rexenv won't open

Almost every “it won’t open” is Gatekeeper, and it is fixed once, permanently, in about five seconds.

“rexenv can’t be opened” / “unidentified developer”

This is expected on the disk-image install. rexenv is ad-hoc code-signed and not yet notarized by Apple, so Gatekeeper blocks the first plain double-click.

Right-click (or Control-click) rexenv in Applications → Open → Open again in the dialog.

If you already double-clicked and got blocked: System Settings → Privacy & Security, scroll down to the rexenv message, Open Anyway, then Open.

After once, rexenv opens normally from then on.

Installing with Homebrew skips this

The cask removes the quarantine attribute during install, so there is no first-launch dialog at all — and rex lands on your PATH while it is there. See Install.

“The app is damaged and can’t be opened”

Same cause, different wording: macOS is reporting the quarantine attribute on a downloaded file, not actual damage. The right-click → Open above clears it.

If you want to confirm nothing is actually wrong with the download, check it against the published checksum:

shasum -a 256 rexenv_*_universal.dmg

and compare with the .dmg.sha256 published next to it.

Nothing happens at all

Check your macOS version — rexenv needs macOS 15 (Sequoia) or later:

sw_vers -productVersion

Below 15, rexenv installs and then cannot run its own web server, because the pinned nginx and cloudflared builds will not launch there. See Requirements.

It opens, but nothing will start

That is a different problem from this page — the app is running and something in the stack is refusing. From a terminal:

rex doctor

It reports what is wrong and, where a fix exists, what to do about it. The most common answer is that another program already holds port 80 or 443 — often a system nginx, or another local dev environment that is still running.

Still stuck

Open an issue at github.com/rexenv, and include the output of rex doctor and your sw_vers -productVersion. Those two answer most of the first round of questions.