mongodump should prompt for username when --authenticationMechanism=GSSAPI and --username is not provided

I want to treat my kerberos username as if it were a password, and not have it display on a "ps -ef" output, so i want mongodump to prompt me for the username when i fail to provide it on the command line. This would be the same as mongodump does for MONGODB-CR when given the username without a password.Here is what it would look like in my script:

mongodump --host hostname --port port# --authenticationMechanism=GSSAPI --authenticationDatabase='$external' --username --out backupdir <<EOF
provide-my-kerberos-id-here
EOF