First stab at implementing shell command.

.gitignore

@@ -1,3 +1,3 @@
 gophernotes
 .ipynb_checkpoints
-Untitled.ipynb
+Untitled*.ipynb

kernel.go

@@ -10,6 +10,7 @@ import (
 	"io/ioutil"
 	"log"
 	"os"
+	"os/exec"
 	"reflect"
 	"runtime"
 	"strings"
@@ -403,16 +404,17 @@ func (kernel *Kernel) handleExecuteRequest(receipt msgReceipt) error {
 	var writersWG sync.WaitGroup
 	writersWG.Add(2)
 
+	jupyterStdOut := JupyterStreamWriter{StreamStdout, &receipt}
+	jupyterStdErr := JupyterStreamWriter{StreamStderr, &receipt}
+
 	// Forward all data written to stdout/stderr to the front-end.
 	go func() {
 		defer writersWG.Done()
-		jupyterStdOut := JupyterStreamWriter{StreamStdout, &receipt}
 		io.Copy(&jupyterStdOut, rOut)
 	}()
 
 	go func() {
 		defer writersWG.Done()
-		jupyterStdErr := JupyterStreamWriter{StreamStderr, &receipt}
 		io.Copy(&jupyterStdErr, rErr)
 	}()
 
@@ -426,7 +428,7 @@ func (kernel *Kernel) handleExecuteRequest(receipt msgReceipt) error {
 	}()
 
 	// eval
-	vals, types, executionErr := doEval(ir, code)
+	vals, types, executionErr := doEval(ir, jupyterStdOut, jupyterStdErr, code)
 
 	// Close and restore the streams.
 	wOut.Close()
@@ -468,7 +470,7 @@ func (kernel *Kernel) handleExecuteRequest(receipt msgReceipt) error {
 
 // doEval evaluates the code in the interpreter. This function captures an uncaught panic
 // as well as the values of the last statement/expression.
-func doEval(ir *interp.Interp, code string) (val []interface{}, typ []xreflect.Type, err error) {
+func doEval(ir *interp.Interp, jupyterStdOut, jupyterStdErr JupyterStreamWriter, code string) (val []interface{}, typ []xreflect.Type, err error) {
 
 	// Capture a panic from the evaluation if one occurs and store it in the `err` return parameter.
 	defer func() {
@@ -480,7 +482,7 @@ func doEval(ir *interp.Interp, code string) (val []interface{}, typ []xreflect.T
 		}
 	}()
 
-	code = evalSpecialCommands(ir, code)
+	code = evalSpecialCommands(ir, jupyterStdOut, jupyterStdErr, code)
 
 	// Prepare and perform the multiline evaluation.
 	compiler := ir.Comp
@@ -626,21 +628,34 @@ func startHeartbeat(hbSocket Socket, wg *sync.WaitGroup) (shutdown chan struct{}
 }
 
 // find and execute special commands in code, remove them from returned string
-func evalSpecialCommands(ir *interp.Interp, code string) string {
+func evalSpecialCommands(ir *interp.Interp, jupyterStdOut, jupyterStdErr JupyterStreamWriter, code string) string {
 	lines := strings.Split(code, "\n")
 	for i, line := range lines {
 		line = strings.TrimSpace(line)
-		if len(line) != 0 && line[0] == '%' {
-			evalSpecialCommand(ir, line)
-			lines[i] = ""
+		if len(line) != 0 {
+			switch line[0] {
+			case '%':
+				evalSpecialCommand(ir, line)
+				lines[i] = ""
+			case '$':
+				evalShellCommand(ir, jupyterStdOut, jupyterStdErr, line)
+				lines[i] = ""
+			}
 		}
 	}
 	return strings.Join(lines, "\n")
 }
 
 // execute special command
 func evalSpecialCommand(ir *interp.Interp, line string) {
-	const help string = "available special commands:\n  %go111module {on|off}\n  %help"
+	const help string = `
+available special commands (%):
+%help
+%go111module {on|off}
+
+execute shell commands ($):
+$ls -l
+	`
 
 	args := strings.SplitN(line, " ", 2)
 	cmd := args[0]
@@ -664,3 +679,49 @@ func evalSpecialCommand(ir *interp.Interp, line string) {
 		panic(fmt.Errorf("unknown special command: %q\n%s", line, help))
 	}
 }
+
+// execute shell command
+func evalShellCommand(ir *interp.Interp, jupyterStdOut, jupyterStdErr JupyterStreamWriter, line string) {
+	args := strings.Split(line, " ")
+	if len(args) <= 0 {
+		return
+	}
+
+	var writersWG sync.WaitGroup
+	writersWG.Add(2)
+
+	command := strings.Replace(args[0], "$", "", 1)
+	cmd := exec.Command(command, args[1:]...)
+
+	stdout, err := cmd.StdoutPipe()
+	if err != nil {
+		panic(err)
+	}
+
+	stderr, err := cmd.StderrPipe()
+	if err != nil {
+		panic(err)
+	}
+
+	go func() {
+		defer writersWG.Done()
+		io.Copy(&jupyterStdOut, stdout)
+	}()
+
+	go func() {
+		defer writersWG.Done()
+		io.Copy(&jupyterStdErr, stderr)
+	}()
+
+	err = cmd.Start()
+	if err != nil {
+		panic(err)
+	}
+
+	err = cmd.Wait()
+	if err != nil {
+		panic(err)
+	}
+
+	writersWG.Wait()
+}