Linux User and Group Account Management
User and Group Accounts Overview
Linux controls resource access through user identities. Accounts are categorized in to user accounts and group accounts.
User Account Roles
- Superuser (root): UID 0 with full system privileges
- Regular Users: UID 1000+ (CentOS7) with restricted permissions
- System Users: UID 1-999 (CentOS7) for service operations without login capability
Group Account Types
- Primary Group: Each user's default group (GID same as UID)
- Supplementary Group: Additional groups for shared resource access
User Account Configuration Files
/etc/passwd
root:x:0:0:root:/root:/bin/bash
- Format:
username:password_placeholder:UID:GID:comment:home_directory:login_shell - Password stored in /etc/shadow
/etc/shadow
root:$6$TgG...:18983:0:99999:7:::
- Fields: username, encrypted password, last password change, min/max password age, warning period, inactivity period, expiration date
Password Policy Management
chage -m 7 -M 90 -W 7 testuser
| Option | Description |
|---|---|
| -m | Minimum days between password changes |
| -M | Maximum password validity period |
| -W | Warning days before expiration |
Group Management Commands
Group Creation
groupadd -g 1001 developers
Member Management
gpasswd -a testuser developers
User Account Operations
User Creation
useradd -m -u 1001 -g users -G wheel -s /bin/bash testuser
Account Configuration Files
~/.bash_profile: Executes at login~/.bashrc: Runs for interactive shells~/.bash_logout: Executes at logout
Account Modification
usermod -l newuser -d /home/newuser -s /bin/zsh testuser
File System Permissions
Permission Types
- Read (r): View content/list directory
- Write (w): Modify content/manage files
- Execute (x): Run programs/traverse directories
Permission Modification
chmod u=rw,g=r,o= file.txt
chown testuser:developers project/
UMASK Defautls
umask 022 # Default directory:755, file:644
