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CompTIA Linux+ XK0-005 - 2.4 - SSH: Commands - ssh-add

The ssh-add command is a tool used to manage SSH identities (private keys) and add them to the authentication agent. It allows users to securely store their private keys in memory during a session, eliminating the need to repeatedly enter passphrases when authenticating with SSH.

Purpose of the command

The main purpose of the ssh-add command is to add private keys to the SSH authentication agent, enabling seamless and passwordless authentication for SSH connections. By adding private keys to the agent, users can avoid entering passphrases for their keys each time they establish an SSH connection.

Key Command Options

Using ssh-add involves a few simple steps. Here's the basic syntax of the command:

ssh-add [options] [identity_file]

The command can be used with various options to perform specific tasks. Here are some commonly used options:

  • -l: Lists all the identities currently added to the agent.
  • -d: Deletes a specific identity from the agent.
  • -D: Deletes all identities from the agent.
  • -t: Sets the maximum lifetime for an identity added to the agent.
  • -c: Enables confirmation prompts before using an identity.

Example Command Usage

To use ssh-add, follow these steps:

  • If you have a private key protected with a passphrase, you may need to unlock it first. Run the following command, replacing identity_file with the path to your private key file:

    ssh-add ~/.ssh/id_rsa

    You will be prompted to enter the passphrase for the private key.

  • To list the identities currently added to the agent, run:

    ssh-add -l

    This will display a list of the identities along with their key fingerprint information.

  • To delete a specific identity from the agent, run:

    ssh-add -d ~/.ssh/another_key

    Replace ~/.ssh/another_key with the path to the private key file you want to remove from the agent.

  • To delete all identities from the agent, run:

    ssh-add -D

    This will remove all the private keys from the agent.

Conclusion

The ssh-add command is a useful tool for managing SSH identities and adding them to the authentication agent. By using this command, users can conveniently store and manage their private keys during a session, enabling passwordless authentication for SSH connections. Understanding the usage of ssh-add is essential for Linux administrators and users who frequently work with SSH and want to streamline the authentication process.