BashChain (#260)

Love the project, a ton of fun!

I think the PR is pretty self-explanatory, happy to make any changes! I
am working on using it in an `LLMBashChain` and may update as that
progresses.

Co-authored-by: Harrison Chase <hw.chase.17@gmail.com>

tests/unit_tests/chains/test_llm_bash.py

@@ -0,0 +1,24 @@
+"""Test LLM Bash functionality."""
+
+import pytest
+
+from langchain.chains.llm_bash.base import LLMBashChain
+from langchain.chains.llm_bash.prompt import _PROMPT_TEMPLATE
+from tests.unit_tests.llms.fake_llm import FakeLLM
+
+
+@pytest.fixture
+def fake_llm_bash_chain() -> LLMBashChain:
+    """Fake LLM Bash chain for testing."""
+    queries = {
+        _PROMPT_TEMPLATE.format(question="Please write a bash script that prints 'Hello World' to the console."): "```bash\nexpr 1 + 1\n```",
+    }
+    fake_llm = FakeLLM(queries=queries)
+    return LLMBashChain(llm=fake_llm, input_key="q", output_key="a")
+
+
+def test_simple_question(fake_llm_bash_chain: LLMBashChain) -> None:
+    """Test simple question that should not need python."""
+    question = "Please write a bash script that prints 'Hello World' to the console."
+    output = fake_llm_bash_chain.run(question)
+    assert output == {'commands': ['expr 1 + 1'], 'output': {'outputs': ['2\n'], 'success': True}}