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ssh-add – Add private key to ssh-agent’s cache

The ssh-add command is used to add private keys to the ssh-agent’s cache. The ssh-agent is a program that runs on your local machine and stores your ssh keys. The keys are then used to authenticate you when you connect to a remote server using ssh. By default, ssh will look for your keys in the ~/.ssh directory. However, if you have a passphrase on your key, you will need to enter it each time you use the key. This is where ssh-add comes in. It adds your key to the ssh-agent’s cache so that you only need to enter your passphrase once.

Overview

The ssh-add command has a few different options that can be used to add keys to the ssh-agent’s cache. Here are some examples:

# Add the default key (~/.ssh/id_rsa)
ssh-add

# Add a specific key
ssh-add ~/.ssh/my_key

# Add a key with a passphrase
ssh-add -K ~/.ssh/my_key

The first example will add the default key (~/.ssh/id_rsa) to the ssh-agent’s cache. The second example will add a specific key (~/.ssh/my_key) to the cache. The third example will add a key (~/.ssh/my_key) to the cache and store the passphrase in your keychain (macOS only).

You can also use the -t option to specify how long the key should be added to the cache. For example, the following command will add the key for 1 hour:

ssh-add -t 3600 ~/.ssh/my_key

If you have multiple keys and want to list them, you can use the -l option:

ssh-add -l

If you want to remove a key from the cache, you can use the -d option:

ssh-add -d ~/.ssh/my_key

Options

The ssh-add command has the following options:

Option Description
-t Set a lifetime for the key in seconds
-l List the keys currently held by the agent
-d Remove a key from the agent
-K Store the passphrase in your keychain (macOS only)
-E Specify a new passphrase when adding the key

Troubleshooting tips

If you are having trouble adding a key to the ssh-agent’s cache, here are some things to check:

  • Make sure the key exists and is in the correct location (~/.ssh/ by default).
  • Make sure the key has the correct permissions (chmod 600 ~/.ssh/my_key).
  • Make sure the ssh-agent is running (eval "$(ssh-agent -s)").
  • Make sure the key is not already in the cache (ssh-add -l).
  • If you have a passphrase on your key, make sure you enter it correctly.

Notes

  • If you use ssh-add to add a key to the ssh-agent’s cache, it will only be available until you log out or restart your computer. To make it persistent, you can add the key to your ssh config file (~/.ssh/config) or add it to the ssh-agent on startup.
  • If you have multiple keys and want to use a specific key for a connection, you can use the -i option with ssh (ssh -i ~/.ssh/my_key user@host).

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