Command-line tool existsuser is enhanced by the option -d/--displayname and now has following usage:

existsuser
-h,--help Prints a help text
--environment Show info about commandline environment
--nonl Remove all newlines (\n) from output
--responsetimeout <responsetimeout> response timeout in seconds for reading response from the backend (default 0s; infinite)
-c,--contextid <contextid> * The id of the context
-A,--adminuser <adminuser> ? Admin username
-P,--adminpass <adminpass> ? Admin password
-i,--userid <userid> | Id of the user
-u,--username <username> | Username of the user
-d,--displayname <displayname> | Display name of the user

Entries marked with an asterisk (*) are mandatory.
Entries marked with an question mark (?) are mandatory depending on your
configuration.
Entries marked with a pipe (|) are mandatory for one another which means that
at least one of them must be set.