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.

 1tmpdir  := `mktemp`  # Create a tmp file
 2version := "0.2.7"   
 3tardir  := tmpdir / "awesomesauce-" + version
 4tarball := tardir + ".tar.gz"  # use tmpfile path to create a tarball
 5
 6publish:
 7  rm -f {{tarball}}
 8  mkdir {{tardir}}
 9  cp README.md *.c {{tardir}}
10  tar zcvf {{tarball}} {{tardir}}
11  scp {{tarball}} me@server.com:release/
12  rm -rf {{tarball}} {{tardir}}

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

1# in terminal:
2export REPOSITORY=gitlab.com
3
4# in justfile 
5REPOSITORY    :=  env_var_or_default('REPOSITORY', "github.com")

Sources

Offical Documentation

Blog