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users-and-groups.rst

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Users and groups

  • UID
  • GID
  • GECOS

Paths

  • /etc/passwd
  • /etc/shadow
  • /etc/group
  • /etc/skel

Etc Passwd

  • File /etc/passwd

Structure:

  • Username
  • Password: x indicates that shadow passwords are used
  • UID: User ID number
  • GID: User's group ID number
  • GECOS: Full name of the user
  • Home directory
  • Login shell
root:x:0:0:root:/root:/bin/bash
bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
adm:x:3:4:adm:/var/adm:/sbin/nologin
shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown
halt:x:7:0:halt:/sbin:/sbin/halt
nobody:x:99:99:Nobody:/:/sbin/nologin
sshd:x:74:74:Privilege-separated SSH:/var/empty/sshd:/sbin/nologin
watney:x:1000:1000:Mark Watney:/home/watney:/bin/bash
lewis:x:1001:1001:Melissa Lewis:/home/lewis:/bin/bash
martinez:x:1002:1002:Rick Martinez:/home/martinez:/bin/bash

Etc Shadow

  • File /etc/shadow

Structure:

  • Username: from /etc/passwd
  • Password
  • Last Password Change: Days since 1970-01-01
  • Minimum days between password changes: 0 - changed at any time
  • Password validity: Days after which password must be changed, 99999 - many, many years
  • Warning threshold: Days to warn user of an expiring password, 7 - full week
  • Account inactive: Days after password expires and account is disabled
  • Time since account is disabled: Days since 1970-01-01
  • A reserved field for possible future use

Password field (split by $):

  • algorithm
  • salt
  • password hash

Password algorithms:

  • 1 - MD5
  • 2a - Blowfish
  • 2y - Blowfish
  • 5 - SHA-256
  • 6 - SHA-512

Password special chars:

  • `` `` (blank entry) - password is not required to log in
  • * (asterisk) - account is disabled, cannot be unlocked, no password has ever been set
  • ! (exclamation mark) - account is locked, can be unlocked, no password has ever been set
  • !<password_hash> - account is locked, can be unlocked, but password is set
  • !! (two exclamation marks) - account created, waiting for initial password to be set by admin
root:$6$Ke02nYgo.9v0SF4p$hjztYvo/M4buqO4oBX8KZTftjCn6fE4cV5o/I95QPekeQpITwFTRbDUBYBLIUx2mhorQoj9bLN8v.w6btE9xy1:16431:0:99999:7:::
adm:$6$5H0QpwprRiJQR19Y$bXGOh7dIfOWpUb/Tuqr7yQVCqL3UkrJns9.7msfvMg4ZO/PsFC5Tbt32PXAw9qRFEBs1254aLimFeNM8YsYOv.:16431:0:99999:7:::
watney:!!:16550::::::
lewis:$6$P9zn0KwR$tgfvvFWJJ5FKmoXiP5rXWOjwoEBOEoAuBi3EphRbJqqjWYvhEM2wa67L9XgQ7W591FxUNklkDIQsk4kijuhE50:16632:0:99999:7:::
martinez:$1$.QKDPc5E$SWlkjRWexrXYgc98F.:12825:0:90:5:30:13096:

Etc Group

  • File /etc/group

Structure:

  • Group Name: from /etc/passwd
  • Group Password: x indicates that shadow passwords are used)
  • GID: Group ID
  • Members: usernames from /etc/passwd
root::0:root
other::1:
bin::2:root,bin,daemon
sys::3:root,bin,sys,adm
adm::4:root,adm,daemon
mail::6:root
astronauts::10:watney,lewis,martinez
daemon::12:root,daemon
sysadmin::14:martinez,lewis
mars::1000:watney
moon::1001:lewis
nobody::60001:
noaccess::60002:
nogroup::65534:

useradd vs. adduser

Commands

User, Password and Groups
Command Type Description
addgroup
add a user or group to the system
adduser
add a user or group to the system
chgpasswd
update group passwords in batch mode
chpasswd
update passwords in batch mode
delgroup
remove a user or group from the system
deluser
remove a user or group from the system
groupadd
create a new group
groupdel
delete a group
groupmod
modify a group definition on the system
groups
print the groups a user is in
id
print real and effective user and group IDs
last
show a listing of last logged in users
login
begin session on the system
login
write utmp and wtmp entries
logname
print user's login name
logout
write utmp and wtmp entries
nologin
prevent unprivileged users from logging into the system
nologin
politely refuse a login
passwd
change user password
passwd
the password file
su
run a command with substitute user and group ID
sudo
execute a command as another user
useradd
create a new user or update default new user information
userdel
delete a user account and related files
usermod
modify a user account
users
print the user names of users currently logged in to the current host
visudo
edit the sudoers file
w
Show who is logged on and what they are doing.
who
show who is logged on
whoami
print effective userid
ulimit
get and set user limits
mesg
display (or do not display) messages from other users
wall
write a message to all users
write
send a message to another user