SSH Remote Command Execution

Basic SSH Connection

# Basic SSH connection
ssh username@hostname
ssh -p 2222 username@hostname    # Connect to specific port
ssh -i /path/to/key.pem username@hostname    # Use private key

Remote Command Execution Syntax

# Basic remote command execution
ssh username@hostname 'command'
ssh username@hostname "command with spaces"
 
# Multiple commands
ssh username@hostname 'command1 && command2'
ssh username@hostname 'command1; command2'    # Execute sequentially regardless of success

Environment Variables

# Using local environment variables
ssh username@hostname "MYVAR=$MYVAR command"
ssh username@hostname 'export MYVAR=value; command'
 
# Preserving environment variables
ssh -t username@hostname 'bash -l -c "command"'

File Operations

# Execute local script on remote host
ssh username@hostname 'bash -s' < local_script.sh
 
# Combine with sudo
ssh username@hostname 'sudo bash -s' < local_script.sh
 
# Execute and redirect output
ssh username@hostname 'command' > local_output.txt

Interactive Sessions

# Force pseudo-terminal allocation
ssh -t username@hostname 'command'
 
# Keep session alive
ssh -o ServerAliveInterval=60 username@hostname

Background Processes

# Run command in background
ssh username@hostname 'nohup command &'
 
# Run command that survives SSH session
ssh username@hostname 'screen -dm command'
ssh username@hostname 'tmux new-session -d "command"'

SSH Options

# Disable strict host key checking
ssh -o StrictHostKeyChecking=no username@hostname 'command'
 
# Compression for slow connections
ssh -C username@hostname 'command'
 
# Quiet mode
ssh -q username@hostname 'command'

X11 Forwarding

# Enable X11 forwarding
ssh -X username@hostname 'gui_application'
ssh -Y username@hostname 'gui_application'    # Trusted X11 forwarding

Port Forwarding

# Local port forwarding
ssh -L local_port:remote_host:remote_port username@hostname
 
# Remote port forwarding
ssh -R remote_port:local_host:local_port username@hostname

Performance Optimization

# Enable compression
ssh -C username@hostname 'command'
 
# Use faster ciphers
ssh -c [email protected] username@hostname 'command'

Security

# Use specific identity file
ssh -i ~/.ssh/specific_key username@hostname 'command'
 
# Specify cipher
ssh -c [email protected] username@hostname 'command'
 
# Use specific SSH key algorithm
ssh -o PubkeyAcceptedKeyTypes=ssh-rsa username@hostname 'command'

Troubleshooting

# Verbose output for debugging
ssh -v username@hostname 'command'    # Verbose
ssh -vv username@hostname 'command'   # More verbose
ssh -vvv username@hostname 'command'  # Even more verbose
 
# Check connection without executing command
ssh -q username@hostname 'exit 0'

Best Practices

1. Always use quotes around remote commands
2. Use double quotes when variable expansion is needed
3. Use single quotes for literal strings
4. Consider using nohup or screen/tmux for long-running processes
5. Always handle error cases in scripts
6. Use SSH config file for frequently used connections
7. Implement proper key management
8. Use SSH agent for key management

SSH Config File Example

# ~/.ssh/config
Host myserver
    HostName server.example.com
    User username
    Port 2222
    IdentityFile ~/.ssh/specific_key
    ServerAliveInterval 60