Sequential Backslashes are removed during interpretation.

[cfogrady]

Sequential Backslashes seem to be reduced to a single backslash. Is there a purposeful reason behind this or am I correct in believing this to be a bug?

Example:
gosh -c 'echo "hello \\\\\\ world"' yields hello \ world

Running the same thing in bash:
bash -c 'echo "hello \\\\\\ world"' yields hello \\\ world matching my expectations.

In code:

func TestMvdanShellPreservesSequentialBackslashes(t *testing.T) {
	parser := syntax.NewParser(syntax.Variant(syntax.LangBash))
	prog, _ := parser.Parse(strings.NewReader(`echo "Hello \\\\\\ World"`), "$0")
	writer := &bytes.Buffer{}
	bash, _ := interp.New(interp.StdIO(nil, writer, writer))
	bash.Run(context.Background(), prog)
	want := `Hello \\\ World`
	if writer.String() != want {
		t.Errorf(`echo "Hello \\\\\\ World" = %v, want %v`, writer.String(), want)
	}
}

I believe this is an issue in the interp as running a syntax.DebugPrint on prog yields:

Parts: []syntax.WordPart (len = 1) {
.  .  .  .  .  .  .  .  .  0: *syntax.Lit {
.  .  .  .  .  .  .  .  .  .  ValuePos: 1:7
.  .  .  .  .  .  .  .  .  .  ValueEnd: 1:25
.  .  .  .  .  .  .  .  .  .  Value: "Hello \\\\\\\\\\\\ World"
.  .  .  .  .  .  .  .  .  }
.  .  .  .  .  .  .  .  }

[mvdan]

Thanks for reporting, this does seem like a bug.

Out of curiosity, how did you find this? Are you using the interpreter via the Go API?

[cfogrady]

Yup. Using the interp Go API for some shell scripts with gnarly arguments including some regexps. Some of the regexps were off after going through my program and after debugging, I managed to localize the issue to the library as above.

[cfogrady]

Further investigation seems to indicate this is a problem specific to double quoted literals.
gosh -c "echo 'hello \\\\\\ world'" yields what I would expect: hello \\\ world.

[cfogrady]

I'll prep a PR after lunch.
Offending code seems to be: https://github.com/mvdan/sh/blob/392da98b03853d889822bc2f62d61f4f92f37fa4/expand/expand.go#L505C4-L518C5

Changing to the below seems to fix it and still pass all the test cases I could think of as relevant.

buf := cfg.strBuilder()
for i := 0; i < len(s); i++ {
	b := s[i]
	if b == '\\' && i+1 < len(s) {
		switch s[i+1] {
		case '"', '\\', '$', '`': // special chars
			i++
			b = s[i] // write the special char, skipping the backslash
		}
	}
	buf.WriteByte(b)
}

[cfogrady]

Got side tracked by some other issues Friday afternoon. PR to fix the backslash issue: #1107