Linux Commands for Admins

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Managing a Linux server is largely about knowing the right command‑line tools and when to use them.
This guide walks you through the most common commands every system administrator needs in day‑to‑day work, from user management and file handling to networking diagnostics, process control, package maintenance, and security hardening. It assumes you already have SSH access or a console on a running Linux distribution (Ubuntu, CentOS/RHEL, Debian, Fedora, etc.) and that you’re comfortable typing into a terminal.

1. Why the CLI Still Matters

Graphical interfaces like Cockpit make many tasks approachable, but they are thin wrappers over the same system APIs. Knowing how to run those commands directly gives:

• Speed – one keystroke can often replace several mouse clicks.
• Automation – scripts and cron jobs use plain text, not GUIs.
• Debugging – error messages from native tools are usually more detailed than GUI logs.
• Portability – most servers run headless; you’ll need the CLI to manage them remotely.

Below we group commands by category. For each, we give a short description, syntax, and a practical example that illustrates real‑world usage.

2. User and Group Management

2.1 Adding a New User

sudo useradd -m -s /bin/bash alice
sudo passwd alice          # set password interactively

• -m creates the home directory /home/alice.
• -s sets the default shell.

If you want to add the user to multiple groups at once:

sudo useradd -m -G sudo,developers alice

2.2 Modifying a User

Change the login shell or move the home directory:

sudo usermod -s /bin/zsh alice          # change shell
sudo usermod -d /new/home/alice -m alice   # move home dir

The -m flag copies existing files to the new location.

2.3 Deleting a User

sudo deluser --remove-home alice

On Debian‑based systems use deluser; on RHEL/CentOS use userdel -r.

3. File System Operations

3.1 Listing Files

ls -alh /var/log

• -a: show hidden files
• -l: long format
• -h: human‑readable sizes

To sort by modification time:

ls -lt /var/log

3.2 Searching for a File

find is more powerful than locate, which relies on an index.

sudo find /etc -name 'sshd_config'

If you want to ignore permission errors:

sudo find / -type f -name '*.conf' 2>/dev/null

3.3 Archiving Logs

Rotated logs are often compressed with gzip. To create a tarball of the last seven days’ logs:

tar czf /backup/logs_$(date +%Y%m%d).tar.gz -C /var/log .

• -c: create
• -z: compress with gzip
• -f: file name

4. Networking Diagnostics

4.1 Inspecting Interfaces

ip addr show eth0

Shows IP, MAC, MTU, and status.

To bring an interface up or down:

sudo ip link set eth0 up
sudo ip link set eth0 down

ifconfig is deprecated but still available on many systems; ip is the modern replacement.

4.2 Checking Open Ports

ss -tuln | grep LISTEN

• -t: TCP
• -u: UDP
• -l: listening sockets
• -n: numeric addresses

If you need a more comprehensive scan, install nmap:

sudo apt-get install nmap      # Debian/Ubuntu
sudo yum install nmap          # RHEL/CentOS

Then run:

sudo nmap -sS -O localhost

• -sS: stealth SYN scan
• -O: attempt OS detection

4.3 Tracing a Route

traceroute google.com

On some systems, you may need to install the package first (traceroute or iproute2).

5. Process Management

5.1 Listing Processes

ps aux --sort=-%cpu | head -10

Shows the top ten CPU‑hungry processes.

For an interactive view, use top:

sudo top

Or install htop for a nicer interface:

sudo apt-get install htop   # Debian/Ubuntu
sudo yum install htop       # RHEL/CentOS

5.2 Killing a Process

To kill by PID:

kill -15 1234          # SIGTERM (default)
kill -9 1234           # SIGKILL, force terminate

To kill all instances of a command:

pkill -f nginx

-f matches against the full command line.

5.3 Changing Process Priority

sudo renice +10 -p 1234   # lower priority (nice value +10)

Only root can set negative nice values (higher priority).

6. Package Management

6.1 Updating the System

Debian/Ubuntu:

sudo apt update && sudo apt upgrade -y

RHEL/CentOS 8+ (dnf):

sudo dnf update -y

Arch:

sudo pacman -Syu --noconfirm

Always read the output for any prompts or conflicts.

6.2 Installing a Package

sudo apt install htop

For multiple packages:

sudo yum install vim git curl

6.3 Removing Unused Packages (Clean Up)

Debian/Ubuntu:

sudo apt autoremove -y

RHEL/CentOS:

sudo package-cleanup --leaves

7. Systemd Service Control

7.1 Starting, Stopping, Restarting

sudo systemctl start nginx
sudo systemctl stop nginx
sudo systemctl restart nginx

7.2 Enabling at Boot

sudo systemctl enable nginx

7.3 Checking Status and Logs

systemctl status nginx
journalctl -u nginx --since "2024-10-01" --no-pager

journalctl can filter by time, priority, or unit.

8. Security Hardening Basics

8.1 Configuring UFW (Uncomplicated Firewall)

sudo ufw default deny incoming
sudo ufw default allow outgoing
sudo ufw allow ssh
sudo ufw enable

Verify:

sudo ufw status verbose

On RHEL/CentOS, use firewalld:

sudo firewall-cmd --permanent --add-service=ssh
sudo firewall-cmd --reload

8.2 SSH Hardening

Edit /etc/ssh/sshd_config:

PermitRootLogin no
PasswordAuthentication no
AllowUsers alice bob

Then reload:

sudo systemctl restart sshd

9. Log Monitoring and Rotation

Logs are critical for troubleshooting.

9.1 Viewing Logs in Real Time

sudo tail -f /var/log/syslog          # Debian/Ubuntu
sudo journalctl -f -u nginx           # systemd unit

9.2 Configuring Logrotate

Edit /etc/logrotate.d/nginx:

/var/log/nginx/*.log {
    daily
    missingok
    rotate 14
    compress
    delaycompress
    notifempty
}

Test configuration:

sudo logrotate -d /etc/logrotate.conf

10. Automation with Cron

Cron jobs run scheduled tasks.

Create a cron entry for a daily backup:

crontab -e
# Add:
0 2 * * * tar czf /backups/$(date +\%Y\%m\%d)_nginx.tar.gz /var/www/html

crontab -l lists current jobs.

11. Scripting Quick Tasks

A small bash script can bundle multiple commands:

#!/usr/bin/env bash
# backup_and_notify.sh

set -euo pipefail

DATE=$(date +%Y%m%d)
BACKUP="/backups/nginx_$DATE.tar.gz"
SRC="/var/www/html"

tar czf "$BACKUP" "$SRC"
echo "Backup $BACKUP completed." | mail -s "Backup Success" admin@example.com

Make it executable:

chmod +x backup_and_notify.sh
sudo mv backup_and_notify.sh /usr/local/bin/

Schedule it with cron as shown earlier.

12. Performance Tuning Tips

Install sysstat if missing:

sudo apt-get install sysstat   # Debian/Ubuntu
sudo yum install sysstat       # RHEL/CentOS

Use sar to collect historical data.

13. Troubleshooting Common Issues

When a command returns an error code, consult the manual:

man <command>

Use --help for quick usage:

systemctl --help

14. Resources and Further Reading

• Linux man pages – the definitive source (man <cmd>).
• Official distro documentation – e.g., Ubuntu Server Guide, RHEL System Administration Guide.
• Community sites – Stack Overflow, Unix & Linux Stack Exchange, Reddit r/linuxadmin.

15. Conclusion

Mastering these core commands equips you to manage any Linux server efficiently—whether you’re troubleshooting a single machine or scripting large‑scale deployments. Practice them in a test environment first, then integrate them into your daily workflow. Over time, the command line will become an intuitive extension of your admin toolkit rather than a chore.

Happy administering!

Frequently Asked Questions (FAQ)

1. What is the fastest way to create a new user with sudo privileges?

Use useradd with the -G option to add the user to the sudo group and set a password:

sudo useradd -m -s /bin/bash -G sudo alice
sudo passwd alice

The -m flag creates the home directory, and adding the user to sudo grants administrative rights.

2. How can I quickly find all .conf files on my system while ignoring permission errors?

Run find with redirection of standard error:

sudo find / -type f -name '*.conf' 2>/dev/null

The 2>/dev/null discards “Permission denied” messages, giving you a clean list of configuration files.

3. What command shows all listening sockets on the system?

Use ss, which is the modern replacement for netstat:

ss -tuln | grep LISTEN

The flags display TCP/UDP sockets (-t -u), only listening ones (-l), and numeric addresses (-n).

4. How do I archive recent logs into a compressed tarball with a date‑stamped filename?

Create the tarball in one command:

tar czf /backup/logs_$(date +%Y%m%d).tar.gz -C /var/log .

-c creates, -z compresses with gzip, and -f names the file using the current date.

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