helpmessage from ln when used without parameter or with --help

[emdete]

i added some checks to solve it:

#!/bin/sh -e
# xdowngrade PKGS.xbps... - install XBPS package directly from .xbps file
help() {
    echo "Install XBPS package directly from .xbps file."
    echo "usage: $0 PKGS.xbps ..."
    rm -r $T
    exit 0
}
T=$(mktemp --tmpdir -d xdowngrade-$(date -I).XXXXXXXX)
pkg=
for f; do
    case $f in
        *.xbps)
            ln -s -r -t $T/ "$f"
            f=${f##*/}
            f=${f%.*.*}
            pkg="$pkg ${f%-*}"
        ;;
        *)
            echo "Wrong argument $f."
            help
        ;;
    esac
done
if [ ! -z "$pkg" ]; then
    xbps-rindex -a $T/*
    xbps-install -i -R $T -f $pkg
else
    help
fi
rm -r $T
exit 0