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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def post_process_travis_macos(journal_filename):
 """Post-process a generated journal file on Travis macOS. Args: journal_filename (str):
 The name of the journal file. """

    travis_build_dir = os.environ.get("TRAVIS_BUILD_DIR", "")
    with open(journal_filename, "r") as file_obj:
        content = file_obj.read()
    processed = content.replace(travis_build_dir, "${TRAVIS_BUILD_DIR}")
    with open(journal_filename, "w") as file_obj:
        file_obj.write(processed)