ash: fix handling of negative start value in ${v:start:len}

function                                             old     new   delta
subevalvar                                          1140    1168     +28

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 9 insertions, 1 deletions

diff --git a/shell/ash.c b/shell/ash.c
index 705fe9fa4..90fb00fbd 100644
--- a/shell/ash.c
+++ b/shell/ash.c
@@ -6411,7 +6411,15 @@ subevalvar(char *p, char *varname, int strloc, int subtype,
 				len = number(loc);
 			}
 		}
-		if (pos >= orig_len) {
+		if (pos < 0) {
+			/* ${VAR:$((-n)):l} starts n chars from the end */
+			pos = orig_len + pos;
+		}
+		if ((unsigned)pos >= orig_len) {
+			/* apart from obvious ${VAR:999999:l},
+			 * covers ${VAR:$((-9999999)):l} - result is ""
+			 * (bash-compat)
+			 */
 			pos = 0;
 			len = 0;
 		}