/etc/alternatives

$ readlink /usr/bin/awk
/etc/alternatives/awk

$ readlink /etc/alternatives/awk
/usr/bin/gawk

$ update-alternatives --config awk
There are 2 choices for the alternative awk (providing /usr/bin/awk).

  Selection    Path            Priority   Status
------------------------------------------------------------
* 0            /usr/bin/gawk    10        auto mode
  1            /usr/bin/gawk    10        manual mode
  2            /usr/bin/mawk    5         manual mode

  …