testscript: expose (*TestScript).stdout via SetStdout(string)

testscript/testdata/cmd_stdout_stderr.txt

@@ -0,0 +1,15 @@
+# Print to stdout via a TestMain cmd
+fprintargs stdout hello world from fprintargs
+stdout 'hello world from fprintargs'
+
+# Verify that we can write to stdout via a Params.Cmds cmd
+echoStdout hello world from echoStdout
+stdout 'hello world from echoStdout'
+
+# Print to stderr via a TestMain cmd
+fprintargs stderr the answer is 42 from fprintargs
+stderr 'the answer is 42 from fprintargs'
+
+# Verify that we can write to stderr via a Params.Cmds cmd
+echoStderr the answer is 42 from echoStderr
+stderr 'the answer is 42 from echoStderr'

testscript/testscript.go

@@ -788,6 +788,18 @@ func (ts *TestScript) Check(err error) {
 	}
 }
 
+// SetStdout updates the last stdout.  This is typically used by commands
+// declared via Params.Cmds.
+func (ts *TestScript) SetStdout(s string) {
+	ts.stdout = s
+}
+
+// SetStderr updates the last stderr.  This is typically used by commands
+// declared via Params.Cmds.
+func (ts *TestScript) SetStderr(s string) {
+	ts.stderr = s
+}
+
 // Logf appends the given formatted message to the test log transcript.
 func (ts *TestScript) Logf(format string, args ...interface{}) {
 	format = strings.TrimSuffix(format, "\n")

testscript/testscript_test.go

@@ -249,6 +249,12 @@ func TestScripts(t *testing.T) {
 					ts.Fatalf("testscript unexpectedly failed with errors: %q", &t.log)
 				}
 			},
+			"echoStdout": func(ts *TestScript, neg bool, args []string) {
+				ts.SetStdout(strings.Join(args, " "))
+			},
+			"echoStderr": func(ts *TestScript, neg bool, args []string) {
+				ts.SetStderr(strings.Join(args, " "))
+			},
 		},
 		Setup: func(env *Env) error {
 			infos, err := ioutil.ReadDir(env.WorkDir)