Interesting example from justfile documentation: where it create mktemp and set it in variable then by concatenation you get a full path to the tar.gz.
Then the Recipe “publish” create the artifact again and push it to a server.

tmpdir  := `mktemp`  # Create a tmp file
version := "0.2.7"   
tardir  := tmpdir / "awesomesauce-" + version
tarball := tardir + ".tar.gz"  # use tmpfile path to create a tarball

publish:
  rm -f {{tarball}}
  mkdir {{tardir}}
  cp README.md *.c {{tardir}}
  tar zcvf {{tarball}} {{tardir}}
  scp {{tarball}} me@server.com:release/
  rm -rf {{tarball}} {{tardir}}

This one can be really usefull to define a default value which can be redefine with env variable:

# in terminal:
export REPOSITORY=gitlab.com

# in justfile 
REPOSITORY    :=  env_var_or_default('REPOSITORY', "github.com")

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