It is the (social contract) responsibility of the programmer to document in the usage and in the manual page that -u takes a username as an argument, and where

It is the (social contract) responsibility of the programmer to document in the usage and in the manual page that -u takes a username as an argument, and where such documentation exists, the convention is that username may be provided as -uuser or -u user. This is trivially achievable with getopts(1): while getopts u: Option do case "$Option" in u) User="$OPTARG" ;; esac done The colon tells getopts(1) that the -u option expects an argument. This behavior is also mandated by the POSIX specification.