no shell call on single command exec on *nix

Instead of always using `sh -c` to run command lines on *nix check if it
is a single command (no spaces) and, if so, run that command directly.

This will give users on systems without 'sh' a way to run their commands.

Fixes #1508

manager/command-prep.go

@@ -1,12 +1,35 @@
+//go:build !windows
 // +build !windows
 
 package manager
 
+import (
+	"os/exec"
+	"strings"
+)
+
 func prepCommand(command string) ([]string, error) {
-	if len(command) == 0 {
+	switch len(strings.Fields(command)) {
+	case 0:
 		return []string{}, nil
+	case 1:
+		return []string{command}, nil
+	}
+
+	// default to 'sh' on path, else try a couple common absolute paths
+	shell := "sh"
+	if _, err := exec.LookPath(shell); err != nil {
+		for _, sh := range []string{"/bin/sh", "/usr/bin/sh"} {
+			if sh, err := exec.LookPath(sh); err == nil {
+				shell = sh
+				break
+			}
+		}
+	}
+	if shell == "" {
+		return []string{}, exec.ErrNotFound
 	}
 
-	cmd := []string{"sh", "-c", command}
+	cmd := []string{shell, "-c", command}
 	return cmd, nil
 }

manager/command-prep_windows.go

@@ -1,3 +1,4 @@
+//go:build windows
 // +build windows
 
 package manager
@@ -8,11 +9,11 @@ import (
 )
 
 func prepCommand(command string) ([]string, error) {
-	switch {
-	case len(command) == 0:
+	switch len(strings.Fields(command)) {
+	case 0:
 		return []string{}, nil
-	case len(strings.Fields(command)) > 1:
-		return []string{}, fmt.Errorf("only single commands supported on windows")
+	case 1:
+		return []string{command}, nil
 	}
-	return []string{command}, nil
+	return []string{}, fmt.Errorf("only single commands supported on windows")
 }

manager/runner_test.go

@@ -5,6 +5,8 @@ import (
 	"fmt"
 	"io/ioutil"
 	"os"
+	"os/exec"
+	"path/filepath"
 	"reflect"
 	"strings"
 	"testing"
@@ -1118,7 +1120,7 @@ func TestRunner_command(t *testing.T) {
 		{
 			name:   "single",
 			input:  "echo",
-			parsed: []string{"sh", "-c", "echo"},
+			parsed: []string{"echo"},
 			out:    "\n",
 		},
 		{
@@ -1209,3 +1211,19 @@ func TestRunner_command(t *testing.T) {
 			})
 	}
 }
+
+func TestRunner_commandPath(t *testing.T) {
+	PATH := os.Getenv("PATH")
+	defer os.Setenv("PATH", PATH)
+	os.Setenv("PATH", "")
+	cmd, err := prepCommand("echo hi")
+	if err != nil && err != exec.ErrNotFound {
+		t.Fatal(err)
+	}
+	if len(cmd) != 3 {
+		t.Fatalf("unexpected command: %#v\n", cmd)
+	}
+	if filepath.Base(cmd[0]) != "sh" {
+		t.Fatalf("unexpected shell: %#v\n", cmd)
+	}
+}