How Go Tests "go test"
Adapted from a talk given at GopherCon Israel 2024
Introduction
How does Go, the project, and team behind it, test go test, the Go tool's command for running tests?
Does Go test go test using the go test command? In this article, we explore the evolution of how the Go
team tests the Go tool (go) and discuss strategies for testing command-line tools written in Go in general.
CLIs and Go
If you are a software engineer in 2024, you are most likely
using a CLI tool written a tool to perform some critical part of your work. Perhaps you're using
docker, to build and run container images or kubectl to interact with a kubernetes cluster.
Maybe you're using terraform to manage your infrastructure as code. Maybe you're using atlas
(the project I work on) to manage your database schema as code. You could be
using trivy to scan your code for vulnerabilities or gh to interact with your code on GitHub.
Go is a fantastic language for writing CLI tools, and today we're going to try and study some of the
strategies that you can employ to test CLI tools by looking at how the Go team tests the go tool.
Motivation
My personal motivation for digging into this topic arose from my work on Atlas, a database schema as code tool. Atlas is a CLI written in Go (see our GitHub repo), that has sometimes been described as a "Terraform for databases." It is used by many companies big and small to streamline their database schema management process.
One of the first decisions Ariel (my co-founder) and I made when we started to work on Atlas was that we were going to be employing a continuous delivery strategy, shipping new features and bug fixes to our users as soon as they were ready, often times multiple times a day. This meant that we needed to have a robust testing strategy in place to ensure that we were shipping high-quality software to our users. After all, we were building a tool that was going to be used by other developers to manage their most critical data assets.
Testing CLI tools
Before we dive into how the Go team tests the go tool, let's take a step back and think about what
CLI testing is all about. Testing CLIs has it's unique challenges, but at the end of the day, it's
very similar to testing any other piece of software.
As with all automated tests, we can identify four discrete phases with CLI tests which I characterize as the "Quadruple A" of testing:
- Arrange: We setup the environment for the test. For CLI tests this typically involves creating temporary files, and setting up environment variables.
- Act: When testing server software we would issue a request, but when testing CLIs, this means executing the binary under test, often supplying it with command-line arguments, flags, and potentially piping data into STDIN.
- Assert: We consume the output streams (STDOUT, STDERR) and compare them to expected values. We also check the exit code of the process, and any side effects that the command may have had on the environment.
- And... cleanup: We clean up the environment, removing any temporary files, and resetting any environment variables that we may have changed. Failing to do this can lead to flaky tests - which debugging is arguably one of the worst things in software development.
How the Go team tests go test
With that in mind let's now explore how testing the go tool has evolved over time.
This section is mostly built upon a terrific and detailed commit message on CL #123577 by Russ Cox. I highly recommend reading the original commit message for a more detailed explanation of the evolution of the Go test suite.
2012-2015: test.bash
In the early days of Go, the Go test suite was tested using a shell script called test.bash. This script
started out as a simple 30-40 line script that ran the go tool with various flags and options and checked
the output. Over time, as the Go tool grew in complexity, so did the test.bash script. It eventually grew
to be a 1500+ line shell script that tested the go tool in a variety of ways. The tests looked something
like this:
TEST 'file:line in error messages'
# Test that error messages have file:line information at beginning of
# the line. Also test issue 4917: that the error is on stderr.
d=$(TMPDIR=/var/tmp mktemp -d -t testgoXXX)
fn=$d/err.go
echo "package main" > $fn
echo 'import "bar"' >> $fn
./testgo run $fn 2>$d/err.out || true
if ! grep -q "^$fn:" $d/err.out; then
echo "missing file:line in error message"
cat $d/err.out
ok=false
fi
rm -r $d
If you examine the test above, you will see that it is comprised of the same four phases that we discussed earlier: Arrange, Act, Assert, and Cleanup:
- Arrange: The test creates a temporary directory and a temporary file.
- Act: The test runs the
gotool with therunsubcommand and pipes the output to a file. - Assert: The test checks that the output contains the filename and line number of the error.
- Cleanup: The test removes the temporary directory.
Russ writes about the test.bash script:
The original cmd/go tests were tiny shell scripts written against a library of shell functions.
They were okay to write but difficult to run: you couldn't select individual tests (with -run) they didn't run on Windows, they were slow, and so on.
The tests had always been awkward to write.
2015-2018: testgo
In June 2015, CL #10464 introduced go_test.go. This file
contained a basic framework for writing Go tests for the go tool named testgo. The same test from above, written
in Go, looked something like this:
func TestFileLineInErrorMessages(t *testing.T) {
tg := testgo(t)
defer tg.cleanup()
tg.parallel()
tg.tempFile("err.go", `package main; import "bar"`)
path := tg.path("err.go")
tg.runFail("run", path)
shortPath := path
if rel, err := filepath.Rel(tg.pwd(), path);
err == nil && len(rel) < len(path) {
shortPath = rel
}
tg.grepStderr(
"^"+regexp.QuoteMeta(shortPath)+":",
"missing file:line in error message",
)
}
As you can see, the test is still comprised of the same four phases: Arrange, Act, Assert, and Cleanup:
- Arrange: The test creates a temporary file.
- Act: The test runs the
gotool with therunsubcommand. - Assert: The test checks that the output contains the filename and line number of the error.
- Cleanup: The test removes the temporary file. (this happens in the
defer tg.cleanup()call)
Russ writes about the testgo framework:
“CL 10464 introduced go_test.go's testgo framework and later CLs translated the test shell script over to individual go tests. This let us run tests selectively, run tests on Windows, run tests in parallel, isolate different tests, and so on.
It was a big advance. It's better but still quite difficult to skim.”
2018-?: script_test.go
Most teams and projects that I know would stop here. Go's testing infrastructure, the testing package, as well as
the accompanying go test tool is terrific. When coupled with some thoughtful library code, testing CLIs in Go can
be a breeze. But the Go team didn't stop there. In 2018, CL #123577
introduced a new testing framework for the go tool called script_test.go.
Russ writes about it:
script_test.go brings back the style of writing tests as little scripts, but they are now scripts in a built-for-purpose shell-like language, not bash itself.
Under script_test.go, test cases are described as txt files which are txtar
archives containing the test script and any accompanying files. Here's the "Hello, world" example for script_test:
# src/cmd/go/testdata/script/run_hello.txt
# this is a txtar archive
# run hello.go (defined below)
go run hello.go
# assert ‘hello world’ was printed to stderr
stderr 'hello world'
-- hello.go --
package main
func main() { println("hello world") }
As before, the test comprises the same four phases: Arrange, Act, Assert, and Cleanup:
- Arrange: The test creates a temporary file, defined by the
-- hello.go --section. - Act: The test runs the
gotool with therunsubcommand on the temporary file. - Assert: The test checks that the output contains the string
hello world. - Cleanup: Where is the cleanup code? We'll explore that in a moment.
How does script_test.go work?
script_test does a lot of cool things under the hood that makes it ideal for testing a CLI tool:
- Each script becomes a Go sub-test, which means from the perspective of
go test, it's a normal test, that can be run in parallel, skipped, or run with the-runflag. script_testcreates an isolated sandbox for each test, so that tests can't interfere with each other. Doing so enables it to run tests in parallel, which can significantly speed up the test suite.- The files defined in the
txtararchive are created in the sandbox, and the test is run in that directory.
After setting up, script_test runs the test script commands, line by line:
- Commands are run in sequence, and the output is captured into
stdoutandstderrbuffers. - Commands are expected to succeed, unless explicitly negated with
!at the beginning of the line. - Many helpful assertion commands such as
stdout(to check thestdoutbuffer),stderr(to check thestderrbuffer), andcmp(to compare the contents of two files) are available.
As for cleanup, script_test automatically cleans up the sandbox after each test, removing all files and directories
created during the test.
Can I use script_test.go for my CLI?
If you are writing a CLI tool in Go, I hope by now you are pretty excited about script_test.go. Wouldn't
you love to have a testing framework that allows you to write tests in a shell-like language, that can be run
in parallel, and that automatically cleans up after itself?
You are probably asking yourself, "Can I use script_test.go for my CLI?"
Well, surprisingly enough, the answer is:
No, you can't.
script_test.go is under an internal package in the Go repository, and it is pretty tightly coupled to the
go tool.
The End.
Or is it?
Introducing testscript
In late 2018, Roger Peppe, a long time Go user, contributor and member of the Go community
created a repo named rogpeppe/go-internal to factor out some useful
internal packages from within the Go codebase. One of these packages is testscript, which is based on the work the
Go team created for script_test.
Roger was kind enough to speak with me in preparation for this talk, so I hope that even if you've read about it before, I can share some new things you haven't heard.
script_test.go, as we mentioned, never exposed a public API, and so over the past 6 years, the package gained
steam and popularity, especially among Go "insiders" - people who knew about script_test, but couldn't use it.
Today, according to public GitHub data, go-internal is depended upon by over 100K repositories on GitHub.
(As a side-note, Roger pointed out to me that it's difficult to get the exact number of projects that use testscript itself, as the go.dev site omits any dependencies that run through test code. If you look at go.dev it shows that only 14 (!) packages import it)
Because script_test never had a public API, and was very tightly coupled to testing the Go tool codebase,
testscript should be thought of as more of a conceptual "factoring out" than a 1:1 exporting.
Over time, many features that weren't available in the original implementation, such as generating coverage data, a standalone CLI, and auto-updated of Golden files was added.
As I will show later, testscript is a fantastic tool and we have been utilizing it in the Atlas codebase
for a long time with great success. However, it is worth mentioning that in November 2023, Russ Cox published
a similar package named rsc.io/script which is also based on the script_test codebase.
I haven't used it myself, but it's worth checking out.
Our example CLI: wordwrap
To demonstrate how testscript works, I've created a simple CLI tool named wordwrap. wordwrap is a simple
tool that takes a path and applies simple word wrapping to all .txt files in that path. You can find the code
on GitHub. Wordwrap has a few features that we would like to test:
On the simple case, suppose our current working directory contains a file named example.txt with the following
content:
This is a text file with some text in it. To demonstrate wordwrap, it has more than 40 chars.
Running wordwrap:
go run ./cmd/wordwrap -path ./dir-with-txt-files
Our example.txt file would be transformed into:
This is a text file with some text in
it. To demonstrate wordwrap, it has more
than 40 chars.
By default, wordwrap wraps lines at 40 characters, but you can specify a different line length with the -width
flag:
go run ./cmd/wordwrap -path ./dir-with-txt-files -width 20
Would wrap the lines at 20 characters:
This is a text file
with some text in
it. To demonstrate
wordwrap, it has
more
than 40 chars.
To make things more interesting, we have also added a -strict flag that will cause wordwrap to fail if any
line in the file is longer than the specified width. For example, suppose our example.txt file contains a word
that is 34 characters long:
It's supercalifragilisticexpialidocious
Even though the sound of it is something quite atrocious
If you say it loud enough you'll always sound precocious
Running wordwrap with the -strict flag and a width of 20:
go run ./cmd/wordwrap -path ./hack -width 20 -strict
Would fail with an error message:
file hack/example.txt: line 2 exceeds specified width 20
exit status 1
Writing tests with testscript
Let's see how to write tests for wordwrap using testscript.
To set up, first create a file named wordwrap_test.go in your projects and create the following
boilerplate code:
package wordwrap_test
import (
"bufio"
"os"
"testing"
"github.com/rogpeppe/go-internal/testscript"
"rotemtam.com/wordwrap"
)
func TestMain(m *testing.M) {
os.Exit(testscript.RunMain(m, map[string]func() int{
"wordwrap": wordwrap.Run,
}))
}
func TestScript(t *testing.T) {
testscript.Run(t, testscript.Params{
Dir: "testdata",
})
}
Here's what's happening in the code above:
- Our
TestMainfunction is a setup function that prepares the test environment. It usestestscript.RunMainto tell testscript that it should create a custom commandwordwrapthat runs thewordwrap.Runfunction. This simulates having a binary namedwordwrapthat runs our program's main function. TestScriptis where the actual magic happens. It usestestscript.Runto run the tests in thetestdatadirectory. Thetestdatadirectory contains the test scripts that we will write in the next step.
Our first test script
Let's create a file named testdata/basic.txt with the following content:
wordwrap
cmp basic.txt basic.golden
-- basic.txt --
This is a text file with some text in it. To demonstrate wordwrap, it has more than 40 chars.
-- basic.golden --
This is a text file with some text in
it. To demonstrate wordwrap, it has more
than 40 chars.
As before, you will find our test script is comprised of the same four phases: Arrange, Act, Assert, and Cleanup:
- Arrange: The test creates a temporary file, defined by the
-- basic.txt --section. - Act: The test runs the
wordwrapcommand. - Assert: The test compares the output to the contents of the
basic.goldenfile. This is done using the includedcmpcommand. - Cleanup: There is no explicit cleanup in this test, as
testscriptwill automatically clean up the sandbox after the test.
The awesome thing about testscript, is that from go test's perspective, basic is just a regular Go test.
This means that we can execute it as we would any other test:
go test -v ./... -run TestScript/basic
This is the output you should see:
=== RUN TestScript
=== RUN TestScript/basic
=== PAUSE TestScript/basic
=== CONT TestScript/basic
testscript.go:558: WORK=$WORK
# --- redacted for brevity ---
> wordwrap
> cmp basic.txt basic.golden
PASS
--- PASS: TestScript (0.00s)
--- PASS: TestScript/basic (0.15s)
PASS
ok rotemtam.com/wordwrap (cached)
A more involved test script
Next, let's create a more involved test script that verifies additional behavior in wordwrap. Create a file named
testdata/dont-touch.txt with the following content:
wordwrap -path p1.txt
! stderr .
cmp p1.txt p1.golden
exec cat dont-touch.txt
stdout 'This file shouldn''t be modified, because we invoke wordwrap with a path argument.'
-- p1.txt --
Don't communicate by sharing memory, share memory by communicating.
-- p1.golden --
Don't communicate by sharing memory,
share memory by communicating.
-- dont-touch.txt --
This file shouldn't be modified, because we invoke wordwrap with a path argument.
This test verifies that wordwrap doesn't modify files that are not passed as arguments. The test script is
comprised of the same phases.
- Arrange: The test creates
p1.txt, which is the file we are going to modify, anddont-touch.txt, which is the file we don't want to modify. - Act: The test runs the
wordwrapcommand with the-pathflag. - Assert: The test compares the output to the contents of the
p1.goldenfile. This is done using the includedcmpcommand. The test also verifies that thedont-touch.txtfile hasn't been modified. - Cleanup: There is no explicit cleanup in this test, as
testscriptwill automatically clean up the sandbox after
Testing the -width flag
In addition, we should probably verify that the -width flag works as expected. Create a file named testdata/width.txt:
skip
wordwrap -width 60
cmp effective.txt effective.golden
-- effective.txt --
This document gives tips for writing clear, idiomatic Go code. It augments the language specification, the Tour of Go, and How to Write Go Code, all of which you should read first.
Note added January, 2022: This document was written for Go's release in 2009, and has not been updated significantly since.
-- effective.golden --
This document gives tips for writing clear, idiomatic Go
code. It augments the language specification, the Tour of
Go, and How to Write Go Code, all of which you should read
first.
Note added January, 2022: This document was written for Go's
release in 2009, and has not been updated significantly
since.
This test script verifies that the -width flag works as expected. The test script is comprised of the same phases.
This works, but I didn't love writing it. Creating the .golden file by hand is a bit tedious, and it's easy to
make mistakes. In this case, wouldn't it be great if we could create a custom command that verifies that the
output is wrapped at 60 characters?
Thankfully, testscript allows us to create custom commands. Let's create a custom command named maxlen that
verifies that the output is wrapped at a maximum of n characters. Add the following code to wordwrap_test.go:
// maxline verifies that the longest line in args[0] is shorter than args[1] chars.
// Usage: maxline <path> <maxline>
func maxline(ts *testscript.TestScript, neg bool, args []string) {
if len(args) != 2 {
ts.Fatalf("usage: maxline <path> <maxline>")
}
l, ok := strconv.Atoi(args[1])
if ok != nil {
ts.Fatalf("usage: maxline <path> <maxline>")
}
scanner := bufio.NewScanner(
strings.NewReader(
ts.ReadFile(args[0]),
),
)
tooLong := false
for scanner.Scan() {
if len(scanner.Text()) > l {
tooLong = true
break
}
}
if tooLong && !neg {
ts.Fatalf("line too long in %s", args[0])
}
if !tooLong && neg {
ts.Fatalf("no line too long in %s", args[0])
}
}
In order to use the maxline command in our test scripts, we need to register it with testscript. Update the
TestScript function in wordwrap_test.go to include the following code:
func TestScript(t *testing.T) {
testscript.Run(t, testscript.Params{
Dir: "testdata",
Cmds: map[string]func(ts *testscript.TestScript, neg bool, args []string){
"maxline": maxline,
},
})
}
Now we can use the maxline command in our test scripts. Create a new test named testdata/width-custom.txt with the
following content:
wordwrap -width 60
! maxline effective.txt 20
maxline effective.txt 60
wordwrap -width 40
! maxline effective.txt 20
maxline effective.txt 40
wordwrap -width 20
maxline effective.txt 20
-- effective.txt --
This document gives tips for writing clear, idiomatic Go code. It augments the language specification, the Tour of Go, and How to Write Go Code, all of which you should read first.
Note added January, 2022: This document was written for Go's release in 2009, and has not been updated significantly since.
Running this test script will verify that the output is wrapped at 60 characters, 40 characters, and 20 characters:
go test -v ./... -run TestScript/width-custom
Output:
? rotemtam.com/wordwrap/cmd/wordwrap [no test files]
=== RUN TestScript
=== RUN TestScript/width-custom
=== PAUSE TestScript/width-custom
=== CONT TestScript/width-custom
testscript.go:558: WORK=$WORK
# --- redacted for brevity ---
> wordwrap -width 60
> ! maxline effective.txt 20
> maxline effective.txt 60
> wordwrap -width 40
> ! maxline effective.txt 20
> maxline effective.txt 40
> wordwrap -width 20
> maxline effective.txt 20
PASS
--- PASS: TestScript (0.00s)
--- PASS: TestScript/width-custom (0.19s)
PASS
ok rotemtam.com/wordwrap 0.626s
Testing strict mode
Finally, let's create a test script that verifies that the -strict flag works as expected. Create a file named
testdata/strict.txt with the following content:
! wordwrap -path poppins.txt -width 20 -strict
stderr 'line 2 exceeds specified width 20'
wordwrap -path poppins.txt -width 20
cmp poppins.txt poppins.golden
-- poppins.txt --
It's supercalifragilisticexpialidocious
Even though the sound of it is something quite atrocious
-- poppins.golden --
It's
supercalifragilisticexpialidocious
Even though the
sound of it is
something quite
atrocious
This test script verifies that wordwrap fails when a line exceeds the specified width in strict mode.
Awesome!
Personal impact
Aside from being a super cool tool for writing tests for CLI tools, testscript has had a significant impact on
my team. We have been using it in the Atlas codebase for a long time, and it has been a game-changer for us.
Atlas, as a schema-as-code tool, is a bridge between code (files) and databases. Thus, being able to easily write tests to verify our tool's behavior in a way that is close to how our users interact with it has been invaluable.
Over the years, we have accumulated a set of custom testscript commands that allow us to write test scripts in
a fluent and intuitive way. You can see this in action in the Atlas codebase, but just to give you a taste, here
is how our testscript entrypoint looks like for MySQL integration tests:
func TestMySQL_Script(t *testing.T) {
myRun(t, func(t *myTest) {
testscript.Run(t.T, testscript.Params{
Dir: "testdata/mysql",
Setup: t.setupScript,
Cmds: map[string]func(ts *testscript.TestScript, neg bool, args []string){
"only": cmdOnly,
"apply": t.cmdApply,
"exist": t.cmdExist,
"synced": t.cmdSynced,
"cmphcl": t.cmdCmpHCL,
"cmpshow": t.cmdCmpShow,
"cmpmig": t.cmdCmpMig,
"execsql": t.cmdExec,
"atlas": t.cmdCLI,
"clearSchema": t.clearSchema,
"validJSON": validJSON,
},
})
})
}
Having these commands, allow us to write test scripts that are easy to read and understand, and that verify the behavior of our tool is correct. For example:
apply 1.hcl
cmpshow users 1.sql
-- 1.hcl --
schema "main" {}
table "users" {
schema = schema.main
column "id" {
null = false
type = integer
auto_increment = true
}
primary_key {
columns = [column.id]
}
}
-- 1.sql --
CREATE TABLE `users` (`id` integer NOT NULL PRIMARY KEY AUTOINCREMENT)
This test applies an Atlas DDL schema on a SQLite database and verifies that the schema is created correctly.
Conclusion
In this article, we have explored how the Go team tests the go tool, and how you can apply similar strategies
to test your CLI tools using testscript.
As a team that develops tools for other developers, we take the reliability of our tools very seriously. The key to this, we have found over the years, is to have a robust testing strategy in place. This allows us to move fast (without breaking things) and to ship high-quality software to our users.
Resources
- Go CL #123577
- rogpeppe/go-internal
testscriptpackage docs- Test scripts on the Atlas codebase
