diff --git a/README.md b/README.md
index 259c308..2d8c3f8 100644
--- a/README.md
+++ b/README.md
@@ -50,6 +50,29 @@ If you use Nix with flakes, you can simply add it to your program's devshell ins
      )
 ```
 
+## What do you put in your commit messages?
+
+Simply add a bash script enclosed in a `git-check-assertions` block to a commit message, e.g.:
+
+~~~
+```git-check-assertions
+dotnet build
+dotnet test --no-build
+```
+~~~
+
+This script will be run with `set -euo pipefail`, and the commit will be considered correct if the script exits succesfully.
+
+You can technically assert that a command fails by writing `! ... || exit 1`, or write assertions about a command's output by piping it to `grep`, but doing so won't lead to very useful error messages when things go wrong. To make those things easier, there are some helper functions included, which are inspired by [bats](https://github.com/bats-core/bats-core) and [bats-assert](https://github.com/bats-core/bats-assert):
+
+* `run <command>`: run a command, capturing its exit status in `status` and combined stdout/stderr in `output`.
+* `assert_success`: succeed if `run` produced a zero exit status.
+* `assert_failure`: succeed if `run` produced a non-zero exit status.
+* `assert_output <string>`: succeed if `output` exactly matches the string.
+* `assert_output --partial <string>`: succeed if `output` contains the string.
+
+I'm considering taking `bats-assert` as a dependency, but for now, this very minimal set of functions with a similar interface should get you on your way.
+
 ## Examples of commit messages
 
 Assert that a commit builds:
@@ -65,16 +88,18 @@ Assert that a commit builds and that the tests succeed:
 ~~~
 ```git-check-assertions
 dotnet build
-dotnet test --no-build
+run dotnet test --no-build
+assert_success
 ```
 ~~~
 
-Assert that a commit builds, but that the tests do not succeed (`assert_fails` is a helper function included in `git-check-assertions`):
+Assert that a commit builds, but that the tests do not succeed:
 
 ~~~
 ```git-check-assertions
 dotnet build
-assert_fails dotnet test --no-build
+run dotnet test --no-build
+assert_failure
 ```
 ~~~
 
@@ -83,7 +108,9 @@ Assert that a commit builds, and that the tests fail with exactly the error that
 ~~~
 ```git-check-assertions
 dotnet build
-(assert_fails dotnet test --no-build) | grep "Invalid URL"
+run dotnet test --no-build
+assert_failure
+assert_output --partial "Invalid URL"
 ```
 ~~~
 
@@ -94,6 +121,7 @@ Assert that a commit builds, and that a specific change breaks the tests (as dis
 dotnet test
 sed -i '/crucial code/d' Main.fs
 dotnet build
-assert_fails dotnet test --no-build
+run dotnet test --no-build
+assert_failure
 ```
 ~~~
diff --git a/bin/git-check-assertions b/bin/git-check-assertions
index f7fe936..5501358 100755
--- a/bin/git-check-assertions
+++ b/bin/git-check-assertions
@@ -24,14 +24,45 @@
 
 set -euo pipefail
 
-assert_fails() {
-  ! "$@" || {
+# helper functions inspired by bats/bats-assert
+run() {
+  set +e
+  output="$("$@" 2>&1)"
+  status=$?
+  set -e
+  return 0
+}
+assert_success() {
+  if [ "$status" -ne 0 ]; then
+    echo "Expected command to succeed, but it failed."
+    exit 1
+  fi
+}
+assert_failure() {
+  if [ "$status" -eq 0 ]; then
     echo "Expected command to fail, but it succeeded."
     exit 1
-  }
+  fi
 }
-export -f assert_fails
+assert_output() {
+  if [ "${1:-}" = "--partial" ]; then
+    local expected="$2"
+    if [[ "$output" != *"$expected"* ]]; then
+      echo "Expected output to contain: $expected"
+      echo "Actual output: $output"
+      exit 1
+    fi
+    return 0
+  fi
+  if [ "$output" != "$1" ]; then
+    echo "Expected output to equal: $1"
+    echo "Actual output: $output"
+    exit 1
+  fi
+}
+export -f run assert_success assert_failure assert_output
 
+# main flow
 orig_ref="$(git symbolic-ref --quiet --short HEAD || git rev-parse HEAD)"
 base="$(git merge-base main HEAD)"
 mapfile -t commits < <(git rev-list --reverse "${base}..HEAD")
diff --git a/test/git-check-assertions.bats b/test/git-check-assertions.bats
index f58c3ea..697d47d 100755
--- a/test/git-check-assertions.bats
+++ b/test/git-check-assertions.bats
@@ -112,3 +112,90 @@ commit_with_assertion() {
 
   assert_success
 }
+
+@test "assert_success should succeed if the executed command succeeded" {
+  git checkout -b feature
+  commit_with_assertion $'run exit 0\nassert_success'
+
+  run git-check-assertions
+
+  assert_success
+}
+
+@test "assert_success should fail if the executed command failed" {
+  git checkout -b feature
+  commit_with_assertion $'run exit 1\nassert_success'
+
+  run git-check-assertions
+
+  assert_failure
+  assert_output --partial "Expected command to succeed, but it failed."
+}
+
+@test "assert_failure should succeed if the executed command failed" {
+  git checkout -b feature
+  commit_with_assertion $'run exit 1\nassert_failure'
+
+  run git-check-assertions
+
+  assert_success
+}
+
+@test "assert_failure should fail if the executed command succeeded" {
+  git checkout -b feature
+  commit_with_assertion $'run exit 0\nassert_failure'
+
+  run git-check-assertions
+
+  assert_failure
+  assert_output --partial "Expected command to fail, but it succeeded."
+}
+
+@test "assert_output should succeed if the output matches the given string" {
+  git checkout -b feature
+  commit_with_assertion $'run echo hello\nassert_output hello'
+
+  run git-check-assertions
+
+  assert_success
+}
+
+@test "assert_output should fail if the output does not match the given string" {
+  git checkout -b feature
+  commit_with_assertion $'run echo hello\nassert_output goodbye'
+
+  run git-check-assertions
+
+  assert_failure
+  assert_output --partial "Expected output to equal: goodbye"
+  assert_output --partial "Actual output: hello"
+}
+
+@test "assert_output should also check stderr output" {
+  git checkout -b feature
+  commit_with_assertion $'run sh -c "echo err 1>&2"\nassert_output err'
+
+  run git-check-assertions
+
+  assert_success
+}
+
+@test "assert_output --partial should succeed if output contains the given string" {
+  git checkout -b feature
+  commit_with_assertion $'run echo hello\nassert_output --partial ell'
+
+  run git-check-assertions
+
+  assert_success
+}
+
+@test "assert_output --partial should fail if output does not contain the given string" {
+  git checkout -b feature
+  commit_with_assertion $'run echo hello\nassert_output --partial xyz'
+
+  run git-check-assertions
+
+  assert_failure
+  assert_output --partial "Expected output to contain: xyz"
+  assert_output --partial "Actual output: hello"
+}