How to use the command "ssh-agent" (with examples)

The ssh-agent command is a useful tool for managing SSH keys. It spawns an SSH Agent process that securely holds SSH keys decrypted in memory until they are removed or the process is killed. This allows users to conveniently manage their keys and establish secure connections to remote servers without having to repeatedly enter their passphrase.

In this article, we will explore two common use cases of the ssh-agent command and provide code examples to illustrate each one. The first use case will demonstrate how to start an SSH Agent for the current shell, and the second use case will show how to kill the currently running agent.

Use Case 1: Starting an SSH Agent for the Current Shell

To start an SSH Agent for the current shell, we can use the eval $(ssh-agent) command. This command will start the agent process, set the necessary environment variables, and output the necessary commands to our shell for evaluation.

Motivation: Starting an SSH Agent is useful when we want to manage our SSH keys without having to repeatedly enter the passphrase for each key. By starting an agent, we can add our keys to it and establish SSH connections without having to provide the passphrase every time.

Explanation: The eval command evaluates the output of the ssh-agent command and runs it as a series of shell commands. This allows the necessary environment variables to be set and the agent process to be started for the current shell session.

Example:

eval $(ssh-agent)

Output:

Agent pid 1234

In this example, starting the SSH Agent process creates a new agent with a process ID of 1234. The agent is now ready to manage our SSH keys securely.

Use Case 2: Killing the Currently Running Agent

To kill the currently running SSH Agent, we can use the ssh-agent -k command. This command will send a signal to the agent process to terminate it and remove the keys from memory.

Motivation: Sometimes we may need to stop the SSH Agent process, for example, if we no longer need to use SSH keys or if we want to ensure that our keys are not kept in memory unnecessarily. By killing the agent, we can terminate the processes and remove sensitive information from memory.

Explanation: The -k option is used to specify that we want to kill the SSH Agent process. When this option is provided, ssh-agent will send a signal to the running agent process to terminate it gracefully.

Example:

ssh-agent -k

Output:

Agent pid 1234 killed.

In this example, killing the SSH Agent process with PID 1234 terminates the agent and removes the keys from memory. The agent is no longer active, and any SSH connections relying on it will fail.

Conclusion

The ssh-agent command is a versatile tool for managing SSH keys securely. In this article, we explored two common use cases: starting an SSH Agent for the current shell and killing the currently running agent. We provided code examples, motivations, explanations, and example outputs to illustrate each use case. Incorporating ssh-agent in our SSH workflow can greatly improve convenience and security when working with remote servers.