main: escape commands while printing them with the -x flag

This should make issues such as the one in
https://github.com/tinygo-org/tinygo/issues/1910 more obvious.

builder/build.go

@@ -547,8 +547,8 @@ func Build(pkgName, outpath string, config *compileopts.Config, action func(Buil
 				}
 				ldflags = append(ldflags, dependency.result)
 			}
-			if config.Options.PrintCommands {
+			if config.Options.PrintCommands != nil {
-				fmt.Printf("%s %s\n", config.Target.Linker, strings.Join(ldflags, " "))
+				config.Options.PrintCommands(config.Target.Linker, ldflags...)
 			}
 			err = link(config.Target.Linker, ldflags...)
 			if err != nil {

builder/cc.go

@@ -56,7 +56,7 @@ import (
 //   depfile but without invalidating its name. For this reason, the depfile is
 //   written on each new compilation (even when it seems unnecessary). However, it
 //   could in rare cases lead to a stale file fetched from the cache.
-func compileAndCacheCFile(abspath, tmpdir string, cflags []string, printCommands bool) (string, error) {
+func compileAndCacheCFile(abspath, tmpdir string, cflags []string, printCommands func(string, ...string)) (string, error) {
 	// Hash input file.
 	fileHash, err := hashFile(abspath)
 	if err != nil {
@@ -128,8 +128,8 @@ func compileAndCacheCFile(abspath, tmpdir string, cflags []string, printCommands
 		// flags (for the assembler) is a compiler error.
 		flags = append(flags, "-Qunused-arguments")
 	}
-	if printCommands {
+	if printCommands != nil {
-		fmt.Printf("clang %s\n", strings.Join(flags, " "))
+		printCommands("clang", flags...)
 	}
 	err = runCCompiler(flags...)
 	if err != nil {

compileopts/options.go

@@ -25,7 +25,7 @@ type Options struct {
 	PrintIR         bool
 	DumpSSA         bool
 	VerifyIR        bool
-	PrintCommands   bool
+	PrintCommands   func(cmd string, args ...string)
 	Debug           bool
 	PrintSizes      string
 	PrintAllocs     *regexp.Regexp // regexp string

main.go

@@ -95,12 +95,31 @@ func copyFile(src, dst string) error {
 
 // executeCommand is a simple wrapper to exec.Cmd
 func executeCommand(options *compileopts.Options, name string, arg ...string) *exec.Cmd {
-	if options.PrintCommands {
+	if options.PrintCommands != nil {
-		fmt.Printf("%s %s\n", name, strings.Join(arg, " "))
+		options.PrintCommands(name, arg...)
 	}
 	return exec.Command(name, arg...)
 }
 
+// printCommand prints a command to stdout while formatting it like a real
+// command (escaping characters etc). The resulting command should be easy to
+// run directly in a shell, although it is not guaranteed to be a safe shell
+// escape. That's not a problem as the primary use case is printing the command,
+// not running it.
+func printCommand(cmd string, args ...string) {
+	command := append([]string{cmd}, args...)
+	for i, arg := range command {
+		// Source: https://www.oreilly.com/library/view/learning-the-bash/1565923472/ch01s09.html
+		const specialChars = "~`#$&*()\\|[]{};'\"<>?! "
+		if strings.ContainsAny(arg, specialChars) {
+			// See: https://stackoverflow.com/questions/15783701/which-characters-need-to-be-escaped-when-using-bash
+			arg = "'" + strings.ReplaceAll(arg, `'`, `'\''`) + "'"
+			command[i] = arg
+		}
+	}
+	fmt.Fprintln(os.Stderr, strings.Join(command, " "))
+}
+
 // Build compiles and links the given package and writes it to outpath.
 func Build(pkgName, outpath string, options *compileopts.Options) error {
 	config, err := builder.NewConfig(options)
@@ -1008,7 +1027,6 @@ func main() {
 		PrintSizes:      *printSize,
 		PrintStacks:     *printStacks,
 		PrintAllocs:     printAllocs,
-		PrintCommands:   *printCommands,
 		Tags:            *tags,
 		GlobalValues:    globalVarValues,
 		WasmAbi:         *wasmAbi,
@@ -1016,6 +1034,9 @@ func main() {
 		OpenOCDCommands: ocdCommands,
 		LLVMFeatures:    *llvmFeatures,
 	}
+	if *printCommands {
+		options.PrintCommands = printCommand
+	}
 
 	os.Setenv("CC", "clang -target="+*target)