krillancello@hjuki ~ $ cat bin/zdoom.sh                                      
#!/bin/bash
DOOMPATH="."
DOOMPATH="${DOOMPATH} /opt/zdoom/"
DOOMPATH="${DOOMPATH} /usr/share/games/doom-data/"
DOOMPATH="${DOOMPATH} /usr/local/games/doom-data/"
DOOMPATH="${DOOMPATH} ${HOME}/.zdoom/"

EXPANEXT="wad"
EXPANEXT="${EXPANEXT} pk3"
#EXPANEXT="${EXPANEXT} zip"
EXPANEXT="${EXPANEXT} deh"

FINDPARAM="-maxdepth 1 -type f \\( "
#FINDPARAM="\\( -maxdepth 1 -type f -iregex '.*\\.\\("

for EXT in ${EXPANEXT}; do
        if [ -z "${FINDEXPR}" ]; then
                FINDEXPR="-iname '*.${EXT}'"
#               FINDEXPR="${EXT}"
        else
                FINDEXPR="${FINDEXPR} -or -iname '*.${EXT}'"
#               FINDEXPR="${FINDEXPR}\\|${EXT}"
        fi
        if [ -z "${EXTLIST}" ]; then
                EXTLIST="'${EXT}'"
        else
                EXTLIST="${EXTLIST}, '${EXT}'"
        fi
done

FINDPARAM="${FINDPARAM}${FINDEXPR} \\) -printf '%f\\t'"
#FINDPARAM="${FINDPARAM}${FINDEXPR}\\)$' \\) -printf '%f\\t'"

for DIR in ${DOOMPATH}; do
        if [ -z "${PATHLIST}" ]; then
                PATHLIST="'${DIR}'"
        else
                PATHLIST="${PATHLIST}, '${DIR}'"
        fi
done

echo "Available expansion files (${EXTLIST}) in known directories (${PATHLIST}):"
echo "find ${DOOMPATH} ${FINDPARAM}"
find ${DOOMPATH} ${FINDPARAM}
echo

krillancello@hjuki ~ $ zdoom.sh
Available expansion files ('wad', 'pk3', 'deh') in known directories ('.', '/opt/zdoom/', '/usr/share/games/doom-data/', '/usr/local/games/doom-data/', '/home/krillancello/.zdoom/'):
find . /opt/zdoom/ /usr/share/games/doom-data/ /usr/local/games/doom-data/ /home/krillancello/.zdoom/ -maxdepth 1 -type f \( -iname '*.wad' -or -iname '*.pk3' -or -iname '*.deh' \) -printf '%f\t'
find: paths must precede expression
Usage: find [path...] [expression]

krillancello@hjuki ~ $ find . /opt/zdoom/ /usr/share/games/doom-data/ /usr/local/games/doom-data/ /home/krillancello/.zdoom/ -maxdepth 1 -type f \( -iname '*.wad' -or -iname '*.pk3' -or -iname '*.deh' \) -printf '%f\t'
freedoom.wad    freedoom_sprites.wad    freedoom_textures.wad   freedoom_graphics.wad   freedoom_levels.wad     freedoom_sounds.wad     doom1.wad       doom2.wad       doomu.wad       cube.wad        ww-ana.wad      Wolf3D.pk3      zdoomcmp1.wad   CZ_181b.wad     Mock2.wad       doom2.wad       krillancello@hjuki ~ $