feat: Support preview markdown from stdin (#71)

Author: haya14busa

README.md

@@ -51,6 +51,14 @@ Or this command will detect README file in the directory automatically.
 gh markdown-preview
 ```
 
+You can also preview Markdown from stdin by piping or using `-`:
+
+```
+echo "# Hello" | gh markdown-preview
+cat README.md | gh markdown-preview
+gh markdown-preview - < README.md
+```
+
 Then access the local web server such as `http://localhost:3333` with Chrome, Firefox, or Safari.
 
 Available options:

cmd/app_test.go

@@ -75,7 +75,7 @@ func TestGh(t *testing.T) {
 
 func TestToHTML(t *testing.T) {
 	markdown := "text"
-	html, err := toHTML(markdown)
+	html, err := toHTML(markdown, &Param{})
 	if err != nil {
 		t.Errorf(err.Error())
 	}
@@ -91,7 +91,7 @@ func TestGfmCheckboxes(t *testing.T) {
 	if err != nil {
 		t.Errorf(err.Error())
 	}
-	html, err := toHTML(string)
+	html, err := toHTML(string, &Param{})
 	if err != nil {
 		t.Errorf(err.Error())
 	}
@@ -126,7 +126,7 @@ func TestGfmAlerts(t *testing.T) {
 	if err != nil {
 		t.Errorf(err.Error())
 	}
-	html, err := toHTML(string)
+	html, err := toHTML(string, &Param{})
 	if err != nil {
 		t.Errorf(err.Error())
 	}

cmd/cli.go

@@ -2,6 +2,7 @@ package cmd
 
 import (
 	"fmt"
+	"io"
 	"log"
 	"os"
 
@@ -19,6 +20,9 @@ type Param struct {
 	forceLightMode bool
 	forceDarkMode  bool
 	autoOpen       bool
+	// stdin support
+	useStdin     bool
+	stdinContent string
 }
 
 var rootCmd = &cobra.Command{
@@ -61,13 +65,36 @@ var rootCmd = &cobra.Command{
 			autoOpen = false
 		}
 
+		// Detect stdin usage
+		useStdin := false
+		stdinContent := ""
+		if filename == "-" {
+			data, err := io.ReadAll(os.Stdin)
+			if err != nil {
+				log.Fatalf("Error reading stdin: %v", err)
+			}
+			stdinContent = string(data)
+			useStdin = true
+		} else if filename == "" {
+			if fi, _ := os.Stdin.Stat(); (fi.Mode() & os.ModeCharDevice) == 0 {
+				data, err := io.ReadAll(os.Stdin)
+				if err != nil {
+					log.Fatalf("Error reading stdin: %v", err)
+				}
+				stdinContent = string(data)
+				useStdin = true
+			}
+		}
+
 		param := &Param{
 			filename:       filename,
 			markdownMode:   markdownMode,
 			reload:         reload,
 			forceLightMode: forceLightMode,
 			forceDarkMode:  forceDarkMode,
 			autoOpen:       autoOpen,
+			useStdin:       useStdin,
+			stdinContent:   stdinContent,
 		}
 
 		err := server.Serve(param)

cmd/server.go

@@ -40,9 +40,14 @@ func (server *Server) Serve(param *Param) error {
 		port = server.port
 	}
 
-	filename, err := targetFile(param.filename)
-	if err != nil {
-		return err
+	// Use a empty filename for stdin
+	filename := ""
+	if !param.useStdin {
+		var err error
+		filename, err = targetFile(param.filename)
+		if err != nil {
+			return err
+		}
 	}
 
 	dir := filepath.Dir(filename)
@@ -96,10 +101,15 @@ func handler(filename string, param *Param, h http.Handler) http.Handler {
 			return
 		}
 
-		markdown, err := slurp(filename)
-		if err != nil {
-			http.Error(w, err.Error(), http.StatusInternalServerError)
-			return
+		var markdown string
+		if param.useStdin && param.stdinContent != "" && filename == "" {
+			markdown = param.stdinContent
+		} else {
+			markdown, err = slurp(filename)
+			if err != nil {
+				http.Error(w, err.Error(), http.StatusInternalServerError)
+				return
+			}
 		}
 
 		html, err := toHTML(markdown, param)
@@ -118,11 +128,16 @@ func handler(filename string, param *Param, h http.Handler) http.Handler {
 
 func mdResponse(w http.ResponseWriter, filename string, param *Param) {
 	w.Header().Set("Content-Type", "text/html; charset=utf-8")
-
-	markdown, err := slurp(filename)
-	if err != nil {
-		http.Error(w, err.Error(), 500)
-		return
+	var markdown string
+	var err error
+	if param.useStdin && param.stdinContent != "" && filename == "" {
+		markdown = param.stdinContent
+	} else {
+		markdown, err = slurp(filename)
+		if err != nil {
+			http.Error(w, err.Error(), 500)
+			return
+		}
 	}
 
 	html, err := toHTML(markdown, param)

cmd/server_test.go

@@ -43,7 +43,8 @@ func TestHandler(t *testing.T) {
 
 func TestMdHandler(t *testing.T) {
 	filename := "../testdata/markdown-demo.md"
-	ts := httptest.NewServer(mdHandler(filename))
+	param := &Param{reload: false}
+	ts := httptest.NewServer(mdHandler(filename, param))
 	defer ts.Close()
 
 	res, err := http.Get(ts.URL)