shell: fix arithmentic evaluation of "++7" and such (it is + + 7, i.e. 7)

function                                             old     new   delta
evaluate_string                                      945     988     +43

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

1 files changed, 15 insertions, 8 deletions

diff --git a/shell/math.c b/shell/math.c
index 2942cdd26..049d5703b 100644
--- a/shell/math.c
+++ b/shell/math.c
@@ -668,19 +668,26 @@ evaluate_string(arith_state_t *math_state, const char *expr)
 
                 /* Should be an operator */
 
-                /* Special case: NUM-- and NUM++ are not recognized if NUM
-                 * is a literal number, not a variable. IOW:
+                /* Special case: XYZ--, XYZ++, --XYZ, ++XYZ are recognized
+                 * only if XYZ is a variable name, not a number or EXPR. IOW:
                  * "a+++v" is a++ + v.
                  * "7+++v" is 7 + ++v, not 7++ + v.
+                 * "--7" is - - 7, not --7.
+                 * "++++a" is + + ++a, not ++ ++ a.
+                 * (we still mishandle "(a)+++7", should be treated as (a) + + + 7, but we do increment a)
                  */
-                if (lasttok == TOK_NUM && !numstackptr[-1].var /* number literal */
-                 && (expr[0] == '+' || expr[0] == '-')
+                if ((expr[0] == '+' || expr[0] == '-')
                  && (expr[1] == expr[0])
                 ) {
-                        //bb_error_msg("special %c%c", expr[0], expr[0]);
-                        op = (expr[0] == '+' ? TOK_ADD : TOK_SUB);
-                        expr += 1;
-                        goto tok_found1;
+                        if (numstackptr == numstack || !numstackptr[-1].var) { /* not a VAR++ */
+                                char next = skip_whitespace(expr + 2)[0];
+                                if (!(isalpha(next) || next == '_')) { /* not a ++VAR */
+                                        //bb_error_msg("special %c%c", expr[0], expr[0]);
+                                        op = (expr[0] == '+' ? TOK_ADD : TOK_SUB);
+                                        expr++;
+                                        goto tok_found1;
+                                }
+                        }
                 }
 
                 p = op_tokens;