Introduction

A System Administrator (sysadmin) is responsible for maintaining, configuring, and ensuring the reliable operation of a UNIX system. The system administrator has special privileges and uses the root (superuser) account to manage the system.

The Root User (Superuser)

• The root user has UID (User ID) = 0.
• Root has unrestricted access to all files, commands, and system resources.
• The root prompt is typically # while a normal user prompt is $.
• Caution: Using root carelessly can damage the system. Always use with care.

Accessing Root Privileges

1. Direct Login as Root:

• Login with username root and its password.
• Not recommended for security reasons.

2. Using su (Switch User):

• su — Switches to root user (requires root password).
• su - username — Switches to a specific user.

3. Using sudo (Superuser Do):

• Allows a permitted user to execute a command as root.
• sudo command — Runs a single command with root privileges.
• The user must be listed in the /etc/sudoers file.
• Preferred method as it provides an audit trail.

Key System Administration Tasks

1. User Management:

• useradd username — Create a new user.
• userdel username — Delete a user.
• usermod -aG group username — Add user to a group.
• passwd username — Set or change a user's password.
• User info is stored in /etc/passwd and /etc/shadow.

2. Group Management:

• groupadd groupname — Create a new group.
• groupdel groupname — Delete a group.
• Group info is stored in /etc/group.

3. File System Management:

• mount /dev/sda1 /mnt — Mount a file system.
• umount /mnt — Unmount a file system.
• df -h — Check disk space usage.
• du -sh /path — Check size of a directory.
• fsck /dev/sda1 — File system consistency check and repair.

4. Process Management:

• ps aux — List all running processes.
• top / htop — Real-time process monitoring.
• kill PID — Terminate a process by PID.
• kill -9 PID — Force kill a process.
• nice -n value command — Run a process with modified priority.
• renice value PID — Change priority of a running process.

5. System Monitoring:

• uptime — System uptime and load averages.
• free -h — Memory usage.
• vmstat — Virtual memory statistics.
• iostat — I/O statistics.
• dmesg — Kernel ring buffer messages.

6. Backup and Recovery:

• tar -cvf backup.tar /directory — Create a backup archive.
• tar -xvf backup.tar — Restore from a backup.
• cpio — Another backup utility in UNIX.
• crontab -e — Schedule automated backups using cron jobs.

7. Network Administration:

• ifconfig / ip addr — View network interfaces.
• ping host — Test connectivity.
• netstat — Network connections and statistics.
• ssh user@host — Secure remote login.
• scp file user@host:/path — Secure file copy.

8. System Startup and Shutdown:

• shutdown -h now — Halt the system immediately.
• shutdown -r now — Reboot the system.
• init 0 — Shutdown (runlevel 0).
• init 6 — Reboot (runlevel 6).

Important Configuration Files

Summary

• The system administrator manages the UNIX system using root privileges.
• sudo is the preferred way to execute administrative commands.
• Key tasks include user/group management, file system maintenance, process control, backup, and network administration.
• Important config files are located in /etc/ directory.