fix: allows to capture cmd out/err

OutOrStderr() introduces an inconsistent behavior.
This commit adds a feature to get the right behavior, without breaking changes

closes: https://github.com/spf13/cobra/issues/1708

Co-authored-by: Mislav Marohnić <git@mislav.net>

.github/workflows/test.yml

@@ -43,13 +43,13 @@ jobs:
 
       - uses: actions/setup-go@v3
         with:
-          go-version: '^1.20'
+          go-version: '^1.21'
           check-latest: true
           cache: true
 
       - uses: actions/checkout@v3
 
-      - uses: golangci/golangci-lint-action@v3.6.0
+      - uses: golangci/golangci-lint-action@v3.7.0
         with:
           version: latest
           args: --verbose
@@ -67,6 +67,7 @@ jobs:
         - 18
         - 19
         - 20
+        - 21
     name: '${{ matrix.platform }} | 1.${{ matrix.go }}.x'
     runs-on: ${{ matrix.platform }}-latest
     steps:

command.go

@@ -181,6 +181,9 @@ type Command struct {
 	// versionTemplate is the version template defined by user.
 	versionTemplate string
 
+	// errPrefix is the error message prefix defined by user.
+	errPrefix string
+
 	// inReader is a reader defined by the user that replaces stdin
 	inReader io.Reader
 	// outWriter is a writer defined by the user that replaces stdout
@@ -188,6 +191,11 @@ type Command struct {
 	// errWriter is a writer defined by the user that replaces stderr
 	errWriter io.Writer
 
+	// outFallbackWriter is a writer defined by the user that is used if outWriter is nil
+	outFallbackWriter io.Writer
+	// errFallbackWriter is a writer defined by the user that is used if errWriter is nil
+	errFallbackWriter io.Writer
+
 	// FParseErrWhitelist flag parse errors to be ignored
 	FParseErrWhitelist FParseErrWhitelist
 
@@ -279,16 +287,30 @@ func (c *Command) SetOutput(output io.Writer) {
 
 // SetOut sets the destination for usage messages.
 // If newOut is nil, os.Stdout is used.
+// Deprecated: Use SetOutFallback and/or SetErrFallback instead (see https://github.com/spf13/cobra/issues/1708)
 func (c *Command) SetOut(newOut io.Writer) {
 	c.outWriter = newOut
 }
 
 // SetErr sets the destination for error messages.
 // If newErr is nil, os.Stderr is used.
+// Deprecated: Use SetOutFallback and/or SetErrFallback instead (see https://github.com/spf13/cobra/issues/1708)
 func (c *Command) SetErr(newErr io.Writer) {
 	c.errWriter = newErr
 }
 
+// SetOutFallback sets the destination for usage messages when SetOut() was not used.
+// If newOut is nil, os.Stdout is used.
+func (c *Command) SetOutFallback(newOut io.Writer) {
+	c.outFallbackWriter = newOut
+}
+
+// SetErrFallback sets the destination for error messages when SetErr() was not used.
+// If newErr is nil, os.Stderr is used.
+func (c *Command) SetErrFallback(newErr io.Writer) {
+	c.errFallbackWriter = newErr
+}
+
 // SetIn sets the source for input data
 // If newIn is nil, os.Stdin is used.
 func (c *Command) SetIn(newIn io.Reader) {
@@ -346,6 +368,11 @@ func (c *Command) SetVersionTemplate(s string) {
 	c.versionTemplate = s
 }
 
+// SetErrPrefix sets error message prefix to be used. Application can use it to set custom prefix.
+func (c *Command) SetErrPrefix(s string) {
+	c.errPrefix = s
+}
+
 // SetGlobalNormalizationFunc sets a normalization function to all flag sets and also to child commands.
 // The user should not have a cyclic dependency on commands.
 func (c *Command) SetGlobalNormalizationFunc(n func(f *flag.FlagSet, name string) flag.NormalizedName) {
@@ -363,9 +390,10 @@ func (c *Command) OutOrStdout() io.Writer {
 	return c.getOut(os.Stdout)
 }
 
-// OutOrStderr returns output to stderr
+// OutOrStderr returns output to stderr.
+// Deprecated: Use OutOrStdout or ErrOrStderr instead
 func (c *Command) OutOrStderr() io.Writer {
-	return c.getOut(os.Stderr)
+	return c.getOutFallbackToErr(os.Stderr)
 }
 
 // ErrOrStderr returns output to stderr
@@ -382,6 +410,9 @@ func (c *Command) getOut(def io.Writer) io.Writer {
 	if c.outWriter != nil {
 		return c.outWriter
 	}
+	if c.outFallbackWriter != nil {
+		return c.outFallbackWriter
+	}
 	if c.HasParent() {
 		return c.parent.getOut(def)
 	}
@@ -392,12 +423,31 @@ func (c *Command) getErr(def io.Writer) io.Writer {
 	if c.errWriter != nil {
 		return c.errWriter
 	}
+	if c.errFallbackWriter != nil {
+		return c.errFallbackWriter
+	}
 	if c.HasParent() {
 		return c.parent.getErr(def)
 	}
 	return def
 }
 
+// getOutFallbackToErr should only be used inside OutOrStderr.
+// Deprecated: this function exists to allow for backwards compatibility only
+// (see https://github.com/spf13/cobra/issues/1708)
+func (c *Command) getOutFallbackToErr(def io.Writer) io.Writer {
+	if c.outWriter != nil {
+		return c.outWriter
+	}
+	if c.errFallbackWriter != nil {
+		return c.errFallbackWriter
+	}
+	if c.HasParent() {
+		return c.parent.getOutFallbackToErr(def)
+	}
+	return def
+}
+
 func (c *Command) getIn(def io.Reader) io.Reader {
 	if c.inReader != nil {
 		return c.inReader
@@ -595,6 +645,18 @@ func (c *Command) VersionTemplate() string {
 `
 }
 
+// ErrPrefix return error message prefix for the command
+func (c *Command) ErrPrefix() string {
+	if c.errPrefix != "" {
+		return c.errPrefix
+	}
+
+	if c.HasParent() {
+		return c.parent.ErrPrefix()
+	}
+	return "Error:"
+}
+
 func hasNoOptDefVal(name string, fs *flag.FlagSet) bool {
 	flag := fs.Lookup(name)
 	if flag == nil {
@@ -752,7 +814,9 @@ func (c *Command) findNext(next string) *Command {
 	}
 
 	if len(matches) == 1 {
-		return matches[0]
+		// Temporarily disable gosec G602, which produces a false positive.
+		// See https://github.com/securego/gosec/issues/1005.
+		return matches[0] // #nosec G602
 	}
 
 	return nil
@@ -1048,7 +1112,7 @@ func (c *Command) ExecuteC() (cmd *Command, err error) {
 			c = cmd
 		}
 		if !c.SilenceErrors {
-			c.PrintErrln("Error:", err.Error())
+			c.PrintErrln(c.ErrPrefix(), err.Error())
 			c.PrintErrf("Run '%v --help' for usage.\n", c.CommandPath())
 		}
 		return c, err
@@ -1077,7 +1141,7 @@ func (c *Command) ExecuteC() (cmd *Command, err error) {
 		// If root command has SilenceErrors flagged,
 		// all subcommands should respect it
 		if !cmd.SilenceErrors && !c.SilenceErrors {
-			c.PrintErrln("Error:", err.Error())
+			c.PrintErrln(cmd.ErrPrefix(), err.Error())
 		}
 
 		// If root command has SilenceUsage flagged,

command_test.go

@@ -1099,6 +1099,39 @@ func TestShorthandVersionTemplate(t *testing.T) {
 	checkStringContains(t, output, "customized version: 1.0.0")
 }
 
+func TestRootErrPrefixExecutedOnSubcommand(t *testing.T) {
+	rootCmd := &Command{Use: "root", Run: emptyRun}
+	rootCmd.SetErrPrefix("root error prefix:")
+	rootCmd.AddCommand(&Command{Use: "sub", Run: emptyRun})
+
+	output, err := executeCommand(rootCmd, "sub", "--unknown-flag")
+	if err == nil {
+		t.Errorf("Expected error")
+	}
+
+	checkStringContains(t, output, "root error prefix: unknown flag: --unknown-flag")
+}
+
+func TestRootAndSubErrPrefix(t *testing.T) {
+	rootCmd := &Command{Use: "root", Run: emptyRun}
+	subCmd := &Command{Use: "sub", Run: emptyRun}
+	rootCmd.AddCommand(subCmd)
+	rootCmd.SetErrPrefix("root error prefix:")
+	subCmd.SetErrPrefix("sub error prefix:")
+
+	if output, err := executeCommand(rootCmd, "--unknown-root-flag"); err == nil {
+		t.Errorf("Expected error")
+	} else {
+		checkStringContains(t, output, "root error prefix: unknown flag: --unknown-root-flag")
+	}
+
+	if output, err := executeCommand(rootCmd, "sub", "--unknown-sub-flag"); err == nil {
+		t.Errorf("Expected error")
+	} else {
+		checkStringContains(t, output, "sub error prefix: unknown flag: --unknown-sub-flag")
+	}
+}
+
 func TestVersionFlagExecutedOnSubcommand(t *testing.T) {
 	rootCmd := &Command{Use: "root", Version: "1.0.0"}
 	rootCmd.AddCommand(&Command{Use: "sub", Run: emptyRun})

site/content/user_guide.md

@@ -596,6 +596,12 @@ Running an application with the '--version' flag will print the version to stdou
 the version template. The template can be customized using the
 `cmd.SetVersionTemplate(s string)` function.
 
+## Error Message Prefix
+
+Cobra prints an error message when receiving a non-nil error value.
+The default error message is `Error: <error contents>`.
+The Prefix, `Error:` can be customized using the `cmd.SetErrPrefix(s string)` function.
+
 ## PreRun and PostRun Hooks
 
 It is possible to run functions before or after the main `Run` function of your command. The `PersistentPreRun` and `PreRun` functions will be executed before `Run`. `PersistentPostRun` and `PostRun` will be executed after `Run`.  The `Persistent*Run` functions will be inherited by children if they do not declare their own.  These functions are run in the following order: