What does set -a do?

In most POSIX‐style shells (bash, zsh, etc.), including the ones you’ll find on macOS, set can be used to turn various shell options on or off.

• set -a This turns on the allexport option (same as set -o allexport). From that point on, every variable you assign is automatically exported into the environment.

  $ set -a
  $ FOO=bar      # automatically exported
  $ export -p    # you’ll see FOO in the list of exported vars
  

• set +a This turns allexport back off (same as set +o allexport). Variables you assign after this will not be exported unless you explicitly use export.

  xxxxxxxxxx
  $ set +a
  $ BAZ=qux      # not exported
  $ export -p   # BAZ won’t be in the exported list
  

A common use-case is when you have a plain text file of variable assignments:

xxxxxxxxxx
set -a
source my-env-file.sh
set +a

That way everything defined in my-env-file.sh ends up in your environment without having to prefix each line with export.