Linux manage users and groups

To list all users in linux, execute cat /etc/passwd, file passwd contains the essential information required during login.

its format:

# format for file: /etc/passwd
# IFS=:     # separated by colon (:)
# in total sever fields

# for example:
xiang:x:1000:1000:xiang,,,:/home/xiang:/bin/bash

# 1:2:3:4:5:6:7
#
# 1: username
# 2: password
# 3: UID (User ID)
# 4: GID (Group ID)
# 5: User ID Info (GECOS), comments, additional info
# 6: HomePath
# 7: Command/shellpath

passwd              # change password for current user
sudo passwd user    # change password for specified user
sudo passwd         # change password for root

# show info
chage -l user

# check validation and integrity
pwck

passwords are saved in /etc/shadow file:

# format for file: /etc/shadow
# IFS=:     # separated by colon (:)
# in total sever fields

# for example:
xiang:$6$uK/Aw0kgk36CUM1:18984:0:99999:7:::

# 1:2:3:4:5:6:7:8
#
# 1: username
# 2: password
#    $1$ means MD5 encryption
#    $2a$ is Blowfish
#    $2y$ is Blowfish
#    $5$ is SHA-256
#    $6$ is SHA-512
# 3: days since last time password changed, since Jan 1, 1970
# 4: minimum number of days required between password changes
# 5: maximum number of days that the password is valid
# 6: number of days for warning before the password is to expire
# 7: number of days after password expires that account is disabled
# 8: number of exact days when the account is disabled, since Jan 1, 1970

groups are saved in /etc/group file:

# format for file: /etc/group
# IFS=:     # separated by colon (:)
# in total sever fields

group_name:password:GID:user_list

# user_list: a list of usernames that are members of this group, separate by comma

command getent (acronym: get entries) is very useful

# for example: get all users
getent passwd

some useful commands may relate to this subject

create restricted group/user in linux

read: sshd_config

setup procedures:

if any login error happens, check file /var/log/auth.log for detail info.