If stdout is a pipe, output to the pipe

If you run micro as `micro | cat` for example, micro will disallow
you from saving the file, and when you quit the buffer, the contents
will be sent to the pipe. This allows one to use micro as part of
an interactive unix pipeline.

Closes #1524

cmd/micro/micro.go

@@ -147,11 +147,16 @@ func LoadInput() []*buffer.Buffer {
 	args := flag.Args()
 	buffers := make([]*buffer.Buffer, 0, len(args))
 
+	btype := buffer.BTDefault
+	if !isatty.IsTerminal(os.Stdout.Fd()) {
+		btype = buffer.BTStdout
+	}
+
 	if len(args) > 0 {
 		// Option 1
 		// We go through each file and load it
 		for i := 0; i < len(args); i++ {
-			buf, err := buffer.NewBufferFromFile(args[i], buffer.BTDefault)
+			buf, err := buffer.NewBufferFromFile(args[i], btype)
 			if err != nil {
 				screen.TermMessage(err)
 				continue
@@ -168,17 +173,22 @@ func LoadInput() []*buffer.Buffer {
 			screen.TermMessage("Error reading from stdin: ", err)
 			input = []byte{}
 		}
-		buffers = append(buffers, buffer.NewBufferFromString(string(input), filename, buffer.BTDefault))
+		buffers = append(buffers, buffer.NewBufferFromString(string(input), filename, btype))
 	} else {
 		// Option 3, just open an empty buffer
-		buffers = append(buffers, buffer.NewBufferFromString(string(input), filename, buffer.BTDefault))
+		buffers = append(buffers, buffer.NewBufferFromString(string(input), filename, btype))
 	}
 
 	return buffers
 }
 
 func main() {
-	defer os.Exit(0)
+	defer func() {
+		if util.Stdout.Len() > 0 {
+			fmt.Fprint(os.Stdout, util.Stdout.String())
+		}
+		os.Exit(0)
+	}()
 
 	// runtime.SetCPUProfileRate(400)
 	// f, _ := os.Create("micro.prof")

internal/buffer/buffer.go

@@ -5,6 +5,7 @@ import (
 	"bytes"
 	"crypto/md5"
 	"errors"
+	"fmt"
 	"io"
 	"io/ioutil"
 	"os"
@@ -60,6 +61,9 @@ var (
 	BTRaw = BufType{4, false, true, false}
 	// BTInfo is a buffer for inputting information
 	BTInfo = BufType{5, false, true, false}
+	// BTStdout is a buffer that only writes to stdout
+	// when closed
+	BTStdout = BufType{6, false, true, true}
 
 	// ErrFileTooLarge is returned when the file is too large to hash
 	// (fastdirty is automatically enabled)
@@ -82,6 +86,8 @@ type SharedBuffer struct {
 	// Name of the buffer on the status line
 	name string
 
+	toStdout bool
+
 	// Settings customized by the user
 	Settings map[string]interface{}
 
@@ -355,6 +361,10 @@ func (b *Buffer) Fini() {
 		b.Serialize()
 	}
 	b.RemoveBackup()
+
+	if b.Type == BTStdout {
+		fmt.Fprint(util.Stdout, string(b.Bytes()))
+	}
 }
 
 // GetName returns the name that should be displayed in the statusline

internal/util/util.go

@@ -1,6 +1,7 @@
 package util
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
 	"os"
@@ -34,6 +35,9 @@ var (
 	// FakeCursor is used to disable the terminal cursor and have micro
 	// draw its own (enabled for windows consoles where the cursor is slow)
 	FakeCursor = false
+
+	// Stdout is a buffer that is written to stdout when micro closes
+	Stdout *bytes.Buffer
 )
 
 func init() {
@@ -46,6 +50,7 @@ func init() {
 	if runtime.GOOS == "windows" {
 		FakeCursor = true
 	}
+	Stdout = new(bytes.Buffer)
 }
 
 // SliceEnd returns a byte slice where the index is a rune index