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I happen to encounter the same.
Inspecting uglifyjs -h showed that uglifyjs doesn't know what --output=elm.min.js means, but it perfectly understands --output elm.min.js
so the .sh should look like this:

#!/bin/sh

set -e

js="elm.js"
min="elm.min.js"

elm make --optimize --output=$js $@

uglifyjs $js --compress "pure_funcs=[F2,F3,F4,F5,F6,F7,F8,F9,A2,A3,A4,A5,A6,A7,A8,A9],pure_getters,keep_fargs=false,unsafe_comps,unsafe" | uglifyjs --mangle --output $min

echo "Initial size: $(cat $js | wc -c) bytes  ($js)"
echo "Minified size:$(cat $min | wc -c) bytes  ($min)"
echo "Gzipped size: $(cat $min | gzip -c | wc -c) bytes"

I hope it helps ;]

I ran into this exact problem some time ago when I was first learning Elm. I eventually figured it out. Is there a PR for this?

I can also confirm this works:

uglifyjs elm.js  --compress "pure_funcs=[F2,F3,F4,F5,F6,F7,F8,F9,A2,A3,A4,A5,A6,A7,A8,A9],pure_getters,keep_fargs=false,unsafe_comps,unsafe" | uglifyjs --mangle --output elm.min.js

It should be the same on every system as (if memory serves) it is handled by uglifyjs directly. So I will add a PR.

Thanks to @Caleb-Irwin for the fix.

No problem! Happy to help Elm.