logname: Print current login name
logname prints the calling user’s name, as found in a
system-maintained file (often /var/run/utmp or
/etc/utmp), and exits with a status of 0. If there is no entry
for the calling process,
an error message and exits with a status of 1.
The only options are --help and --version. See Common options.
An exit status of zero indicates success, and a nonzero value indicates failure.