README for yarn, a scenario testing tool


  • You need Python 2. Yarn's dependencies do not work with Python 3. (Yet.)
  • Install ttystatus:
  • Install cliapp:
  • Install Python-Markdown:
  • You can install them from source, or using your operating system's package manager (if they've been packaged for your operating system; Debian has them: python-ttystatus, python-cliapp, python-markdown).
  • Similarly, install yarn from source or with the package manager (in Debian: cmdtest).
  • Installation from source requires cloning the git repository and running python install, perhaps with additional options (see --help) to specify installation location, etc. Read also the installation documentation of the libraries.


yarn is a scenario testing tool: you write a scenario describing how a user uses your software and what should happen, and express, using very lightweight syntax, the scenario in such a way that it can be tested automatically. The scenario has a simple, but strict structure:

SCENARIO name of scenario
GIVEN some setup for the test
WHEN thing that is to be tested happens
THEN the post-conditions must be true

As an example, consider a very short test scenario for verifying that a backup program works, at least for one simple case.

SCENARIO basic backup and restore
GIVEN some live data in a directory
AND an empty backup repository
WHEN a backup is made
THEN the data can be restored

(Note the addition of AND: you can have multiple GIVEN, WHEN, and THEN statements. The AND keyword makes the text be more readable.)

Scenarios are meant to be written in mostly human readable language. However, they are not free form text. In addition to the GIVEN/WHEN/THEN structure, the text for each of the steps needs a computer-executable implementation. This is done by using IMPLEMENTS. The backup scenario from above might be implemented as follows:

IMPLEMENTS GIVEN some live data in a directory
rm -rf "$DATADIR/data"
mkdir "$DATADIR/data"
echo foo > "$DATADIR/data/foo"

IMPLEMENTS GIVEN an empty backup repository
rm -rf "$DATADIR/repo"
mkdir "$DATADIR/repo"

IMPLEMENTS WHEN a backup is made
backup-program -r "$DATADIR/repo" "$DATADIR/data"

IMPLEMENTS THEN the data can be restored
mkdir "$DATADIR/restored"
restore-program -r "$DATADIR/repo" "$DATADIR/restored"
diff -rq "$DATADIR/data" "$DATADIR/restored"

Each "IMPLEMENT GIVEN" (or WHEN, THEN) is followed by a regular expression on the same line, and then a shell script that gets executed to implement any step that matches the regular expression. The implementation can extract data from the match as well: for example, the regular expression might allow a file size to be specified.

The above example seems a bit silly, of course: why go to the effort to obfuscate the various steps? The answer is that the various steps, implemented using IMPLEMENTS, can be combined in many ways, to test different aspects of the program being tested. In effect, the IMPLEMENTS sections provide a vocabulary which the scenario writer can use to express a variety of usefully different scenarios, which together test all the aspects of the software that need to be tested.

Moreover, by making the step descriptions be human language text, matched by regular expressions, most of the test can hopefully be written, and understood, by non-programmers. Someone who understands what a program should do, could write tests to verify its behaviour. The implementations of the various steps need to be implemented by a programmer, but given a well-designed set of steps, with enough flexibility in their implementation, that quite a good test suite can be written.

Test language specification

A test document is written in Markdown, with block quoted code blocks being interpreted specially. Each block must follow the syntax defined here.

  • Every step in a scenario is one logical line, and starts with a keyword. A step can be continued on the next physical line by starting the next line with ... (three dots, followed by a space).

  • Each implementation (IMPLEMENTS) starts as a new block, and continues until there is a block that starts with another keyword.

The following keywords are defined.

  • SCENARIO starts a new scenario. The rest of the line is the name of the scenario. The name is used for documentation and reporting purposes only and has no semantic meaning. SCENARIO MUST be the first keyword in a scenario, with the exception of IMPLEMENTS. The set of documents passed in a test run may define any number of scenarios between them, but there must be at least one or it is a test failure. The IMPLEMENTS sections are shared between the documents and scenarios.

  • ASSUMING defines a condition for the scenario. The rest of the line is "matched text", which gets implemented by an IMPLEMENTS section. If the code executed by the implementation fails, the scenario is skipped.

  • GIVEN prepares the world for the test to run. If the implementation fails, the scenario fails.

  • WHEN makes the change to the world that is to be tested. If the code fails, the scenario fails.

  • THEN verifies that the changes made by the GIVEN steps did the right thing. If the code fails, the scenario fails.

  • FINALLY specifies how to clean up after a scenario. If the code fails, the scenario fails. All FINALLY blocks get run either when encountered in the scenario flow, or at the end of the scenario, regardless of whether the scenario is failing or not.

  • AND acts as ASSUMING, GIVEN, WHEN, THEN, or FINALLY: whichever was used last. It must not be used unless the previous step was one of those, or another AND.

  • IMPLEMENTS is followed by one of ASSUMING, GIVEN, WHEN, or THEN, and a PCRE regular expression, all on one line, and then further lines of shell commands until the end of the block quoted code block. Markdown is unclear whether an empty line (no characters, not even whitespace) between two block quoted code blocks starts a new one or not, so we resolve the ambiguity by specifiying that a code block directly following a code block is a continuation unless it starts with one of the scenario testing keywords.

    The shell commands get parenthesised parts of the match of the regular expression as environment variables ($MATCH_1 etc). For example, if the regexp is "a (\d+) byte file", then $MATCH_1 gets set to the number matched by \d+.

    The test runner creates a temporary directory, whose name is given to the shell code in the DATADIR environment variable.

    The test runner sets the SRCDIR environment variable to the path to the directory it was invoked from (by convention, the root of the source tree of the project).

    The test runner removes all other environment variables, except TERM, USER, USERNAME, LOGNAME, HOME, and PATH. It also forces SHELL set to /bin/sh, and LC_ALL set to C, in order to have as clean an environment as possible for tests to run in.

    The shell commands get invoked with /bin/sh -eu, and need to be written accordingly. Be careful about commands that return a non-zero exit code. There will eventually be a library of shell functions supplied which allow handling the testing of non-zero exit codes cleanly. In addition functions for handling stdout and stderr will be provided.

    The code block of an IMPLEMENTS block fails if the shell invocation exits with a non-zero exit code. Output to stderr is not an indication of failure. Any output to stdout or stderr may or may not be shown to the user.


  • The name of each scenario (given with SCENARIO) must be unique.
  • All names of scenarios and steps will be normalised before use (whitespace collapse, leading and trailing whitespace
  • Every ASSUMING, GIVEN, WHEN, THEN, FINALLY must be matched by exactly one IMPLEMENTS. The test runner checks this before running any code.
  • Every IMPLEMENTS may match any number of ASSUMING, GIVEN, WHEN, THEN, or FINALLY. The test runner may warn if an IMPLEMENTS is unused.
  • If ASSUMING fails, that scenario is skipped, and any FINALLY steps are not run.

See also

Wikipedia has an article on Behaviour Driven Development, which can provide background and further explanation to what this tools tries to do.


  • Add DEFINING, PRODUCING, if they turn out to be useful.
  • Need something like ASSUMING, except fail the scenario if the pre-condition is not true. Useful for testing that you can ssh to localhost when flinging, for example. DJAS: We think this might be 'REQUIRING' and it still does not run the FINALLY group.