# Bats: the Bash Automated Testing System

Bats is a [TAP](http://testanything.org/)-compliant testing framework
for Bash. It provides a simple way to verify that the UNIX programs
you write behave as expected.

A Bats test file is a Bash script with special syntax for defining
test cases. Under the hood, each test case is just a function with a
description.

```bash
#!/usr/bin/env bats

@test "addition using bc" {
  result="$(echo 2+2 | bc)"
  [ "$result" -eq 4 ]
}

@test "addition using dc" {
  result="$(echo 2 2+p | dc)"
  [ "$result" -eq 4 ]
}
```

Test cases consist of standard shell commands. Bats makes use of
Bash's `errexit` (`set -e`) option when running test cases. If every
command in the test case exits with a `0` status code (success), the
test passes. In this way, each line is an assertion of truth.

To run your tests, invoke the `bats` interpreter with a path to a test
file. The file's test cases are run sequentially and in isolation, and
the results are written to standard output in human-readable [TAP
format](http://testanything.org/wiki/index.php/TAP_specification#THE_TAP_FORMAT).
If all the test cases pass, `bats` exits with a `0` status code. If
there are any failures, `bats` exits with a `1` status code.

    $ bats addition.bats
    1..2
    ok 1 addition using bc
    ok 2 addition using dc
    $ echo $?
    0

You can also define special `setup` and `teardown` functions which run
before and after each test case, respectively. Use these to load
fixtures, set up your environment, and clean up when you're done.

Bats is most useful when testing software written in Bash, but you can
use it to test any UNIX program.

### The _run_ helper

You're probably most interested in testing a command's exit status and
output. Bats includes a `run` helper that invokes its arguments as a
command, saves the exit status and output into special global
variables, and then returns with a `0` status code so you can continue
to make assertions in your test case.

For example, let's say you're testing that the `foo` command, when
passed a nonexistent filename, exits with a `1` status code and prints
an error message.

```bash
@test "invoking foo with a nonexistent file prints an error" {
  run foo nonexistent_filename
  [ "$status" -eq 1 ]
  [ "$output" = "foo: no such file 'nonexistent_filename'" ]
}
```

The `$status` variable contains the status code of the command, and
the `$output` variable contains the combined contents of the command's
standard output and standard error streams.

A third special variable, the `$lines` array, is available for easily
accessing individual lines of output. For example, if you want to test
that invoking `foo` without any arguments prints usage information on
the first line:

```bash
@test "invoking foo without arguments prints usage" {
  run foo
  [ "$status" -eq 1 ]
  [ "${lines[0]}" = "usage: foo <filename>" ]
}
```

### The _load_ command

You may want to share common code across multiple test files. Bats
includes a convenient `load` command for sourcing a Bash source file
relative to the location of the current test file. For example, if you
have a Bats test in `test/foo.bats`, the command

```bash
load test_helper
```

will source the script `test/test_helper.bash` in your test file. This
can be useful for sharing functions to set up your environment or load
fixtures.

### Special variables

There are several global variables you can use to introspect on Bats
tests:

* `$BATS_TEST_FILENAME` is the fully expanded path to the Bats test
file.
* `$BATS_TEST_DIRNAME` is the directory in which the Bats test file is
located.
* `$BATS_TEST_NAMES` is an array of function names for each test case.
* `$BATS_TEST_NAME` is the name of the function containing the current
test case.
* `$BATS_TEST_DESCRIPTION` is the description of the current test
case.
* `$BATS_TEST_NUMBER` is the (1-based) index of the current test case
in the test file.
* `$BATS_TMPDIR` is the location to a directory that may be used to
store temporary files.