The result of the authentication by the command is shown in its output string or by its exit code. The command may puts a string to its standard output to show the result in the form of a status response of the FTP protocol, that is, "230" for success and "530" for failure. Otherwise the exit code of the process is used, the value zero for success and non-zero values for failure.
Example: passing username in argument while password in environment variable
[the content of the myauth command]
if [ "$1" = "user1" -a "$MYPASS" = "pass1" ]; then
echo "230 SUCCESS"
echo "530 FAILURE"