interp: support read -p flag for prompts

I have implemented the "read -p" flag, as requested in issue #551.
Included are new test cases to cover the added feature,
as well as the integration of -p with the existing code that handles read.
All test cases old and new are passing.

interp/builtin.go

@@ -518,12 +518,19 @@ func (r *Runner) builtinCode(ctx context.Context, pos syntax.Pos, name string, a
 		}
 		r.setErr(returnStatus(code))
 	case "read":
+		var prompt string
 		raw := false
 		fp := flagParser{remaining: args}
 		for fp.more() {
 			switch flag := fp.flag(); flag {
 			case "-r":
 				raw = true
+			case "-p":
+				prompt = fp.value()
+				if prompt == "" {
+					r.errf("read: -p: option requires an argument\n")
+					return 2
+				}
 			default:
 				r.errf("read: invalid option %q\n", flag)
 				return 2
@@ -538,6 +545,10 @@ func (r *Runner) builtinCode(ctx context.Context, pos syntax.Pos, name string, a
 			}
 		}
 
+		if prompt != "" {
+			r.out(prompt)
+		}
+
 		line, err := r.readLine(raw)
 		if err != nil {
 			return 1

interp/interp_test.go

@@ -2587,6 +2587,22 @@ set +o pipefail
 		"IFS=: read a b c <<< '1\\:2:3'; echo \"$a\"; echo $b; echo $c",
 		"1:2\n3\n\n",
 	},
+	{
+		"read -p",
+		"read: -p: option requires an argument\nexit status 2 #JUSTERR",
+	},
+	{
+		"read -X -p",
+		"read: invalid option \"-X\"\nexit status 2 #JUSTERR",
+	},
+	{
+		"read -p 'Display me as a prompt. Continue? (y/n) ' choice <<< 'y'; echo $choice",
+		"Display me as a prompt. Continue? (y/n) y\n",
+	},
+	{
+		"read -r -p 'Prompt and raw flag together: ' a <<< '\\a\\b\\c'; echo $a",
+		"Prompt and raw flag together: \\a\\b\\c\n",
+	},
 
 	// getopts
 	{