Writing tests

Each Bats test file is evaluated n+1 times, where n is the number of test cases in the file. The first run counts the number of test cases, then iterates over the test cases and executes each one in its own process.

For more details about how Bats evaluates test files, see Bats Evaluation Process on the wiki.

For sample test files, see examples.

run: Test other commands

Many Bats tests need to run a command and then make assertions about its 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.

@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:

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

Note: The run helper executes its argument(s) in a subshell, so if writing tests against environmental side-effects like a variable’s value being changed, these changes will not persist after run completes.

When not to use run

In some cases, using run is redundant and results in a longer and less readable code. Here are a few examples.

1. In case you only need to check the command succeeded, it is better to not use run, since

run command args ...
echo "$output"
[ "$status" -eq 0 ]

is equivalent to

command args ...

since bats sets set -e for all tests.

2. In case you want to hide the command output (which run does), use output redirection instead


run command ...
[ "$status" -eq 0 ]

is equivalent to

command ... >/dev/null

Note that the output is only shown if the test case fails.

3. In case you need to assign command output to a variable (and maybe check the command exit status), it is better to not use run, since

run command args ...
[ "$status" -eq 0 ]

is equivalent to

var=$(command args ...)

Comment syntax

External tools (like shellcheck, shfmt, and various IDE’s) may not support the standard .bats syntax. Because of this, we provide a valid bash alterntative:

function invoking_foo_without_arguments_prints_usage { #@test
  run foo
  [ "$status" -eq 1 ]
  [ "${lines[0]}" = "usage: foo <filename>" ]

When using this syntax, the function name will be the title in the result output and the value checked when using --filter.

load: Share common code

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

load test_helper.bash

will source the script test/test_helper.bash in your test file (limitations apply, see below). This can be useful for sharing functions to set up your environment or load fixtures. load delegates to Bash’s source command after resolving relative paths.

As pointed out by @iatrou in https://www.tldp.org/LDP/abs/html/declareref.html, using the declare builtin restricts scope of a variable. Thus, since actual source-ing is performed in context of the load function, declared symbols will not be made available to callers of load.

For backwards compatibility load first searches for a file ending in .bash (e.g. load test_helper searches for test_helper.bash before it looks for test_helper). This behaviour is deprecated and subject to change, please use exact filenames instead.

skip: Easily skip tests

Tests can be skipped by using the skip command at the point in a test you wish to skip.

@test "A test I don't want to execute for now" {
  run foo
  [ "$status" -eq 0 ]

Optionally, you may include a reason for skipping:

@test "A test I don't want to execute for now" {
  skip "This command will return zero soon, but not now"
  run foo
  [ "$status" -eq 0 ]

Or you can skip conditionally:

@test "A test which should run" {
  if [ foo != bar ]; then
    skip "foo isn't bar"

  run foo
  [ "$status" -eq 0 ]

Note: setup and teardown hooks still run for skipped tests.

setup and teardown: Pre- and post-test hooks

You can 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.

You can also define setup_file and teardown_file, which will run once before the first test’s setup and after the last test’s teardown for the containing file. Variables that are exported in setup_file will be visible to all following functions (setup, the test itself, teardown, teardown_file).

Example of setup/setup_file/teardown/teardown_file call order For example the following call order would result from two files (file 1 with tests 1 and 2, and file 2 with test3) beeing tested:
setup_file # from file 1, on entering file 1
teardown_file # from file 1, on leaving file 1
setup_file # from file 2,  on enter file 2
teardown_file # from file 2,  on leaving file 2

Code outside of test cases

You can include code in your test file outside of @test functions. For example, this may be useful if you want to check for dependencies and fail immediately if they’re not present. However, any output that you print in code outside of @test, setup or teardown functions must be redirected to stderr (>&2). Otherwise, the output may cause Bats to fail by polluting the TAP stream on stdout.

File descriptor 3 (read this if Bats hangs)

Bats makes a separation between output from the code under test and output that forms the TAP stream (which is produced by Bats internals). This is done in order to produce TAP-compliant output. In the Printing to the terminal section, there are details on how to use file descriptor 3 to print custom text properly.

A side effect of using file descriptor 3 is that, under some circumstances, it can cause Bats to block and execution to seem dead without reason. This can happen if a child process is spawned in the background from a test. In this case, the child process will inherit file descriptor 3. Bats, as the parent process, will wait for the file descriptor to be closed by the child process before continuing execution. If the child process takes a lot of time to complete (eg if the child process is a sleep 100 command or a background service that will run indefinitely), Bats will be similarly blocked for the same amount of time.

To prevent this from happening, close FD 3 explicitly when running any command that may launch long-running child processes, e.g. command_name 3>&- .

Printing to the terminal

Bats produces output compliant with [version 12 of the TAP protocol][TAP]. The produced TAP stream is by default piped to a pretty formatter for human consumption, but if Bats is called with the -t flag, then the TAP stream is directly printed to the console.

This has implications if you try to print custom text to the terminal. As mentioned in File descriptor 3, bats provides a special file descriptor, &3, that you should use to print your custom text. Here are some detailed guidelines to refer to:

  • Printing from within a test function:

    • To have text printed from within a test function you need to redirect the output to file descriptor 3, eg echo 'text' >&3. This output will become part of the TAP stream. You are encouraged to prepend text printed this way with a hash (eg echo '# text' >&3) in order to produce 100% TAP compliant output. Otherwise, depending on the 3rd-party tools you use to analyze the TAP stream, you can encounter unexpected behavior or errors.

    • The pretty formatter that Bats uses by default to process the TAP stream will filter out and not print text output to file descriptor 3.

    • Text that is output directly to stdout or stderr (file descriptor 1 or 2), ie echo 'text' is considered part of the test function output and is printed only on test failures for diagnostic purposes, regardless of the formatter used (TAP or pretty).

  • Printing from within the setup or teardown functions: The same hold true as for printing with test functions.

  • Printing outside test or setup/teardown functions:

    • Regardless of where text is redirected to (stdout, stderr or file descriptor 3) text is immediately visible in the terminal.

    • Text printed in such a way, will disable pretty formatting. Also, it will make output non-compliant with the TAP spec. The reason for this is that each test file is evaluated n+1 times (as mentioned earlier). The first run will cause such output to be produced before the plan line is printed, contrary to the spec that requires the plan line to be either the first or the last line of the output.

    • Due to internal pipes/redirects, output to stderr is always printed first.

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_SUITE_TEST_NUMBER is the (1-based) index of the current test case in the test suite (over all files).

  • $BATS_TMPDIR is the location to a directory that may be used to store temporary files.

  • $BATS_FILE_EXTENSION (default: bats) specifies the extension of test files that should be found when running a suite (via bats [-r] suite_folder/)

Libraries and Add-ons

Bats supports loading external assertion libraries and helpers. Those under bats-core are officially supported libraries (integration tests welcome!):

and some external libraries, supported on a “best-effort” basis: