Forcing login shell in terminal overrides PATH on macOS

[dlukes]

Today I learned that on macOS, shells run /usr/libexec/path_helper to prepend entries from /etc/paths{,.d} to the PATH. They only do so when they're invoked as login shells, because otherwise, this would override PATH customizations configured by users whenever they launch a subshell (fish only fixed this relatively recently).

Unfortunately, the Jupyter terminal always launches shells as login:

notebook/notebook/terminal/__init__.py

Lines 24 to 26 in 43df5af

	# Enable login mode - to automatically source the /etc/profile script
	if os.name != 'nt':
	shell.append('-l')

This means that if the user has modified their PATH because of tools like virtualenv or pyenv, these entries will be demoted in shells launched by the Jupyter terminal on macOS, and the system Python in /usr/bin/python will take precedence. (Other binaries will also be affected of course, but Python is likely to be a prominent problem for Jupyter users.)

Running the shell as login makes sense in remote server setups where Jupyter is not launched from an existing shell session, which has already presumably sourced /etc/profile, but from some kind of spawner. But it doesn't make sense when launched from a shell on a local machine.

One possible way to address this would be to only run shells as login when they're not nested, which can be detected via the SHLVL env var:

dlukes@2786c15