Logname

Logname is a Unix-like command that prints the login name of the user who started the current login session. It is often used in shell scripts to determine the original user associated with a session, especially in environments where privilege escalation or user switching occurs. The command outputs a single line containing the login name, followed by a newline, and typically exits with status 0 on success or a nonzero status on failure.

Implementation and behavior vary by system. On many Unix-like systems, logname obtains the login name from the session accounting data, such as the utmp or utmpx file that records active logins, or from environment variables like LOGNAME or USER when those sources are unavailable. This makes logname distinct from commands that report the current effective user, such as whoami or id -un.

Because logname reflects the login session rather than the effective user ID, its value may differ from the user shown by privileges tools after actions like su or sudo, or when the session was not started as a true login. In cases where login information cannot be determined, logname may print nothing and return a nonzero exit status.

Typical usage is simple: logname. It is commonly included in scripts to preserve or verify the original user context, especially in automated tasks that may involve switching identities during execution.

See also: whoami, id, who, tty, utmp.