[PATCH] builtin: Keep backslash on undefined escape sequences

[Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>]

On Mon, Oct 21, 2024 at 06:01:53AM +0100, Harald van Dijk wrote:
>
> This particular use of echo with a backslash character used to work the same
> way across shells and is widely used in configure scripts in a large number
> of packages (specifically: any that use AC_SUBST_FILE). It was changed by
> commit 776424a8 (parser: Add dollar single quote) without that commit
> indicating that this would change; are you really sure this should not be
> viewed as a bug? The impact will be massive.

Fair enough.  This behaviour should be reverted.

---8<---
A lot of scripts (in particular, autoconf) relies on echo keeping
undefined backslash sequences intact.  Preserve this behaviour by
only interpreting the few sequences required for dollar single quote.

Repoted-by: Дилян Палаузов <dilyan.palauzov@xxxxxxxxx>
Fixes: 776424a8f915 ("parser: Add dollar single quote")
Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>

diff --git a/src/bltin/printf.c b/src/bltin/printf.c
index 2c18e93..46c6295 100644
--- a/src/bltin/printf.c
+++ b/src/bltin/printf.c
@@ -339,8 +339,8 @@ unsigned conv_escape(char *str0, char *out0, bool mbchar)
 	switch (ch) {
 	default:
 		if (!isodigit(ch)) {
-			value = ch ?: '\\';
-			str -= !ch;
+			value = '\\';
+			str--;
 			break;
 		}
 
@@ -428,6 +428,12 @@ hex:
 		ch = 8;
 		goto hex;
 
+	case '\\':
+	case '\"':
+	case '\'':
+		value = ch;
+		break;
+
 	case 'a':	value = '\a';	break;	/* alert */
 	case 'b':	value = '\b';	break;	/* backspace */
 	case 'e':	value = '\033';	break;	/* <ESC> */
-- 
Email: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt