interp: support closing stdin, stdout, and stderr files

interp/interp_test.go

@@ -1329,6 +1329,26 @@ var runTests = []runTest{
 		"mkdir a && cd a && echo foo_interp_missing >b && cd .. && cat a/b",
 		"foo_interp_missing\n",
 	},
+	{
+		"echo foo 2>&-; :",
+		"foo\n",
+	},
+	{
+		// `>&-` closes stdout or stderr. Note that any writes result in errors.
+		"echo foo >&- 2>&-; :",
+		"",
+	},
+	{
+		"echo foo | sed $(read line 2>/dev/null; echo 's/o/a/g')",
+		"",
+	},
+	{
+		// `<&-` closes stdin, to e.g. ensure that a subshell does not consume
+		// the standard input shared with the parent shell.
+		// Note that any reads result in errors.
+		"echo foo | sed $(exec <&-; read line 2>/dev/null; echo 's/o/a/g')",
+		"faa\n",
+	},
 
 	// background/wait
 	{"wait", ""},
@@ -2841,6 +2861,10 @@ done <<< 2`,
 		"read </dev/null",
 		"exit status 1",
 	},
+	{
+		"read 1</dev/null",
+		"exit status 1",
+	},
 	{
 		"read -X",
 		"read: invalid option \"-X\"\nexit status 2 #JUSTERR",

interp/runner.go

@@ -855,12 +855,19 @@ func (r *Runner) redir(ctx context.Context, rd *syntax.Redirect) (io.Closer, err
 		r.stdin = pr
 		return pr, nil
 	}
+
 	orig := &r.stdout
 	if rd.N != nil {
 		switch rd.N.Value {
+		case "0":
+			// Note that the input redirects below always use stdin (0)
+			// because we don't support anything else right now.
 		case "1":
+			// The default for the output redirects below.
 		case "2":
 			orig = &r.stderr
+		default:
+			panic(fmt.Sprintf("unsupported redirect fd: %v", rd.N.Value))
 		}
 	}
 	arg := r.literal(rd.Word)
@@ -885,12 +892,23 @@ func (r *Runner) redir(ctx context.Context, rd *syntax.Redirect) (io.Closer, err
 			*orig = r.stdout
 		case "2":
 			*orig = r.stderr
+		case "-":
+			*orig = io.Discard // closing the output writer
+		default:
+			panic(fmt.Sprintf("unhandled %v arg: %q", rd.Op, arg))
 		}
 		return nil, nil
 	case syntax.RdrIn, syntax.RdrOut, syntax.AppOut,
 		syntax.RdrAll, syntax.AppAll:
 		// done further below
-	// case syntax.DplIn:
+	case syntax.DplIn:
+		switch arg {
+		case "-":
+			r.stdin = nil // closing the input file
+		default:
+			panic(fmt.Sprintf("unhandled %v arg: %q", rd.Op, arg))
+		}
+		return nil, nil
 	default:
 		panic(fmt.Sprintf("unhandled redirect op: %v", rd.Op))
 	}