account is disabled
day of account disabling (since Jan 1, 1970)
reserved field

/etc/group
----------
group name
encrypted group password
gid
list, of, usernames


Commands
--------
- User/Group management
adduser
addgroup
deluser
delgroup

- File ownership/permissions management
chown
chgrp
chmod
umask
ls

- User account editing/examination
passwd
chsh
chfn
chage
finger
groups
id

- User/Group access
ssh
su
newgrp


File Permissions
----------------
- 12 file mode bits exist
- User, group, other, special
- Read    (ugo)
- Write   (ugo)
- Execute (ugo)
- Setuid (u)
  If file is a directory:
    Ignored
  If file is a file:
    When executed, the process belongs to the file's owner,
    instead of the user.
- Setgid (g)
  If file is a directory:
    New files in the directory will belong to the same group
    as the directory.
  If file is a file:
    When executed, the process belongs to the file's group,
    instead of the user's group.
- Stick bit (o)
  If file is a directory:
    Users can not delete files in this directory, unless they
    own the file, or directory.


Documentation
-------------
man 5 passwd
man 5 shadow
man 5 group
man command (such as chown or chmod)