conditional bash script that takes one argument and checks for an existing executable

build.sh

#!/usr/bin/env bash
export PATH="$PATH:/usr/texbin/"
which pandoc &>/dev/null
if [ $? -eq 0 ]
then
    if [ ! -n "$1" ]
    then
        echo 'defaulting to pdf'
        file_name='scalingphp.pdf'
    else
        file_name="scalingphp.$1"
    fi
    files=$(ls *.md | grep -vi sample | grep -v README | sort | xargs)
    pandoc $files -o $file_name
else
    echo "you need to install pandoc"
fi