From ea54f87ea06f48035878c1ff2659d6bb7dd4f512 Mon Sep 17 00:00:00 2001
From: Mykhailo Chalyi <mike@chaliy.name>
Date: Fri, 27 Mar 2026 23:41:01 +0000
Subject: [PATCH] fix(interpreter): expand special variables ($#, $?, etc.) in
 arithmetic

Special shell variables like $# were not recognized by
expand_arithmetic_vars_depth because it only consumed alphanumeric/_ chars
after $. Add handling for special variable characters (#, ?, $, !, @, *, -)
so they resolve correctly inside (( )) arithmetic context.

Closes #876
---
 crates/bashkit/src/interpreter/mod.rs         | 11 +++++++++
 .../tests/spec_cases/bash/arithmetic.test.sh  | 23 +++++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/crates/bashkit/src/interpreter/mod.rs b/crates/bashkit/src/interpreter/mod.rs
index aca72bdb..8b261484 100644
--- a/crates/bashkit/src/interpreter/mod.rs
+++ b/crates/bashkit/src/interpreter/mod.rs
@@ -7165,6 +7165,17 @@ impl Interpreter {
                     } else {
                         result.push_str(&expanded);
                     }
+                } else if let Some(&c) = chars.peek()
+                    && matches!(c, '#' | '?' | '$' | '!' | '@' | '*' | '-')
+                {
+                    // Handle special variables: $#, $?, $$, $!, $@, $*, $-
+                    chars.next();
+                    let value = self.expand_variable(&c.to_string());
+                    if value.is_empty() {
+                        result.push('0');
+                    } else {
+                        result.push_str(&value);
+                    }
                 } else {
                     // Handle $var syntax (common in arithmetic)
                     let mut name = String::new();
diff --git a/crates/bashkit/tests/spec_cases/bash/arithmetic.test.sh b/crates/bashkit/tests/spec_cases/bash/arithmetic.test.sh
index 3ca493fe..9ad74ee0 100644
--- a/crates/bashkit/tests/spec_cases/bash/arithmetic.test.sh
+++ b/crates/bashkit/tests/spec_cases/bash/arithmetic.test.sh
@@ -493,3 +493,26 @@ echo $x
 ### expect
 42
 ### end
+
+### arith_special_var_hash
+# $# in arithmetic context
+set -- a b c
+echo "argc: $#"
+(( $# > 0 )) && echo "true" || echo "false"
+(( 3 > 0 )) && echo "true2" || echo "false2"
+x=$#
+(( x > 0 )) && echo "true3" || echo "false3"
+### expect
+argc: 3
+true
+true2
+true3
+### end
+
+### arith_special_var_question
+# $? in arithmetic context
+true
+(( $? == 0 )) && echo "zero" || echo "nonzero"
+### expect
+zero
+### end