Cannot `brew install` from a `bash` script

Question

I want to create a script to setup an environment on a Mac. Here's a simple sample...

#!/bin/bash
brew install gcovr || exit 1
gcovr --version

NOTE: Script assumes brew was previously installed .

When I execute the script I get an error.

$ ./install
Warning: gcovr 4.1 is already installed and up-to-date
To reinstall 4.1, run `brew reinstall gcovr`
Traceback (most recent call last):
  File "/usr/local/Cellar/gcovr/4.1/libexec/bin/gcovr", line 6, in <module>
    from pkg_resources import load_entry_point
ModuleNotFoundError: No module named 'pkg_resources'

However, if I run it directly from the command line, it progresses as expected.

$ brew install gcovr
Warning: gcovr 4.1 is already installed and up-to-date
To reinstall 4.1, run `brew reinstall gcovr`

It should be noted, the script does install gcovr before failing and bailing.

Answer 1

It appears this error comes from Python being misconfigured.

The answer is to brew postinstall python3.

When Googling around, I found a separate project demonstrating a similar issue.

Here is what I saw on my machine:

$ python --version
Python 2.7.10

$ brew install python
Warning: python 3.7.4 is already installed, it's just not linked
You can use `brew link python` to link this version.

$ brew postinstall python3
==> Postinstalling python
==> /usr/local/Cellar/python/3.7.4/bin/python3 -s setup.py --no-user-cfg install
==> /usr/local/Cellar/python/3.7.4/bin/python3 -s setup.py --no-user-cfg install
==> /usr/local/Cellar/python/3.7.4/bin/python3 -s setup.py --no-user-cfg install

After running postinstall, it didn't appear to change Python

$ python --version
Python 2.7.10

However, it did postively affect Homebrew, and allowed the script to run successfully!

$ ./install
Updating Homebrew...
==> Auto-updated Homebrew!
Updated 1 tap (homebrew/cask).
No changes to formulae.

Warning: gcovr 4.1 is already installed and up-to-date
To reinstall 4.1, run `brew reinstall gcovr`
gcovr 4.1

Copyright 2013-2018 the gcovr authors
Copyright 2013 Sandia Corporation
Under the terms of Contract DE-AC04-94AL85000 with Sandia Corporation,
the U.S. Government retains certain rights in this software.