ssh

Overview

The ssh (Secure Shell) command is used to securely log into remote machines and execute commands. It provides encrypted communication between two hosts over an insecure network.

Syntax

ssh [options] [user@]hostname [command]

Common Options

Option Description
-p port Port to connect to on the remote host
-i identity_file Selects a file from which the identity key is read
-L port:host:hostport Local port forwarding
-R port:host:hostport Remote port forwarding
-X Enables X11 forwarding
-v Verbose mode
-q Quiet mode

Key Use Cases

  1. Remote system administration
  2. Secure file transfers
  3. Port forwarding
  4. Remote command execution
  5. Tunneling applications

Examples with Explanations

Example 1: Basic Connection

ssh user@remote.host

Connects to remote.host as ‘user’

Example 2: Run Remote Command

ssh user@remote.host 'ls -l'

Executes ‘ls -l’ on remote host and returns output

Example 3: Port Forwarding

ssh -L 8080:localhost:80 user@remote.host

Forwards local port 8080 to port 80 on remote host

Understanding Output

  • Connection messages
  • Host key verification
  • Authentication methods
  • Warning and error messages
  • Remote command output

Common Usage Patterns

  1. Key-based authentication:

    ssh-keygen -t rsa
ssh-copy-id user@remote.host

  2. Configuration via ~/.ssh/config:

    Host alias
    HostName remote.host
    User username
    Port 22

  3. Persistent connections:

    ssh -o ServerAliveInterval=60 user@remote.host

Performance Analysis

  • Use compression (-C) for slow connections
  • Enable multiplexing for multiple sessions
  • Use ControlMaster for connection sharing
  • Configure proper timeout values

Additional Resources