hush: implement negative start in the ${v: -n[:m]} idiom

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

1 files changed, 6 insertions, 2 deletions

diff --git a/shell/hush.c b/shell/hush.c
index 64b33cf1c..f6b50dec6 100644
--- a/shell/hush.c
+++ b/shell/hush.c
@@ -5619,8 +5619,12 @@ static NOINLINE const char *expand_one_var(char **to_be_freed_pp, char *arg, cha
                                 goto arith_err;
                         debug_printf_varexp("len:'%s'=%lld\n", exp_word, (long long)len);
                         if (len >= 0) { /* bash compat: len < 0 is illegal */
-                                if (beg < 0) /* bash compat */
-                                        beg = 0;
+                                if (beg < 0) {
+                                        /* negative beg counts from the end */
+                                        beg = (arith_t)strlen(val) + beg;
+                                        if (beg < 0) /* ${v: -999999} is "" */
+                                                beg = len = 0;
+                                }
                                 debug_printf_varexp("from val:'%s'\n", val);
                                 if (len == 0 || !val || beg >= strlen(val)) {
  arith_err: