username=$1
if [ -n "$username" ]
then
    echo username: "$username"
fi

$ echo $SHELL
/usr/bin/ksh
$ sh foo.sh humpty
username: humpty
$ sh foo.sh =
foo.sh[2]: test: 0403-004 Specify a parameter with this command.
$ ksh foo.sh =
foo.sh[2]: test: 0403-004 Specify a parameter with this command.
$ ksh93 foo.sh =
username: =
$ bash foo.sh =
username: =

$ ksh93 -c 'echo ${.sh.version}'
Version M-12/28/93e
$ bash --version
GNU bash, version 3.2.16(1)-release (powerpc-ibm-aix5.2.0.0)
Copyright (C) 2005 Free Software Foundation, Inc