$PAGER environment variable sets custom pager

command.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"flag"
 	"io"
+	"os"
 	"os/exec"
 	"strings"
 
@@ -37,9 +38,35 @@ func command(stdin io.Reader, stdout io.Writer) error {
 	if err != nil {
 		return err
 	}
+
+	if pager := os.ExpandEnv("$PAGER"); pager != "" {
+		return usePager(pager, stdout, func(w io.Writer) error {
+			return printDiff(w, profiles, moduleInfo)
+		})
+	}
 	return printDiff(stdout, profiles, moduleInfo)
 }
 
+func usePager(
+	name string,
+	stdout io.Writer,
+	printDiff func(io.Writer) error,
+) error {
+	cmd := exec.Command(name)
+	cmd.Stdout = stdout
+	w, err := cmd.StdinPipe()
+	if err != nil {
+		return err
+	}
+
+	go func() {
+		defer w.Close()
+		printDiff(w)
+	}()
+
+	return cmd.Run()
+}
+
 func runGoTests() (*bytes.Buffer, error) {
 	cmd := exec.Command(
 		"go",

command_test.go

@@ -9,6 +9,9 @@ import (
 )
 
 func TestCommand(t *testing.T) {
+	// ensure only specific pager tests execute with custom pager
+	t.Setenv("PAGER", "")
+
 	t.Run("from-stdin-pipe", func(t *testing.T) {
 		args := os.Args
 		t.Cleanup(func() { os.Args = args })
@@ -119,6 +122,30 @@ func TestCommand(t *testing.T) {
 			t.Errorf("got len=%d, want len=%d", len(got), len(want))
 		}
 	})
+
+	t.Run("print-with-pager", func(t *testing.T) {
+		args := os.Args
+		t.Setenv("PAGER", "less")
+		t.Cleanup(func() { os.Args = args })
+
+		// override os.Args which contains flags from the test binary
+		os.Args = []string{
+			"coverdiff",
+		}
+		flag.Parse()
+
+		var stdout bytes.Buffer
+		stdin := bytes.NewBufferString(readFile(t, "testdata/coverage.out"))
+		if err := command(stdin, &stdout); err != nil {
+			t.Fatal(err)
+		}
+
+		got := stdout.String()
+		want := readFile(t, "testdata/coverdiff.out")
+		if got != want {
+			t.Errorf("got len=%d, want len=%d", len(got), len(want))
+		}
+	})
 }
 
 func readFile(tb testing.TB, name string) string {