MacPorts and /opt/local on Catalina and Big Sur read only volumes

Tabitha McNerney tabithamc at
Sat Jun 26 02:03:23 UTC 2021

On a fresh Catalina or Big Sure system, if you cd to root / then sudo then
try mkdir /opt or something else such as mkdir /hello the system won't
allow it, I get this:

mkdir: /hello: Read-only file system

note: the MacBook I just tried this on also has FileVault enabled and its
got one of those Apple T2 chips with a touch bar

How does the MacPorts Catalina or Big Sur pkg installer work around this

On Fri, Jun 25, 2021 at 3:26 PM Ryan Schmidt <ryandesign at>

> On Jun 25, 2021, at 18:07, Tabitha McNerney wrote:
> > Hi all,
> >
> > I haven't installed a fresh MacPorts system in quite some time but will
> soon be doing so on a few Macs one running Catalina and the other Big Sur.
> Starting with Catalina, the root volume / is read-only so how do the
> MacPorts package installers set things up such that /opt/local can remain
> the default path to MacPorts for both read and write functionality on
> Catalina and Big Sur? Do the MacPorts package installers make use of
> Apple's new bi-directional firmlinks capability defined in
> /etc/synthetic.conf as also described on this page?
> >
> >
> >
> > Thank you.
> I'm not aware of MacPorts doing anything special for this. It just works?
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