SSHAGENTPID

SSH_AGENT_PID is the environment variable used by OpenSSH to store the process ID of the ssh-agent running for the current user session. The canonical name is SSH_AGENT_PID (with an underscore); some sources or typings may refer to it informally as SSHAGENTPID, but the variable in practice is SSH_AGENT_PID. It is set when ssh-agent is started and its value is exported to the shell so that SSH clients can locate and communicate with the agent.

When a user starts ssh-agent, typically via commands like eval "$(ssh-agent -s)" or by using a session manager, the agent outputs a line that assigns SSH_AGENT_PID to the agent’s process ID and SSH_AUTH_SOCK to the path of the agent’s Unix domain socket. Exporting these variables allows subsequent SSH tools to find the agent without knowing its specific PID or socket name.

The SSH_AGENT_PID value is primarily used to manage the agent process. Programs can read the PID to send signals to the agent, for example to terminate it with a controlled shutdown using kill "$SSH_AGENT_PID". It also serves as a sanity check: if the PID no longer corresponds to a running ssh-agent, scripts may need to restart the agent or refresh environment variables.

Security considerations include protecting the agent’s socket path (SSH_AUTH_SOCK) and ensuring environment variables are not exposed to untrusted processes. If the agent dies or is restarted, SSH_AGENT_PID changes and users should reinitialize their environment accordingly.

See also: SSH_AUTH_SOCK, ssh-agent, ssh-add, key management tools.