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MacTeX installs a data structure by Gerben Wierda and Jerome Laurens to support multiple TeX distributions on a machine. Users who installed last year's MacTeX-2020 will discover that their old TeX distribution has not been altered by MacTeX. Instead, a simple method is provided to switch
between active distributions. If there is an error in MacTeX-2021, you can temporarily retreat to MacTeX-2020 until the error is fixed.
In earlier years, MacTeX installed a Preference Pane to select the active distribution. We no longer do that because Apple kept changing the design requirements for Preference Panes. MacTeX-2021 does not remove Preference Panes, so you may still have one. (If you want to remove it, go to /Library/PreferencePanes and
drag TexDist.prefPane to the trash.)
Before El Capitan, this link was /usr/texbin. But Apple no longer permits users to write to the /usr directory, even if they have root support. The links /usr/texbin and /Library/TeX/texbin were otherwise completely equivalent. Older applications may still be configured to use /usr/texbin.
The link /Library/TeX/texbin should never be directly changed by users. It points into the data structure but not directly at the active binary,
and it doesn't change when the active distribution changes.
Changing distributions automatically changes PATH and MANPATH variables, so interaction with TeX via the command line will use the appropriate distribution. It also automatically reconfigures GUI applications so they use the new active distribution.
Incidentally, the data structure does not modify the actual TeX distribution in any way. It is ingeniously constructed so that, for instance, changing a single symbolic link changes the active TeX distribution.
TeX distributions are installed in locations which are usually not displayed in the Finder. But it is possible to inspect the currently active distribution by going to /Library/TeX/Root with Finder. This is a symbolic link to the full currently active distribution. Similarly, /Library/TeX/Documentation is a symbolic link to all documentation folders for the currently active distribution.