ensure single running instance of a shell script

gistfile1.sh

if command -v flock > /dev/null 2>&1; then
    flock -nx "$LOCK" || exit
else
    trap 'rm -f "$LOCK"; exit' INT QUIT TERM
    shlock -f "$LOCK" -p $$ || exit
    trap 'rm -f "$LOCK"; exit' EXIT
fi