Darren Soothill

Useful Commands

A collection of practical Linux commands for system administration and testing

About This Page

This page contains a curated collection of useful Linux commands for system administration, performance testing, and troubleshooting. Click on any command section below to expand and view detailed examples and explanations.

FIO (Flexible I/O Tester)

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FIO is a versatile tool for benchmarking and testing storage devices. It provides comprehensive I/O performance testing capabilities with extensive configuration options.

Installation

# Install FIO on Ubuntu/Debian
sudo apt update
sudo apt install fio

# Install FIO on RHEL/CentOS
sudo yum install fio

# Verify installation
fio --version

Fill a Disk

Use FIO to fill a disk with data for testing purposes.

Sequential Write (Fastest)

# Fill entire disk with sequential writes
sudo fio --name=filltest --filename=/dev/nvme0n1 --rw=write --bs=1M --direct=1 --size=100% --numjobs=1

# Fill with larger block size for maximum speed
sudo fio --name=filltest --filename=/dev/nvme0n1 --rw=write --bs=4M --direct=1 --size=100% --numjobs=1

Random Write (More Realistic)

# Fill disk with random data (more realistic workload)
sudo fio --name=filltest --filename=/dev/nvme0n1 --rw=randwrite --bs=4k --direct=1 --size=100% --numjobs=1

# Fill with multiple jobs for parallel writes
sudo fio --name=filltest --filename=/dev/nvme0n1 --rw=randwrite --bs=4k --direct=1 --size=100% --numjobs=4

⚠️ Data Loss Warning

CRITICAL: The commands above will PERMANENTLY OVERWRITE ALL DATA on the specified device. Always double-check the device path before running these commands.

FIO Parameters Explained

  • --name: Job name for identification in output
  • --filename: Device or file to test
  • --rw: I/O pattern - write, randwrite, read, randread
  • --bs: Block size - 4k, 1M, etc.
  • --direct: Use direct I/O (1 = bypass OS cache)
  • --size: Amount of data - 100%, 50G, etc.
  • --numjobs: Number of parallel jobs

Additional FIO Examples

Random Read Performance Test

# Test random read performance
sudo fio --name=randread --filename=/dev/nvme0n1 --rw=randread --bs=4k --direct=1 --numjobs=4 --time_based --runtime=60

Mixed Workload Test

# 70% read, 30% write mixed workload
sudo fio --name=mixed --filename=/dev/nvme0n1 --rw=randrw --rwmixread=70 --bs=4k --direct=1 --numjobs=4 --time_based --runtime=60

✓ Best Practices

  • Always verify device paths using lsblk before running write operations
  • Unmount any mounted partitions before testing
  • Use --direct=1 to bypass OS cache for accurate results
  • Start with small test sizes before running full disk operations

Smartmontools - Disk Health Monitoring

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Smartmontools is a suite of utilities for monitoring and analyzing the health of storage devices using SMART (Self-Monitoring, Analysis and Reporting Technology).

Installation

# Install on Ubuntu/Debian
sudo apt update
sudo apt install smartmontools

# Install on RHEL/CentOS
sudo yum install smartmontools

# Verify installation
smartctl --version

Setting Up Automatic Monitoring

Configure smartd to automatically detect all devices and send email alerts.

Configure smartd Daemon

# Backup original configuration
sudo cp /etc/smartd.conf /etc/smartd.conf.backup

# Edit configuration file
sudo nano /etc/smartd.conf

Add Configuration

# Monitor all devices, run short self-test daily, long test weekly
# Send email alerts on any SMART errors
DEVICESCAN -a -o on -S on -s (S/../.././02|L/../../6/03) -m darren@soothill.com -M exec /usr/share/smartmontools/smartd-runner

Enable and Start smartd

# Enable smartd to start on boot
sudo systemctl enable smartd

# Start smartd service
sudo systemctl start smartd

# Check service status
sudo systemctl status smartd

Manual Device Testing

# Check if SMART is available and enabled
sudo smartctl -i /dev/sda

# Check overall health status
sudo smartctl -H /dev/sda

# View all SMART attributes
sudo smartctl -A /dev/sda

# Start a short self-test
sudo smartctl -t short /dev/sda

# Start a long self-test
sudo smartctl -t long /dev/sda

# View self-test results
sudo smartctl -l selftest /dev/sda

✓ Best Practices

  • Test email functionality with -M test before relying on alerts
  • Use -M diminishing to avoid alert fatigue
  • Schedule long tests during off-peak hours
  • Monitor smartd logs: sudo journalctl -u smartd

SSH Connection Fix - Legacy Algorithms

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Fix SSH connection issues with legacy servers that use outdated key exchange and host key algorithms.

Common Error Messages

Unable to negotiate with 10.10.201.2 port 22: no matching key exchange method found.
Their offer: diffie-hellman-group1-sha1,diffie-hellman-group14-sha1

Unable to negotiate with 10.10.201.2 port 22: no matching host key type found.
Their offer: ssh-rsa

Quick Fix (One-time Connection)

ssh -oKexAlgorithms=+diffie-hellman-group14-sha1 -oHostKeyAlgorithms=+ssh-rsa user@10.10.201.2

Alternative with Additional Legacy Support

ssh -oKexAlgorithms=+diffie-hellman-group14-sha1,diffie-hellman-group1-sha1 \
    -oHostKeyAlgorithms=+ssh-rsa \
    -oPubkeyAcceptedKeyTypes=+ssh-rsa \
    user@10.10.201.2

Permanent Fix (SSH Config)

Add to ~/.ssh/config:

# For specific host
Host 10.10.201.2
    KexAlgorithms +diffie-hellman-group14-sha1,diffie-hellman-group1-sha1
    HostKeyAlgorithms +ssh-rsa
    PubkeyAcceptedKeyTypes +ssh-rsa

# For entire subnet
Host 10.10.201.*
    KexAlgorithms +diffie-hellman-group14-sha1,diffie-hellman-group1-sha1
    HostKeyAlgorithms +ssh-rsa
    PubkeyAcceptedKeyTypes +ssh-rsa

Create SSH Config File

# Create .ssh directory if it doesn't exist
mkdir -p ~/.ssh
chmod 700 ~/.ssh

# Create or edit config file
nano ~/.ssh/config

# Set proper permissions
chmod 600 ~/.ssh/config

⚠️ Security Warning

Important: These algorithms are disabled by default because they have known security weaknesses:

  • diffie-hellman-group1-sha1: Vulnerable to logjam attacks
  • ssh-rsa with SHA-1: SHA-1 hash collisions have been demonstrated

Only re-enable these if: You're on a trusted network, connecting to legacy equipment that cannot be updated, and you understand the risks.

✓ Best Practices

  • Use SSH config file for regular connections
  • Limit legacy algorithm support to specific hosts
  • Document why you need legacy support
  • Use VPN when accessing legacy systems over untrusted networks

📌 More Commands Coming Soon

Additional useful commands will be added to this page. Check back regularly for updates including networking tools, monitoring commands, and more storage utilities.