Migrating Homebrew install from one Macbook to another (both Intel) with brew bundle: what's a Brewfile, and how to migrate from source filesystem (of a dead MacBook)?

[johnnyutahh]

I want to minmally/efficiently understand macOS /usr/local (presumably where all of Homebrew downloads/builds/etc live... yes?) filesystem layout/structure/etc in order to migrate a /usr/local from one Macbook to another, both running macOS v10.15.7, but with different hardware configurations (both running Intel-based CPUs/cores)... IF a migration (or a /usr/local merge of some kind?) is feasible.

QUESTION: can someone provide guidance for more efficient understanding/learning (for the above purpose)?

Something other than man brew (which appears to go deeper than I need to understand the migration maneuver)?

My background

I am a long-time (over a decade) macOS-Homebrew user. (I've also dabbled a little with Linux-based Homebrew installs as well.) I know enough to install all sorts of brew-based software (including running services like my own DNS server on macOS), but I'm NOT familiar with the guts of Homebrew. Pls do not treat me as a first-time user; rather please consider me a first time "Homebrew 'guts' evaluator"... or something like that.

I certainly have never written a Homebrew software package/formula/etc. I have developed many software packages across many other OS platforms (numerous Unix-es, Linux, Windows, etc), so I am far from "new" to software development and deployment... except on macOS and Homebrew (both).

Related discussion

https://www.reddit.com/r/MacOS/comments/xh59gd/trying_to_learn_homebrewmacos_usrlocal_layout_for/

[carlocab]

This doesn't answer your question about the structure of directories inside HOMEBREW_PREFIX, but perhaps you'd like to consider using brew bundle to migrate your Homebrew installation from one Mac to another.

Using brew bundle does not require any knowledge of the aforementioned structure.

[johnnyutahh]

brew bundle looks promising, thank you! After skimming https://github.com/Homebrew/homebrew-bundle and brew help bundle the first, key question I have is: WHAT is a Brewfile? There's lots of docs/talk about a Brewfile or Brewfiles... but no (at least thus far, for me?) easy-to-find place/discussion about WHAT a Brewfile actually is.

Any help for the above is greatly appreciated. :-)

My first (total) guess: a Brewfile is something akin to a .tar file... but this again is a total guess.

[carlocab]

There is en example Brewfile shown in the README at https://github.com/Homebrew/homebrew-bundle.

[johnnyutahh]

There is en example Brewfile shown in the README at https://github.com/Homebrew/homebrew-bundle.

Yes... I saw that. But that does not answer my question. I'm seeking to understand what the purpose/context/etc of a Brewfile is. Is it an "input" or an "output" (or both) to brew bundle [something] command? Is it "config-ish" file that a human edits? Is it a perpetual config-ish file (that is often found in /etc or /usr/local/etc)? Maybe it's something else?

Is there something I'm clearly missing? I sincerely would very much like to be shown something obvious to tell me I'm "wrong" and that I'm simply not reading the docs correctly.

[carlocab]

Yes... I saw that.

Sorry, I thought you hadn't, given this comment:

My first (total) guess: a Brewfile is something akin to a .tar file... but this again is a total guess.

In any case, you don't need to know what a Brewfile is. Just do

brew bundle dump

then copy the Brewfile in your current working directory to your new machine.

After installing Homebrew on the new machine, do

brew bundle --file=/path/to/Brewfile

[johnnyutahh]

In any case, you don't need to know what a Brewfile is. Just do...

Excellent, thank you! Paraphrashing the procedure:

1. on source-macOS machine: brew bundle dump --> writes Brewfile to current directory.
2. on target-macOS machinebrew bundle --file=/path/to/Brewfile_from_source_macOS_machine --> re-installs Homebrew environment

Based upon the above procedure: here's my current interpretation of what a Brewfile is, outlining the dump-and-rebuild process of brew bundle along the way (one can make a case that adding a paragraph like this to the help/man docs would go a long way):

"The Brewfile [apparently?] is an auto-generated (by brew bundle dump) text-config-file that specifies the entire Homebrew-installed "environment" [but apparently not /usr/local/etc config files, according to my initial tests] and is used as input to brew bundle --file=[input_Brewfile] to rebuild said environment on a new machine. The Brewfile tends to be kilobytes in size and not megabytes big."

[johnnyutahh]

Regardless of the Brewfile debate:

I need a procedure to migrate the HOMEBREW_PREFIX (often /usr/local... I guess?) from one macOS system to another. Apparently brew bundle is designed to provide the core means to do this, BUT I do not know any example procedure for doing this. Hence to me, understanding what the heck a Brewfile is supposed to be (it's purpose, etc) would go a long way for me to figure out he procedure on my own.

Alas, for now: it seems like I have to "reverse engineer" this understanding from the semi-cryptic (to me, thus far) docs.

[SMillerDev]

I need a procedure to migrate the HOMEBREW_PREFIX (often /usr/local... I guess?) from one macOS system to another.

There isn't any, since the differences in device could mean that nothing would work.

Apparently brew bundle is designed to provide the core means to do this, BUT I do not know any example procedure for doing this.

Example:
brew bundle dump on device A, copy the package definition that is a Brewfile to device B, run brew bundle --file=Brefile on device B to install everything again.

man brew and https://github.com/Homebrew/homebrew-bundle have more information about brew bundle

[johnnyutahh]

Next question:

How do I run a brew bundle dump on only the filesystem (or /usr/local copy--if that's all that's needed?) of the original machine?

The original/source MacBook is not currently operational--it died--although target-disk-mode on the "dead" MacBook still works... thus far, but I do not want to count on it. I also have (at least) a full copy of the original-MacBook's /usr/local on my new MacBook and a full filesystem copy of the original MacBook on an external disk.

Reminder: both source and target MacBook's are Intel-based.

[johnnyutahh]

Update: chroot on macOS does not appear to work, and is known to be quite problematic, eg:

1. https://gist.github.com/eddieh/b3c04436eb6b28826939cd57f1b704bb
2. https://apple.stackexchange.com/questions/356970/how-do-i-create-a-minimal-chroot-jail-in-macos

I'm back to having no realistic, automatic option to capture the entire Homebrew environment from the source/original macOS/MacBook... unless I can revive the broken (source/original) MacBook. :-/

My system:

$ sw_vers
ProductName:	Mac OS X
ProductVersion:	10.15.7
BuildVersion:	19H15
$ which chroot
/usr/sbin/chroot
$ sudo chroot /Volumes/external_disk
Killed: 9
$ pwd
/Users/myuser
$ source ~/.bash_profile
$ which chroot
/usr/local/opt/coreutils/libexec/gnubin/chroot
$ sudo chroot /Volumes/external_disk
Killed: 9
$ pwd
/Users/myuser
$ chroot --version
chroot (GNU coreutils) 9.1
Copyright (C) 2022 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <https://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by Roland McGrath.
$

[johnnyutahh]

Update:

a.mv /usr/local to a temporary directory in order to link-to/replace with the copied-from-original/source-MacBook /usr/local also appears not to work. sudo mv /usr/local blocks me.

b. what if I just delete the current /usr/local on the target/new MacBook and try to move the old /usr/local there... might that possibly work??

[johnnyutahh]

Update: chroot on macOS does not appear to work, and is known to be quite problematic...

fyi. This may have been due to a broken environment in/on/within my macOS; I'm not yet convinced that chroot can not work for a similar case in the future.

[johnnyutahh]

b. what if I just [first uninstall Homebrew then] delete the current /usr/local on the target/new MacBook and try to move the old /usr/local there [and then reinstall Homebrew and run brew bundle dump to generate the original/source brew environment]... might that possibly work??

SOLUTION: the above is exactly what I ended up doing.

(Whew... finally got to "the end". 😎)

[johnnyutahh]

How do I mark the above "discussion thread leaf" as The Answer?

[johnnyutahh]

My current perspective: Homebew's inability to run-time manage a non-default-install directory is a major problem for this scenario. It's also arguable that it's not unreasonable for Homebrew to have (some time ago) considered install/file-location modularity (at run time) in its architecture/design, especially given Homebrew's maturity. Alas, mine is a subjective argument.

In general, I'm surprised there (appears to be?) no brew --path-prefix=[non_default_path_prefix]-like feature currently in Homebrew. :-/ I'd be more surprised if there's not a feature-request/roadmap with this (feature) on the books.

Nonetheless, Homebrew has provided me and my team ENORMOUS (and hard-to-measure) value. Pls do not let my small grip here distract from my HUGE appreciation of the project. :-)

[johnnyutahh]

Here's my answer for this problem (a post I made elsewhere in this discussion thread), for today anyway:

https://github.com/orgs/Homebrew/discussions/3664#discussioncomment-3676106