Essential Linux Commands for DevOps Engineers

As a DevOps engineer, mastering Linux commands is crucial for efficient system troubleshooting and management.

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1. System Monitoring & Performance

CPU Usage:

• top – Real-time process and resource usage.
• htop – Enhanced process viewer.
• sar – Historical resource usage.

Memory Usage:

• free -h – Memory usage overview.
• cat /proc/meminfo – Detailed memory statistics.

Disk Usage:

• df -h – Disk usage by filesystem.
• du -sh <path> – Directory size.

Network Usage:

• netstat -tuln or ss -tuln – Active connections.
• iftop – Real-time bandwidth usage.
• ping <IP/hostname> – Connectivity check.

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2. File and Directory Management

File Management:

• ls -lh – List files with details.
• cat, less, more – View file content.
• find /path -name “*.log” – Search files.

Permissions:

• chmod 755 <file> – Change file permissions.
• chown user:group <file> – Change ownership.

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3. User and Group Management

Users:

• id <username> – User details.
• adduser <username> – Add a user.
• passwd <username> – Change user password.

Groups:

• groups <username> – List user groups.
• usermod -aG <group> <username> – Add user to a group.

Logged-In Users:

• who – List logged-in users.
• last – Login history.

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4. Process Management

View Processes:

• ps aux – List processes.
• pgrep <name> – Search for a process.

Manage Processes:

• kill -9 <PID> – Kill a process by ID.
• pkill <name> – Kill a process by name.

Logs:

• journalctl – View system logs.
• dmesg – Kernel messages.
• tail -f /var/log/<file> – Monitor logs in real-time.

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5. Networking

Connectivity:

• curl -I <url> – Test HTTP connection.
• ping <IP/hostname> – Test network connectivity.
• traceroute <hostname> – Trace network route.

Debugging:

• telnet <host> <port> – Test open ports.
• dig <hostname> – DNS lookup.

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6. Disk and Filesystem Management

Filesystem:

• fsck /dev/sda1 – Check filesystem.
• mount and umount – Mount/unmount filesystems.

Disk Partitioning:

• fdisk -l – Partition details.
• lsblk – List block devices.

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7. Package Management

Debian/Ubuntu:

• apt update – Update package list.
• apt install <package> – Install a package.

RHEL/CentOS:

• yum update – Update packages.
• yum install <package> – Install a package.

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8. Backup and Archive

Backup:

• rsync -av /source/ /destination/ – Sync files/directories.

Archiving:

• tar -cvf archive.tar /path – Archive files.
• gzip archive.tar – Compress the archive.

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10. General Troubleshooting Commands

• uptime – System uptime.
• uname -a – Kernel and OS details.
• tcpdump – Network packet capture.

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End-to-End Troubleshooting Example

1. Identify the Issue:
   Use commands such as top, df -h, or ping to gather initial insights.

2. Narrow Down Root Cause:
   Utilize ps aux, netstat, or iotop to trace the source of the issue.

3. Apply Fixes:
   Restart services or update packages as necessary.

4. Monitor Post-Fix:
   Continuously monitor the system with journalctl or tail -f /var/log/<file> to ensure the issue is resolved.