diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..d406517
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,15 @@
+.PHONY: build test unit integration clean
+
+build:
+	go build -o shout .
+
+test: unit integration
+
+unit:
+	go test ./...
+
+integration: build
+	./shout test test/
+
+clean:
+	rm -f shout
diff --git a/cmd.go b/cmd.go
new file mode 100644
index 0000000..6cc9f7f
--- /dev/null
+++ b/cmd.go
@@ -0,0 +1,361 @@
+package main
+
+import (
+	"fmt"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"sort"
+	"strconv"
+	"strings"
+	"sync"
+	"time"
+
+	"github.com/spf13/cobra"
+)
+
+var version = "dev"
+
+var rootCmd = &cobra.Command{
+	Use:   "shout",
+	Short: "$ shell output tester",
+}
+
+func init() {
+	rootCmd.AddCommand(testCmd())
+	rootCmd.AddCommand(versionCmd())
+	rootCmd.AddCommand(exampleCmd())
+}
+
+func testCmd() *cobra.Command {
+	var (
+		update   bool
+		keep     bool
+		cleanEnv bool
+		pathDirs []string
+		timeout  string
+		verbose  bool
+		portFrom int
+		parallel bool
+	)
+
+	cmd := &cobra.Command{
+		Use:   "test [files...]",
+		Short: "Run .shout test files",
+		Args:  cobra.ArbitraryArgs,
+		RunE: func(cmd *cobra.Command, args []string) error {
+			timeoutDur, err := parseDuration(timeout)
+			if err != nil {
+				return err
+			}
+
+			paths := args
+			if len(paths) == 0 {
+				paths = []string{"."}
+			}
+
+			files, err := findShoutFiles(paths)
+			if err != nil {
+				return err
+			}
+			if len(files) == 0 {
+				fmt.Fprintln(os.Stderr, "No .shout files found")
+				os.Exit(1)
+			}
+
+			start := time.Now()
+			var results []TestResult
+			cwd, _ := os.Getwd()
+			nextPort := portFrom
+
+			runOne := func(filePath string, port int) TestResult {
+				content, err := os.ReadFile(filePath)
+				if err != nil {
+					relPath, _ := filepath.Rel(cwd, filePath)
+					return TestResult{Path: relPath, Error: err.Error()}
+				}
+
+				relPath, _ := filepath.Rel(cwd, filePath)
+				parsed, err := parse(relPath, string(content))
+				if err != nil {
+					return TestResult{Path: relPath, Error: err.Error()}
+				}
+
+				// Resolve directives
+				envVars := make(map[string]string)
+				setupEnvVars := make(map[string]string)
+				userEnvVars := make(map[string]string)
+				var setupCommands []Command
+
+				for _, d := range parsed.Directives {
+					switch d.Type {
+					case "setup":
+						setupPath := filepath.Join(filepath.Dir(filePath), d.Path)
+						setupContent, err := os.ReadFile(setupPath)
+						if err != nil {
+							return TestResult{Path: parsed.Path, Error: err.Error()}
+						}
+						setupRelPath, _ := filepath.Rel(cwd, setupPath)
+						setupParsed, err := parse(setupRelPath, string(setupContent))
+						if err != nil {
+							return TestResult{Path: parsed.Path, Error: err.Error()}
+						}
+						for _, sd := range setupParsed.Directives {
+							if sd.Type == "setup" {
+								return TestResult{
+									Path:  parsed.Path,
+									Error: fmt.Sprintf("%s: @setup not allowed in setup files", setupRelPath),
+								}
+							}
+							if sd.Type == "env" {
+								setupEnvVars[sd.Key] = sd.Value
+							}
+						}
+						setupCommands = append(setupCommands, setupParsed.Commands...)
+
+					case "env":
+						userEnvVars[d.Key] = d.Value
+					}
+				}
+
+				// Setup env < user env
+				for k, v := range setupEnvVars {
+					envVars[k] = v
+				}
+				for k, v := range userEnvVars {
+					envVars[k] = v
+				}
+				if port > 0 {
+					if _, ok := userEnvVars["PORT"]; !ok {
+						if _, ok := setupEnvVars["PORT"]; !ok {
+							envVars["PORT"] = strconv.Itoa(port)
+						}
+					}
+				}
+
+				merged := ShoutFile{
+					Path:       parsed.Path,
+					Commands:   append(setupCommands, parsed.Commands...),
+					Directives: parsed.Directives,
+				}
+
+				var onCommand func(Command)
+				if verbose {
+					onCommand = func(c Command) {
+						fmt.Fprintf(os.Stderr, dim("  $ %s\n"), c.Cmd)
+					}
+				}
+
+				fileResult := runFile(merged, RunOptions{
+					CleanEnv:  cleanEnv,
+					PathDirs:  pathDirs,
+					EnvVars:   envVars,
+					Timeout:   timeoutDur,
+					Verbose:   verbose,
+					OnCommand: onCommand,
+				})
+
+				// Check setup commands for failures
+				for i := 0; i < len(setupCommands) && i < len(fileResult.Results); i++ {
+					r := fileResult.Results[i]
+					sc := setupCommands[i]
+					ok := false
+					switch sc.ExitCodeType {
+					case ExitCodeNone:
+						ok = r.ExitCode == 0
+					case ExitCodeWildcard:
+						ok = r.ExitCode != 0
+					case ExitCodeExact:
+						ok = r.ExitCode == sc.ExitCodeValue
+					}
+					if !ok {
+						if keep {
+							fmt.Fprintln(os.Stderr, fileResult.TmpDir)
+						} else {
+							cleanupTmpDir(fileResult.TmpDir)
+						}
+						return evaluateFile(
+							parsed.Path,
+							nil,
+							fmt.Sprintf("setup command failed (exit %d): $ %s", r.ExitCode, sc.Cmd),
+						)
+					}
+				}
+
+				fileOwnResults := fileResult.Results
+				if len(setupCommands) > 0 && len(fileResult.Results) >= len(setupCommands) {
+					fileOwnResults = fileResult.Results[len(setupCommands):]
+				}
+
+				testResult := evaluateFile(parsed.Path, fileOwnResults, fileResult.Error)
+
+				if update && len(fileOwnResults) > 0 {
+					updated := rewriteFile(parsed, fileOwnResults, string(content))
+					if updated != string(content) {
+						_ = os.WriteFile(filePath, []byte(updated), 0o644)
+					}
+				}
+
+				if keep {
+					fmt.Fprintln(os.Stderr, fileResult.TmpDir)
+				} else {
+					cleanupTmpDir(fileResult.TmpDir)
+				}
+
+				return testResult
+			}
+
+			printDots := func(r TestResult) {
+				if r.Error != "" {
+					fmt.Print(red("F"))
+					return
+				}
+				passed := r.CommandCount - len(r.Failures)
+				for i := 0; i < passed; i++ {
+					fmt.Print(green("."))
+				}
+				for i := 0; i < len(r.Failures); i++ {
+					fmt.Print(red("F"))
+				}
+			}
+
+			if parallel {
+				allResults := make([]TestResult, len(files))
+				var wg sync.WaitGroup
+				for idx, f := range files {
+					wg.Add(1)
+					go func(i int, filePath string, port int) {
+						defer wg.Done()
+						allResults[i] = runOne(filePath, port)
+					}(idx, f, nextPort)
+					if nextPort > 0 {
+						nextPort++
+					}
+				}
+				wg.Wait()
+				for _, r := range allResults {
+					printDots(r)
+					results = append(results, r)
+				}
+				fmt.Println()
+			} else {
+				for _, filePath := range files {
+					port := 0
+					if nextPort > 0 {
+						port = nextPort
+						nextPort++
+					}
+					r := runOne(filePath, port)
+					printDots(r)
+					results = append(results, r)
+				}
+				fmt.Println()
+			}
+
+			// Print failures
+			var failures []TestResult
+			for _, r := range results {
+				if !r.Passed {
+					failures = append(failures, r)
+				}
+			}
+			if len(failures) > 0 {
+				fmt.Println()
+				for _, f := range failures {
+					fmt.Println(formatFailure(f))
+					fmt.Println()
+				}
+			}
+
+			elapsed := time.Since(start)
+			fmt.Println(formatSummary(results, elapsed))
+
+			if len(failures) > 0 {
+				os.Exit(1)
+			}
+			return nil
+		},
+	}
+
+	cmd.Flags().BoolVarP(&update, "update", "u", false, "Rewrite expected output in-place with actual output")
+	cmd.Flags().BoolVarP(&keep, "keep", "k", false, "Keep temp directories after run")
+	cmd.Flags().BoolVar(&cleanEnv, "clean-env", false, "Start with empty environment")
+	cmd.Flags().StringArrayVar(&pathDirs, "path", nil, "Prepend <path> to PATH (repeatable)")
+	cmd.Flags().StringVar(&timeout, "timeout", "10s", "Per-command timeout")
+	cmd.Flags().BoolVarP(&verbose, "verbose", "v", false, "Print each command as it runs")
+	cmd.Flags().IntVar(&portFrom, "port-from", 0, "Auto-assign $PORT starting from <n>")
+	cmd.Flags().BoolVar(&parallel, "parallel", false, "Run files in parallel")
+
+	return cmd
+}
+
+func versionCmd() *cobra.Command {
+	return &cobra.Command{
+		Use:   "version",
+		Short: "Print the version",
+		Run: func(cmd *cobra.Command, args []string) {
+			fmt.Println(version)
+		},
+	}
+}
+
+func exampleCmd() *cobra.Command {
+	return &cobra.Command{
+		Use:   "example",
+		Short: "Print an example .shout file",
+		Run: func(cmd *cobra.Command, args []string) {
+			fmt.Print(`# Example .shout file
+$ echo hello
+hello
+
+$ echo "one"; echo "two"; echo "three"
+one
+...
+three
+
+$ cat nonexistent
+cat: nonexistent: ...
+[1]
+
+$ true
+[0]
+`)
+		},
+	}
+}
+
+func findShoutFiles(paths []string) ([]string, error) {
+	var files []string
+
+	for _, p := range paths {
+		abs, err := filepath.Abs(p)
+		if err != nil {
+			continue
+		}
+
+		info, err := os.Stat(abs)
+		if err != nil {
+			if strings.HasSuffix(abs, ".shout") {
+				files = append(files, abs)
+			}
+			continue
+		}
+
+		if info.IsDir() {
+			_ = filepath.WalkDir(abs, func(path string, d fs.DirEntry, err error) error {
+				if err != nil {
+					return nil
+				}
+				if !d.IsDir() && strings.HasSuffix(path, ".shout") {
+					files = append(files, path)
+				}
+				return nil
+			})
+		} else if strings.HasSuffix(abs, ".shout") {
+			files = append(files, abs)
+		}
+	}
+
+	sort.Strings(files)
+	return files, nil
+}
diff --git a/color.go b/color.go
new file mode 100644
index 0000000..806422a
--- /dev/null
+++ b/color.go
@@ -0,0 +1,40 @@
+package main
+
+import "os"
+
+var colorEnabled = detectColor()
+
+func detectColor() bool {
+	if os.Getenv("NO_COLOR") != "" {
+		return false
+	}
+	if os.Getenv("TERM") == "dumb" {
+		return false
+	}
+	fi, err := os.Stdout.Stat()
+	if err != nil {
+		return false
+	}
+	return fi.Mode()&os.ModeCharDevice != 0
+}
+
+func red(s string) string {
+	if !colorEnabled {
+		return s
+	}
+	return "\033[31m" + s + "\033[0m"
+}
+
+func green(s string) string {
+	if !colorEnabled {
+		return s
+	}
+	return "\033[32m" + s + "\033[0m"
+}
+
+func dim(s string) string {
+	if !colorEnabled {
+		return s
+	}
+	return "\033[2m" + s + "\033[0m"
+}
diff --git a/duration.go b/duration.go
new file mode 100644
index 0000000..9afbaf5
--- /dev/null
+++ b/duration.go
@@ -0,0 +1,29 @@
+package main
+
+import (
+	"fmt"
+	"regexp"
+	"strconv"
+	"time"
+)
+
+var durationRe = regexp.MustCompile(`^(\d+(?:\.\d+)?)(ms|s|m)$`)
+
+func parseDuration(s string) (time.Duration, error) {
+	m := durationRe.FindStringSubmatch(s)
+	if m == nil {
+		return 0, fmt.Errorf("invalid duration: %s", s)
+	}
+
+	value, _ := strconv.ParseFloat(m[1], 64)
+	switch m[2] {
+	case "ms":
+		return time.Duration(value * float64(time.Millisecond)), nil
+	case "s":
+		return time.Duration(value * float64(time.Second)), nil
+	case "m":
+		return time.Duration(value * float64(time.Minute)), nil
+	default:
+		return 0, fmt.Errorf("unknown unit: %s", m[2])
+	}
+}
diff --git a/duration_test.go b/duration_test.go
new file mode 100644
index 0000000..914e830
--- /dev/null
+++ b/duration_test.go
@@ -0,0 +1,60 @@
+package main
+
+import (
+	"testing"
+	"time"
+)
+
+func TestParseDurationMs(t *testing.T) {
+	d, err := parseDuration("500ms")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if d != 500*time.Millisecond {
+		t.Errorf("got %v, want 500ms", d)
+	}
+}
+
+func TestParseDurationSeconds(t *testing.T) {
+	d, err := parseDuration("10s")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if d != 10*time.Second {
+		t.Errorf("got %v, want 10s", d)
+	}
+}
+
+func TestParseDurationDecimal(t *testing.T) {
+	d, err := parseDuration("1.5s")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if d != 1500*time.Millisecond {
+		t.Errorf("got %v, want 1.5s", d)
+	}
+}
+
+func TestParseDurationMinutes(t *testing.T) {
+	d, err := parseDuration("1m")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if d != time.Minute {
+		t.Errorf("got %v, want 1m", d)
+	}
+}
+
+func TestParseDurationInvalid(t *testing.T) {
+	_, err := parseDuration("abc")
+	if err == nil {
+		t.Error("expected error for invalid duration")
+	}
+}
+
+func TestParseDurationNoUnit(t *testing.T) {
+	_, err := parseDuration("10")
+	if err == nil {
+		t.Error("expected error for missing unit")
+	}
+}
diff --git a/format.go b/format.go
new file mode 100644
index 0000000..4c984c1
--- /dev/null
+++ b/format.go
@@ -0,0 +1,139 @@
+package main
+
+import (
+	"fmt"
+	"strings"
+	"time"
+)
+
+func evaluateFile(path string, results []CommandResult, fileError string) TestResult {
+	if fileError != "" {
+		return TestResult{
+			Path:         path,
+			Passed:       false,
+			CommandCount: len(results),
+			Error:        fileError,
+		}
+	}
+
+	var failures []FailedCommand
+
+	for _, r := range results {
+		cmd := r.Command
+		outputMatches := matchOutput(cmd.Expected, r.Actual)
+
+		var exitCodeMismatch bool
+		switch cmd.ExitCodeType {
+		case ExitCodeNone:
+			exitCodeMismatch = r.ExitCode != 0
+		case ExitCodeWildcard:
+			exitCodeMismatch = r.ExitCode == 0
+		case ExitCodeExact:
+			exitCodeMismatch = r.ExitCode != cmd.ExitCodeValue
+		}
+
+		if !outputMatches || exitCodeMismatch {
+			var diffLines []DiffLine
+			if !outputMatches {
+				diffLines = diff(cmd.Expected, r.Actual)
+			}
+			failures = append(failures, FailedCommand{
+				Result:           r,
+				DiffLines:        diffLines,
+				ExitCodeMismatch: exitCodeMismatch,
+			})
+		}
+	}
+
+	return TestResult{
+		Path:         path,
+		Passed:       len(failures) == 0,
+		CommandCount: len(results),
+		Failures:     failures,
+	}
+}
+
+func formatFailure(t TestResult) string {
+	var lines []string
+
+	lines = append(lines, red("FAIL "+t.Path))
+
+	if t.Error != "" {
+		lines = append(lines, "  "+red(t.Error))
+		return strings.Join(lines, "\n")
+	}
+
+	for _, f := range t.Failures {
+		lines = append(lines, "")
+		lines = append(lines, "  "+dim("$")+" "+f.Result.Command.Cmd)
+
+		if len(f.DiffLines) > 0 {
+			lines = append(lines, red("  expected:"))
+			for _, dl := range f.DiffLines {
+				switch dl.Kind {
+				case "expected":
+					lines = append(lines, red("  > ")+dl.Text)
+				case "equal":
+					lines = append(lines, "    "+dl.Text)
+				case "context":
+					lines = append(lines, "    "+dim(dl.Text))
+				}
+			}
+			lines = append(lines, green("  actual:"))
+			for _, dl := range f.DiffLines {
+				switch dl.Kind {
+				case "actual":
+					lines = append(lines, green("  > ")+dl.Text)
+				case "equal":
+					lines = append(lines, "    "+dl.Text)
+				case "context":
+					lines = append(lines, "    "+dim(dl.Text))
+				}
+			}
+		}
+
+		if f.ExitCodeMismatch {
+			cmd := f.Result.Command
+			var expectedStr string
+			switch cmd.ExitCodeType {
+			case ExitCodeNone:
+				expectedStr = "0"
+			case ExitCodeWildcard:
+				expectedStr = "non-zero"
+			case ExitCodeExact:
+				expectedStr = fmt.Sprintf("%d", cmd.ExitCodeValue)
+			}
+			lines = append(lines, red(fmt.Sprintf("  expected exit code: %s", expectedStr)))
+			lines = append(lines, green(fmt.Sprintf("  actual exit code: %d", f.Result.ExitCode)))
+		}
+	}
+
+	return strings.Join(lines, "\n")
+}
+
+func formatSummary(results []TestResult, elapsed time.Duration) string {
+	totalCommands := 0
+	failedCommands := 0
+	for _, r := range results {
+		totalCommands += r.CommandCount
+		failedCommands += len(r.Failures)
+	}
+	passedCommands := totalCommands - failedCommands
+
+	var parts []string
+	if passedCommands > 0 {
+		parts = append(parts, green(fmt.Sprintf("%d passed", passedCommands)))
+	}
+	if failedCommands > 0 {
+		parts = append(parts, red(fmt.Sprintf("%d failed", failedCommands)))
+	}
+
+	var timeStr string
+	if elapsed < time.Second {
+		timeStr = fmt.Sprintf("%dms", elapsed.Milliseconds())
+	} else {
+		timeStr = fmt.Sprintf("%.1fs", elapsed.Seconds())
+	}
+
+	return fmt.Sprintf("%s in %s", strings.Join(parts, ", "), timeStr)
+}
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..437126a
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,10 @@
+module shout
+
+go 1.24.1
+
+require github.com/spf13/cobra v1.10.2
+
+require (
+	github.com/inconshreveable/mousetrap v1.1.0 // indirect
+	github.com/spf13/pflag v1.0.9 // indirect
+)
diff --git a/go.sum b/go.sum
new file mode 100644
index 0000000..a6ee3e0
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,10 @@
+github.com/cpuguy83/go-md2man/v2 v2.0.6/go.mod h1:oOW0eioCTA6cOiMLiUPZOpcVxMig6NIQQ7OS05n1F4g=
+github.com/inconshreveable/mousetrap v1.1.0 h1:wN+x4NVGpMsO7ErUn/mUI3vEoE6Jt13X2s0bqwp9tc8=
+github.com/inconshreveable/mousetrap v1.1.0/go.mod h1:vpF70FUmC8bwa3OWnCshd2FqLfsEA9PFc4w1p2J65bw=
+github.com/russross/blackfriday/v2 v2.1.0/go.mod h1:+Rmxgy9KzJVeS9/2gXHxylqXiyQDYRxCVz55jmeOWTM=
+github.com/spf13/cobra v1.10.2 h1:DMTTonx5m65Ic0GOoRY2c16WCbHxOOw6xxezuLaBpcU=
+github.com/spf13/cobra v1.10.2/go.mod h1:7C1pvHqHw5A4vrJfjNwvOdzYu0Gml16OCs2GRiTUUS4=
+github.com/spf13/pflag v1.0.9 h1:9exaQaMOCwffKiiiYk6/BndUBv+iRViNW+4lEMi0PvY=
+github.com/spf13/pflag v1.0.9/go.mod h1:McXfInJRrz4CZXVZOBLb0bTZqETkiAhM9Iw0y3An2Bg=
+go.yaml.in/yaml/v3 v3.0.4/go.mod h1:DhzuOOF2ATzADvBadXxruRBLzYTpT36CKvDb3+aBEFg=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
diff --git a/main.go b/main.go
new file mode 100644
index 0000000..a5a50d9
--- /dev/null
+++ b/main.go
@@ -0,0 +1,13 @@
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+func main() {
+	if err := rootCmd.Execute(); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+}
diff --git a/match.go b/match.go
new file mode 100644
index 0000000..e00d996
--- /dev/null
+++ b/match.go
@@ -0,0 +1,103 @@
+package main
+
+import (
+	"regexp"
+	"strings"
+)
+
+func matchLine(pattern, actual string) bool {
+	if !strings.Contains(pattern, "...") {
+		return pattern == actual
+	}
+
+	parts := strings.Split(pattern, "...")
+	escaped := make([]string, len(parts))
+	for i, p := range parts {
+		escaped[i] = regexp.QuoteMeta(p)
+	}
+	re := regexp.MustCompile("^" + strings.Join(escaped, ".*") + "$")
+	return re.MatchString(actual)
+}
+
+func matchOutput(expected, actual []string) bool {
+	return doMatch(expected, 0, actual, 0)
+}
+
+func doMatch(expected []string, ei int, actual []string, ai int) bool {
+	if ei == len(expected) && ai == len(actual) {
+		return true
+	}
+	if ei == len(expected) {
+		return false
+	}
+
+	exp := expected[ei]
+
+	// Multi-line wildcard
+	if exp == "..." {
+		for skip := ai; skip <= len(actual); skip++ {
+			if doMatch(expected, ei+1, actual, skip) {
+				return true
+			}
+		}
+		return false
+	}
+
+	if ai == len(actual) {
+		return false
+	}
+
+	if matchLine(exp, actual[ai]) {
+		return doMatch(expected, ei+1, actual, ai+1)
+	}
+
+	return false
+}
+
+func diff(expected, actual []string) []DiffLine {
+	var result []DiffLine
+	ei, ai := 0, 0
+
+	for ei < len(expected) || ai < len(actual) {
+		if ei < len(expected) && expected[ei] == "..." {
+			nextExp := ""
+			hasNext := ei+1 < len(expected)
+			if hasNext {
+				nextExp = expected[ei+1]
+			}
+
+			if !hasNext {
+				result = append(result, DiffLine{Kind: "context", Text: "..."})
+				break
+			}
+
+			result = append(result, DiffLine{Kind: "context", Text: "..."})
+			ei++
+			for ai < len(actual) && !matchLine(nextExp, actual[ai]) {
+				ai++
+			}
+			continue
+		}
+
+		if ei < len(expected) && ai < len(actual) {
+			if matchLine(expected[ei], actual[ai]) {
+				result = append(result, DiffLine{Kind: "equal", Text: actual[ai]})
+				ei++
+				ai++
+			} else {
+				result = append(result, DiffLine{Kind: "expected", Text: expected[ei]})
+				result = append(result, DiffLine{Kind: "actual", Text: actual[ai]})
+				ei++
+				ai++
+			}
+		} else if ei < len(expected) {
+			result = append(result, DiffLine{Kind: "expected", Text: expected[ei]})
+			ei++
+		} else {
+			result = append(result, DiffLine{Kind: "actual", Text: actual[ai]})
+			ai++
+		}
+	}
+
+	return result
+}
diff --git a/match_test.go b/match_test.go
new file mode 100644
index 0000000..26ae213
--- /dev/null
+++ b/match_test.go
@@ -0,0 +1,127 @@
+package main
+
+import "testing"
+
+func TestMatchLineExact(t *testing.T) {
+	if !matchLine("hello", "hello") {
+		t.Error("exact match should pass")
+	}
+	if matchLine("hello", "world") {
+		t.Error("exact mismatch should fail")
+	}
+}
+
+func TestMatchLineInlineWildcard(t *testing.T) {
+	if !matchLine("Homebrew 5...", "Homebrew 5.1.0") {
+		t.Error("trailing wildcard should match")
+	}
+	if !matchLine("...world", "hello world") {
+		t.Error("leading wildcard should match")
+	}
+	if !matchLine("a...b...c", "aXXbYYc") {
+		t.Error("multiple wildcards should match")
+	}
+	if matchLine("a...c", "aXXd") {
+		t.Error("should not match when suffix differs")
+	}
+}
+
+func TestMatchLinePreservesLiteralDots(t *testing.T) {
+	if !matchLine("match ...", "match ...") {
+		t.Error("literal ... should match itself")
+	}
+}
+
+func TestMatchOutputEmpty(t *testing.T) {
+	if !matchOutput(nil, nil) {
+		t.Error("both empty should match")
+	}
+	if !matchOutput([]string{}, []string{}) {
+		t.Error("both empty slices should match")
+	}
+}
+
+func TestMatchOutputExact(t *testing.T) {
+	if !matchOutput([]string{"hello", "world"}, []string{"hello", "world"}) {
+		t.Error("exact match should pass")
+	}
+	if matchOutput([]string{"hello"}, []string{"world"}) {
+		t.Error("mismatch should fail")
+	}
+}
+
+func TestMatchOutputExtraActual(t *testing.T) {
+	if matchOutput([]string{"hello"}, []string{"hello", "world"}) {
+		t.Error("extra actual lines should fail")
+	}
+}
+
+func TestMatchOutputExtraExpected(t *testing.T) {
+	if matchOutput([]string{"hello", "world"}, []string{"hello"}) {
+		t.Error("extra expected lines should fail")
+	}
+}
+
+func TestMatchOutputMultilineWildcard(t *testing.T) {
+	if !matchOutput([]string{"first", "...", "last"}, []string{"first", "a", "b", "c", "last"}) {
+		t.Error("multiline wildcard should match multiple lines")
+	}
+	if !matchOutput([]string{"first", "...", "last"}, []string{"first", "last"}) {
+		t.Error("multiline wildcard should match zero lines")
+	}
+	if !matchOutput([]string{"..."}, []string{"a", "b", "c"}) {
+		t.Error("standalone wildcard should match anything")
+	}
+	if !matchOutput([]string{"..."}, nil) {
+		t.Error("standalone wildcard should match empty")
+	}
+}
+
+func TestMatchOutputMixed(t *testing.T) {
+	expected := []string{"one", "...", "three"}
+	actual := []string{"one", "two", "three"}
+	if !matchOutput(expected, actual) {
+		t.Error("mixed wildcard should match")
+	}
+}
+
+func TestMatchOutputWildcardAtEnd(t *testing.T) {
+	if !matchOutput([]string{"start", "..."}, []string{"start", "a", "b"}) {
+		t.Error("trailing wildcard should match remaining lines")
+	}
+}
+
+func TestMatchOutputInlineAndMultiline(t *testing.T) {
+	expected := []string{"Homebrew 5...", "..."}
+	actual := []string{"Homebrew 5.1.0", "extra line"}
+	if !matchOutput(expected, actual) {
+		t.Error("inline + multiline wildcards should work together")
+	}
+}
+
+func TestDiffBasic(t *testing.T) {
+	d := diff([]string{"hello"}, []string{"world"})
+	if len(d) != 2 {
+		t.Fatalf("expected 2 diff lines, got %d", len(d))
+	}
+	if d[0].Kind != "expected" || d[0].Text != "hello" {
+		t.Errorf("d[0] = %+v", d[0])
+	}
+	if d[1].Kind != "actual" || d[1].Text != "world" {
+		t.Errorf("d[1] = %+v", d[1])
+	}
+}
+
+func TestDiffEqual(t *testing.T) {
+	d := diff([]string{"hello"}, []string{"hello"})
+	if len(d) != 1 || d[0].Kind != "equal" {
+		t.Errorf("expected equal, got %+v", d)
+	}
+}
+
+func TestDiffContext(t *testing.T) {
+	d := diff([]string{"..."}, []string{"a", "b"})
+	if len(d) != 1 || d[0].Kind != "context" {
+		t.Errorf("expected context, got %+v", d)
+	}
+}
diff --git a/parse.go b/parse.go
new file mode 100644
index 0000000..623211b
--- /dev/null
+++ b/parse.go
@@ -0,0 +1,128 @@
+package main
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+)
+
+func stripComment(line string) string {
+	inSingle := false
+	inDouble := false
+	for i := 0; i < len(line); i++ {
+		ch := line[i]
+		switch {
+		case ch == '\'' && !inDouble:
+			inSingle = !inSingle
+		case ch == '"' && !inSingle:
+			inDouble = !inDouble
+		case ch == '#' && !inSingle && !inDouble:
+			return strings.TrimRight(line[:i], " \t")
+		}
+	}
+	return line
+}
+
+func parseExitCode(lines []string) ([]string, ExitCodeType, int) {
+	if len(lines) == 0 {
+		return lines, ExitCodeNone, 0
+	}
+
+	last := lines[len(lines)-1]
+	if len(last) < 3 || last[0] != '[' || last[len(last)-1] != ']' {
+		return lines, ExitCodeNone, 0
+	}
+
+	inner := last[1 : len(last)-1]
+	if inner == "*" {
+		return lines[:len(lines)-1], ExitCodeWildcard, 0
+	}
+
+	code, err := strconv.Atoi(inner)
+	if err != nil {
+		return lines, ExitCodeNone, 0
+	}
+	return lines[:len(lines)-1], ExitCodeExact, code
+}
+
+func trimTrailingEmpty(lines []string) []string {
+	end := len(lines)
+	for end > 0 && lines[end-1] == "" {
+		end--
+	}
+	return lines[:end]
+}
+
+func parse(path, content string) (ShoutFile, error) {
+	rawLines := strings.Split(content, "\n")
+
+	// Remove trailing newline
+	if len(rawLines) > 0 && rawLines[len(rawLines)-1] == "" {
+		rawLines = rawLines[:len(rawLines)-1]
+	}
+
+	var commands []Command
+	var directives []Directive
+	var current *Command
+	seenCommand := false
+
+	for i, line := range rawLines {
+		lineNum := i + 1
+
+		if !seenCommand && strings.HasPrefix(line, "@") {
+			if strings.HasPrefix(line, "@setup ") {
+				setupPath := strings.TrimSpace(line[7:])
+				if setupPath == "" {
+					return ShoutFile{}, fmt.Errorf("%s:%d: @setup requires a file path", path, lineNum)
+				}
+				directives = append(directives, Directive{
+					Type: "setup", Path: setupPath, Line: lineNum,
+				})
+			} else if strings.HasPrefix(line, "@env ") {
+				rest := strings.TrimSpace(line[5:])
+				eq := strings.Index(rest, "=")
+				if eq <= 0 {
+					return ShoutFile{}, fmt.Errorf("%s:%d: malformed @env directive (expected KEY=VALUE): %s", path, lineNum, line)
+				}
+				directives = append(directives, Directive{
+					Type: "env", Key: rest[:eq], Value: rest[eq+1:], Line: lineNum,
+				})
+			} else {
+				return ShoutFile{}, fmt.Errorf("%s:%d: unknown directive: %s", path, lineNum, line)
+			}
+			continue
+		}
+
+		if strings.HasPrefix(line, "$ ") {
+			seenCommand = true
+			if current != nil {
+				trimmed := trimTrailingEmpty(current.Expected)
+				remaining, ecType, ecVal := parseExitCode(trimmed)
+				current.Expected = trimTrailingEmpty(remaining)
+				current.ExitCodeType = ecType
+				current.ExitCodeValue = ecVal
+				commands = append(commands, *current)
+			}
+
+			current = &Command{
+				Line:     lineNum,
+				Raw:      line,
+				Cmd:      stripComment(line[2:]),
+				Expected: nil,
+			}
+		} else if current != nil {
+			current.Expected = append(current.Expected, line)
+		}
+	}
+
+	if current != nil {
+		trimmed := trimTrailingEmpty(current.Expected)
+		remaining, ecType, ecVal := parseExitCode(trimmed)
+		current.Expected = trimTrailingEmpty(remaining)
+		current.ExitCodeType = ecType
+		current.ExitCodeValue = ecVal
+		commands = append(commands, *current)
+	}
+
+	return ShoutFile{Path: path, Commands: commands, Directives: directives}, nil
+}
diff --git a/parse_test.go b/parse_test.go
new file mode 100644
index 0000000..6eb7ede
--- /dev/null
+++ b/parse_test.go
@@ -0,0 +1,167 @@
+package main
+
+import (
+	"testing"
+)
+
+func TestParseBasicCommand(t *testing.T) {
+	sf, err := parse("test.shout", "$ echo hello\nhello\n")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(sf.Commands) != 1 {
+		t.Fatalf("expected 1 command, got %d", len(sf.Commands))
+	}
+	c := sf.Commands[0]
+	if c.Cmd != "echo hello" {
+		t.Errorf("cmd = %q, want %q", c.Cmd, "echo hello")
+	}
+	if len(c.Expected) != 1 || c.Expected[0] != "hello" {
+		t.Errorf("expected = %v, want [hello]", c.Expected)
+	}
+	if c.ExitCodeType != ExitCodeNone {
+		t.Errorf("exit code type = %d, want ExitCodeNone", c.ExitCodeType)
+	}
+}
+
+func TestParseMultipleCommands(t *testing.T) {
+	sf, err := parse("test.shout", "$ echo one\none\n\n$ echo two\ntwo\n")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(sf.Commands) != 2 {
+		t.Fatalf("expected 2 commands, got %d", len(sf.Commands))
+	}
+	if sf.Commands[0].Cmd != "echo one" {
+		t.Errorf("cmd[0] = %q", sf.Commands[0].Cmd)
+	}
+	if sf.Commands[1].Cmd != "echo two" {
+		t.Errorf("cmd[1] = %q", sf.Commands[1].Cmd)
+	}
+}
+
+func TestParseExitCode(t *testing.T) {
+	sf, err := parse("test.shout", "$ false\n[1]\n")
+	if err != nil {
+		t.Fatal(err)
+	}
+	c := sf.Commands[0]
+	if c.ExitCodeType != ExitCodeExact || c.ExitCodeValue != 1 {
+		t.Errorf("exit code = (%d, %d), want (Exact, 1)", c.ExitCodeType, c.ExitCodeValue)
+	}
+	if len(c.Expected) != 0 {
+		t.Errorf("expected = %v, want []", c.Expected)
+	}
+}
+
+func TestParseExitCodeWildcard(t *testing.T) {
+	sf, err := parse("test.shout", "$ false\n[*]\n")
+	if err != nil {
+		t.Fatal(err)
+	}
+	c := sf.Commands[0]
+	if c.ExitCodeType != ExitCodeWildcard {
+		t.Errorf("exit code type = %d, want Wildcard", c.ExitCodeType)
+	}
+}
+
+func TestParseExitCodeWithOutput(t *testing.T) {
+	sf, err := parse("test.shout", "$ sh -c \"echo oops && exit 1\"\noops\n[*]\n")
+	if err != nil {
+		t.Fatal(err)
+	}
+	c := sf.Commands[0]
+	if len(c.Expected) != 1 || c.Expected[0] != "oops" {
+		t.Errorf("expected = %v, want [oops]", c.Expected)
+	}
+	if c.ExitCodeType != ExitCodeWildcard {
+		t.Errorf("exit code type = %d, want Wildcard", c.ExitCodeType)
+	}
+}
+
+func TestParseComment(t *testing.T) {
+	sf, err := parse("test.shout", "$ echo hello   # this is a comment\nhello\n")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if sf.Commands[0].Cmd != "echo hello" {
+		t.Errorf("cmd = %q, want %q", sf.Commands[0].Cmd, "echo hello")
+	}
+}
+
+func TestParseCommentInQuotes(t *testing.T) {
+	sf, err := parse("test.shout", "$ echo \"keep # this\"\nkeep # this\n")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if sf.Commands[0].Cmd != `echo "keep # this"` {
+		t.Errorf("cmd = %q", sf.Commands[0].Cmd)
+	}
+}
+
+func TestParseEnvDirective(t *testing.T) {
+	sf, err := parse("test.shout", "@env GREETING=hello\n\n$ echo $GREETING\nhello\n")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(sf.Directives) != 1 {
+		t.Fatalf("expected 1 directive, got %d", len(sf.Directives))
+	}
+	d := sf.Directives[0]
+	if d.Type != "env" || d.Key != "GREETING" || d.Value != "hello" {
+		t.Errorf("directive = %+v", d)
+	}
+}
+
+func TestParseSetupDirective(t *testing.T) {
+	sf, err := parse("test.shout", "@setup shared.shout\n\n$ echo hi\nhi\n")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(sf.Directives) != 1 {
+		t.Fatalf("expected 1 directive, got %d", len(sf.Directives))
+	}
+	d := sf.Directives[0]
+	if d.Type != "setup" || d.Path != "shared.shout" {
+		t.Errorf("directive = %+v", d)
+	}
+}
+
+func TestParseNoExpectedOutput(t *testing.T) {
+	sf, err := parse("test.shout", "$ export FOO=bar\n$ echo $FOO\nbar\n")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(sf.Commands) != 2 {
+		t.Fatalf("expected 2 commands, got %d", len(sf.Commands))
+	}
+	if len(sf.Commands[0].Expected) != 0 {
+		t.Errorf("first command expected = %v, want []", sf.Commands[0].Expected)
+	}
+}
+
+func TestParseTrimsTrailingBlanks(t *testing.T) {
+	sf, err := parse("test.shout", "$ echo hello\nhello\n\n\n$ echo world\nworld\n")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(sf.Commands[0].Expected) != 1 || sf.Commands[0].Expected[0] != "hello" {
+		t.Errorf("expected = %v", sf.Commands[0].Expected)
+	}
+}
+
+func TestParseLineNumbers(t *testing.T) {
+	sf, err := parse("test.shout", "@env X=1\n\n$ echo hello\nhello\n\n$ echo world\nworld\n")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if sf.Directives[0].Line != 1 {
+		t.Errorf("directive line = %d, want 1", sf.Directives[0].Line)
+	}
+	if sf.Commands[0].Line != 3 {
+		t.Errorf("command[0] line = %d, want 3", sf.Commands[0].Line)
+	}
+	if sf.Commands[1].Line != 6 {
+		t.Errorf("command[1] line = %d, want 6", sf.Commands[1].Line)
+	}
+}
diff --git a/run.go b/run.go
new file mode 100644
index 0000000..887c61c
--- /dev/null
+++ b/run.go
@@ -0,0 +1,203 @@
+package main
+
+import (
+	"io"
+	"os"
+	"os/exec"
+	"regexp"
+	"strconv"
+	"strings"
+	"syscall"
+	"time"
+)
+
+const sentinelPrefix = "__SHOUT_SENTINEL_"
+
+type RunOptions struct {
+	CleanEnv  bool
+	PathDirs  []string
+	EnvVars   map[string]string
+	Timeout   time.Duration
+	Verbose   bool
+	OnCommand func(Command)
+}
+
+func buildScript(commands []Command) string {
+	var b strings.Builder
+	b.WriteString("exec 2>&1\n")
+	for i, cmd := range commands {
+		b.WriteString(cmd.Cmd)
+		b.WriteByte('\n')
+		b.WriteString("printf '\\n" + sentinelPrefix + "%s_" + strconv.Itoa(i) + "__\\n' \"$?\"\n")
+	}
+	return b.String()
+}
+
+func parseSentinelOutput(raw string, commandCount int) (outputs [][]string, exitCodes []int) {
+	re := regexp.MustCompile(regexp.QuoteMeta(sentinelPrefix) + `(\d+)_(\d+)__`)
+
+	remaining := raw
+	for i := 0; i < commandCount; i++ {
+		loc := re.FindStringSubmatchIndex(remaining)
+		if loc == nil {
+			lines := strings.Split(remaining, "\n")
+			if len(lines) > 0 && lines[0] == "" {
+				lines = lines[1:]
+			}
+			lines = trimTrailingEmpty(lines)
+			outputs = append(outputs, lines)
+			exitCodes = append(exitCodes, 1)
+			break
+		}
+
+		before := remaining[:loc[0]]
+		exitCodeStr := remaining[loc[2]:loc[3]]
+		ec, _ := strconv.Atoi(exitCodeStr)
+
+		lines := strings.Split(before, "\n")
+		if len(lines) > 0 && lines[0] == "" {
+			lines = lines[1:]
+		}
+		lines = trimTrailingEmpty(lines)
+		if len(lines) == 1 && lines[0] == "" {
+			lines = nil
+		}
+
+		outputs = append(outputs, lines)
+		exitCodes = append(exitCodes, ec)
+
+		afterSentinel := remaining[loc[1]:]
+		if strings.HasPrefix(afterSentinel, "\n") {
+			afterSentinel = afterSentinel[1:]
+		}
+		remaining = afterSentinel
+	}
+
+	for len(outputs) < commandCount {
+		outputs = append(outputs, nil)
+		exitCodes = append(exitCodes, 1)
+	}
+
+	return outputs, exitCodes
+}
+
+func runFile(file ShoutFile, opts RunOptions) FileResult {
+	tmpDir, err := os.MkdirTemp("", "shout-")
+	if err != nil {
+		return FileResult{File: file, TmpDir: "", Error: err.Error()}
+	}
+
+	if len(file.Commands) == 0 {
+		return FileResult{File: file, TmpDir: tmpDir}
+	}
+
+	script := buildScript(file.Commands)
+
+	// Build environment
+	var envMap map[string]string
+	if opts.CleanEnv {
+		envMap = make(map[string]string)
+	} else {
+		envMap = make(map[string]string)
+		for _, e := range os.Environ() {
+			if k, v, ok := strings.Cut(e, "="); ok {
+				envMap[k] = v
+			}
+		}
+	}
+
+	envMap["HOME"] = tmpDir
+	envMap["SHOUT_DIR"] = tmpDir
+
+	for k, v := range opts.EnvVars {
+		envMap[k] = v
+	}
+
+	if len(opts.PathDirs) > 0 {
+		existing := envMap["PATH"]
+		envMap["PATH"] = strings.Join(opts.PathDirs, ":") + ":" + existing
+	}
+
+	envSlice := make([]string, 0, len(envMap))
+	for k, v := range envMap {
+		envSlice = append(envSlice, k+"="+v)
+	}
+
+	cmd := exec.Command("/bin/sh")
+	cmd.Dir = tmpDir
+	cmd.Env = envSlice
+	cmd.Stderr = nil
+	cmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true}
+
+	stdin, err := cmd.StdinPipe()
+	if err != nil {
+		return FileResult{File: file, TmpDir: tmpDir, Error: err.Error()}
+	}
+
+	stdoutPipe, err := cmd.StdoutPipe()
+	if err != nil {
+		return FileResult{File: file, TmpDir: tmpDir, Error: err.Error()}
+	}
+
+	if err := cmd.Start(); err != nil {
+		return FileResult{File: file, TmpDir: tmpDir, Error: err.Error()}
+	}
+
+	defer func() {
+		if cmd.Process != nil {
+			_ = syscall.Kill(-cmd.Process.Pid, syscall.SIGKILL)
+		}
+	}()
+
+	_, _ = io.WriteString(stdin, script)
+	stdin.Close()
+
+	// Read stdout with timeout
+	totalTimeout := opts.Timeout * time.Duration(len(file.Commands))
+	type readResult struct {
+		data []byte
+		err  error
+	}
+	ch := make(chan readResult, 1)
+	go func() {
+		data, err := io.ReadAll(stdoutPipe)
+		ch <- readResult{data, err}
+	}()
+
+	var output []byte
+	select {
+	case r := <-ch:
+		if r.err != nil {
+			return FileResult{File: file, TmpDir: tmpDir, Error: r.err.Error()}
+		}
+		output = r.data
+	case <-time.After(totalTimeout):
+		return FileResult{File: file, TmpDir: tmpDir, Error: "Timeout reading output"}
+	}
+
+	_ = cmd.Wait()
+
+	outputs, exitCodesList := parseSentinelOutput(string(output), len(file.Commands))
+
+	results := make([]CommandResult, len(file.Commands))
+	for i, c := range file.Commands {
+		if opts.Verbose && opts.OnCommand != nil {
+			opts.OnCommand(c)
+		}
+		actual := outputs[i]
+		if actual == nil {
+			actual = []string{}
+		}
+		results[i] = CommandResult{
+			Command:  c,
+			Actual:   actual,
+			ExitCode: exitCodesList[i],
+		}
+	}
+
+	return FileResult{File: file, Results: results, TmpDir: tmpDir}
+}
+
+func cleanupTmpDir(dir string) {
+	os.RemoveAll(dir)
+}
diff --git a/shout b/shout
new file mode 100755
index 0000000..440a587
Binary files /dev/null and b/shout differ
diff --git a/test/basic.shout b/test/basic.shout
new file mode 100644
index 0000000..2d759ed
--- /dev/null
+++ b/test/basic.shout
@@ -0,0 +1,9 @@
+$ echo hello
+hello
+
+$ echo one && echo two
+one
+two
+
+$ echo "working directory: $(basename $PWD)"
+working directory: ...
diff --git a/test/comments.shout b/test/comments.shout
new file mode 100644
index 0000000..54fe5eb
--- /dev/null
+++ b/test/comments.shout
@@ -0,0 +1,5 @@
+$ echo hello   # this is a comment
+hello
+
+$ echo "keep # this"
+keep # this
diff --git a/test/env.shout b/test/env.shout
new file mode 100644
index 0000000..245d02c
--- /dev/null
+++ b/test/env.shout
@@ -0,0 +1,5 @@
+@env GREETING=hello
+@env TARGET=world
+
+$ echo "$GREETING $TARGET"
+hello world
diff --git a/test/features.shout b/test/features.shout
new file mode 100644
index 0000000..92eed91
--- /dev/null
+++ b/test/features.shout
@@ -0,0 +1,28 @@
+$ echo "test exit codes"
+test exit codes
+
+$ false
+[1]
+
+$ sh -c "exit 42"
+[42]
+
+$ sh -c "echo oops && exit 1"
+oops
+[*]
+
+$ export MY_VAR=hello
+$ echo $MY_VAR
+hello
+
+$ cd /tmp
+$ pwd
+/tmp
+
+$ echo "line 1" && echo "" && echo "line 3"
+line 1
+
+line 3
+
+$ echo "match ..."
+match ...
diff --git a/test/setup-shared.shout b/test/setup-shared.shout
new file mode 100644
index 0000000..7ad3eba
--- /dev/null
+++ b/test/setup-shared.shout
@@ -0,0 +1 @@
+$ export READY=yes
diff --git a/test/setup-user.shout b/test/setup-user.shout
new file mode 100644
index 0000000..023c5c2
--- /dev/null
+++ b/test/setup-user.shout
@@ -0,0 +1,4 @@
+@setup setup-shared.shout
+
+$ echo $READY
+yes
diff --git a/types.go b/types.go
new file mode 100644
index 0000000..2e6f295
--- /dev/null
+++ b/types.go
@@ -0,0 +1,65 @@
+package main
+
+// ExitCodeType describes how to match a command's exit code.
+type ExitCodeType int
+
+const (
+	ExitCodeNone     ExitCodeType = iota // not specified — expect 0
+	ExitCodeExact                        // expect specific code
+	ExitCodeWildcard                     // [*] — expect any non-zero
+)
+
+type Command struct {
+	Line          int
+	Raw           string
+	Cmd           string
+	Expected      []string
+	ExitCodeType  ExitCodeType
+	ExitCodeValue int
+}
+
+type Directive struct {
+	Type  string // "setup" or "env"
+	Path  string // for setup
+	Key   string // for env
+	Value string // for env
+	Line  int
+}
+
+type ShoutFile struct {
+	Path       string
+	Commands   []Command
+	Directives []Directive
+}
+
+type CommandResult struct {
+	Command  Command
+	Actual   []string
+	ExitCode int
+}
+
+type FileResult struct {
+	File    ShoutFile
+	Results []CommandResult
+	TmpDir  string
+	Error   string
+}
+
+type DiffLine struct {
+	Kind string // "equal", "expected", "actual", "context"
+	Text string
+}
+
+type TestResult struct {
+	Path         string
+	Passed       bool
+	CommandCount int
+	Failures     []FailedCommand
+	Error        string
+}
+
+type FailedCommand struct {
+	Result           CommandResult
+	DiffLines        []DiffLine
+	ExitCodeMismatch bool
+}
diff --git a/update.go b/update.go
new file mode 100644
index 0000000..aa812b0
--- /dev/null
+++ b/update.go
@@ -0,0 +1,79 @@
+package main
+
+import (
+	"regexp"
+	"strings"
+)
+
+var exitCodeMarkerRe = regexp.MustCompile(`^\[(\d+|\*)\]$`)
+
+func rewriteFile(file ShoutFile, results []CommandResult, originalContent string) string {
+	lines := strings.Split(originalContent, "\n")
+	var output []string
+
+	cmdIdx := 0
+
+	for i := 0; i < len(lines); i++ {
+		line := lines[i]
+
+		if strings.HasPrefix(line, "$ ") {
+			output = append(output, line)
+
+			if cmdIdx >= len(file.Commands) || cmdIdx >= len(results) {
+				cmdIdx++
+				continue
+			}
+
+			cmd := file.Commands[cmdIdx]
+			result := results[cmdIdx]
+
+			// Skip past old expected output lines
+			j := i + 1
+			for j < len(lines) && !strings.HasPrefix(lines[j], "$ ") {
+				j++
+			}
+
+			oldExpectedRaw := lines[i+1 : j]
+
+			// Check for exit code marker
+			oldTrimmed := trimTrailingEmpty(oldExpectedRaw)
+			var oldExitMarker string
+			if len(oldTrimmed) > 0 {
+				last := oldTrimmed[len(oldTrimmed)-1]
+				if exitCodeMarkerRe.MatchString(last) {
+					oldExitMarker = last
+				}
+			}
+
+			// Count trailing blank lines
+			trailingBlanks := 0
+			for k := len(oldExpectedRaw) - 1; k >= 0; k-- {
+				if oldExpectedRaw[k] == "" {
+					trailingBlanks++
+				} else {
+					break
+				}
+			}
+
+			// If wildcards match, keep original
+			if matchOutput(cmd.Expected, result.Actual) {
+				output = append(output, oldExpectedRaw...)
+			} else {
+				output = append(output, result.Actual...)
+				if oldExitMarker != "" {
+					output = append(output, oldExitMarker)
+				}
+				for k := 0; k < trailingBlanks; k++ {
+					output = append(output, "")
+				}
+			}
+
+			i = j - 1
+			cmdIdx++
+		} else if cmdIdx == 0 {
+			output = append(output, line)
+		}
+	}
+
+	return strings.Join(output, "\n")
+}