diff --git a/cmd/liquid/main.go b/cmd/liquid/main.go
index 2b59d00..74ec7ae 100644
--- a/cmd/liquid/main.go
+++ b/cmd/liquid/main.go
@@ -9,10 +9,10 @@
 package main
 
 import (
-	"bytes"
+	"errors"
+	"flag"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"os"
 	"strings"
 
@@ -21,59 +21,79 @@ import (
 
 // for testing
 var (
-	stderr           = os.Stderr
-	stdout io.Writer = os.Stdout
-	stdin  io.Reader = os.Stdin
-	exit             = os.Exit
+	stderr   io.Writer              = os.Stderr
+	stdout   io.Writer              = os.Stdout
+	stdin    io.Reader              = os.Stdin
+	exit     func(int)              = os.Exit
+	env      func() []string        = os.Environ
+	bindings map[string]interface{} = map[string]interface{}{}
 )
 
 func main() {
-	if err := run(os.Args[1:]); err != nil {
-		fmt.Fprintln(stderr, err) // nolint: gas
-		os.Exit(1)
-	}
-}
+	var err error
 
-func run(args []string) error {
-	switch {
-	case len(args) == 0:
-		buf := new(bytes.Buffer)
-		if _, err := io.Copy(buf, stdin); err != nil {
-			return err
+	cmdLine := flag.NewFlagSet(os.Args[0], flag.ContinueOnError)
+	cmdLine.Usage = func() {
+		fmt.Fprintf(stderr, "usage: %s [OPTIONS] [FILE]\n", cmdLine.Name())
+		fmt.Fprint(stderr, "\nOPTIONS\n")
+		cmdLine.PrintDefaults()
+	}
+
+	var bindEnvs bool
+	cmdLine.BoolVar(&bindEnvs, "env", false, "bind environment variables")
+
+	err = cmdLine.Parse(os.Args[1:])
+	if err != nil {
+		if err == flag.ErrHelp {
+			exit(0)
+			return
 		}
-		return render(buf.Bytes(), "")
-	case args[0] == "-h" || args[0] == "--help":
-		usage()
-	case strings.HasPrefix(args[0], "-"):
-		// undefined flag
-		usage()
+		fmt.Fprintln(stderr, err)
 		exit(1)
-	case len(args) == 1:
-		s, err := ioutil.ReadFile(args[0])
-		if err != nil {
-			return err
+		return
+	}
+
+	if bindEnvs {
+		for _, e := range env() {
+			pair := strings.SplitN(e, "=", 2)
+			bindings[pair[0]] = pair[1]
 		}
-		return render(s, args[0])
+	}
+
+	args := cmdLine.Args()
+	switch len(args) {
+	case 0:
+		// use stdin
+	case 1:
+		stdin, err = os.Open(args[0])
 	default:
-		usage()
+		err = errors.New("too many arguments")
+	}
+
+	if err == nil {
+		err = render()
+	}
+
+	if err != nil {
+		fmt.Fprintln(stderr, err)
 		exit(1)
 	}
-	return nil
 }
 
-func render(b []byte, filename string) (err error) {
-	tpl, err := liquid.NewEngine().ParseTemplate(b)
+func render() error {
+	buf, err := io.ReadAll(stdin)
 	if err != nil {
 		return err
 	}
-	out, err := tpl.Render(map[string]interface{}{})
+
+	tpl, err := liquid.NewEngine().ParseTemplate(buf)
+	if err != nil {
+		return err
+	}
+	out, err := tpl.Render(bindings)
 	if err != nil {
 		return err
 	}
 	_, err = stdout.Write(out)
 	return err
 }
-
-func usage() {
-	fmt.Fprintf(stdout, "usage: %s [FILE]\n", os.Args[0]) // nolint: gas
-}
diff --git a/cmd/liquid/main_test.go b/cmd/liquid/main_test.go
index 15caf6a..266aade 100644
--- a/cmd/liquid/main_test.go
+++ b/cmd/liquid/main_test.go
@@ -2,47 +2,112 @@ package main
 
 import (
 	"bytes"
+	"os"
 	"testing"
 
 	"github.com/stretchr/testify/require"
 )
 
 func TestMain(t *testing.T) {
-	exit = func(n int) { t.Fatalf("exit called") }
+	oldArgs := os.Args
+
+	defer func() {
+		os.Args = oldArgs
+		stderr = os.Stderr
+		stdout = os.Stdout
+		stdin = os.Stdin
+		exit = os.Exit
+		env = os.Environ
+		bindings = map[string]interface{}{}
+	}()
+
+	exit = func(n int) {
+		t.Fatalf("exit called")
+	}
+
+	os.Args = []string{"liquid"}
 
 	// stdin
 	src := `{{ "Hello World" | downcase | split: " " | first | append: "!"}}`
-	buf := new(bytes.Buffer)
+	buf := &bytes.Buffer{}
 	stdin = bytes.NewBufferString(src)
 	stdout = buf
-	require.NoError(t, run([]string{}))
+	main()
 	require.Equal(t, "hello!", buf.String())
 
-	// filename
-	buf = new(bytes.Buffer)
-	stdin = bytes.NewBufferString("")
+	// environment binding
+	var envCalled bool
+	env = func() []string {
+		envCalled = true
+		return []string{"TARGET=World"}
+	}
+	src = `Hello, {{ TARGET }}!`
+	// without -e
+	stdin = bytes.NewBufferString(src)
+	buf = &bytes.Buffer{}
 	stdout = buf
-	require.NoError(t, run([]string{"testdata/source.txt"}))
+	os.Args = []string{"liquid"}
+	main()
+	require.False(t, envCalled)
+	require.Equal(t, "Hello, !", buf.String())
+	// with -e
+	stdin = bytes.NewBufferString(src)
+	buf = &bytes.Buffer{}
+	stdout = buf
+	os.Args = []string{"liquid", "--env"}
+	main()
+	require.True(t, envCalled)
+	require.Equal(t, "Hello, World!", buf.String())
+
+	// filename
+	stdin = os.Stdin
+	buf = &bytes.Buffer{}
+	stdout = buf
+	os.Args = []string{"liquid", "testdata/source.txt"}
+	main()
 	require.Contains(t, buf.String(), "file system")
 
-	// missing file
-	require.Error(t, run([]string{"testdata/missing_file"}))
+	// following tests test the exit code
+	var exitCalled bool
+	exitCode := 0
+	exit = func(n int) { exitCalled = true; exitCode = n }
 
+	os.Args = []string{"liquid", "testdata/source.txt"}
+	main()
+	require.Equal(t, 0, exitCode)
+
+	exitCode = 0
+	// missing file
+	buf = &bytes.Buffer{}
+	stderr = buf
+	os.Args = []string{"liquid", "testdata/missing_file"}
+	main()
+	require.Equal(t, 1, exitCode)
+	require.Contains(t, buf.String(), "no such")
+
+	exitCalled = false
 	// --help
-	buf = new(bytes.Buffer)
-	stdout = buf
-	require.NoError(t, run([]string{"--help"}))
+	buf = &bytes.Buffer{}
+	stderr = buf
+	os.Args = []string{"liquid", "--help"}
+	main()
 	require.Contains(t, buf.String(), "usage:")
+	require.True(t, exitCalled)
+	require.Equal(t, 0, exitCode)
 
 	// --undefined-flag
-	exitCode := 0
-	exit = func(n int) { exitCode = n }
-	require.NoError(t, run([]string{"--undefined-flag"}))
+	buf = &bytes.Buffer{}
+	stderr = buf
+	os.Args = []string{"liquid", "--undefined-flag"}
+	main()
 	require.Equal(t, 1, exitCode)
+	require.Contains(t, buf.String(), "defined")
 
 	// multiple args
-	exitCode = 0
-	exit = func(n int) { exitCode = n }
-	require.NoError(t, run([]string{"file1", "file2"}))
+	os.Args = []string{"liquid", "testdata/source.txt", "file2"}
+	buf = &bytes.Buffer{}
+	stderr = buf
+	main()
+	require.Contains(t, buf.String(), "too many")
 	require.Equal(t, 1, exitCode)
 }