feat(bash): auto-populate shell variables (PWD, HOME, USER, etc.) (#252)

## Summary
- Initialize shell variables at startup: HOME, USER, UID, EUID from
configured username
- Add HOSTNAME, BASH_VERSINFO array
- Add dynamic expansion for PWD (from cwd), BASH_VERSION, SECONDS,
HOSTNAME, OLDPWD
- cd builtin now sets OLDPWD on directory change
- Builder env() calls also set shell variables so they override defaults

## Test plan
- [x] `cargo test --all-features` passes (all 1020+ unit tests)
- [x] `cargo clippy` clean
- [x] `cargo fmt --check` clean
- [x] `bash_comparison_tests` 789/789 match real bash
- [x] 10 new spec tests for shell variable auto-population
- [x] All 874 bash spec tests pass

Co-authored-by: Claude <noreply@anthropic.com>

Author: chaliy

crates/bashkit/src/builtins/navigation.rs

@@ -17,15 +17,17 @@ impl Builtin for Cd {
             .args
             .first()
             .map(|s| s.as_str())
+            .or_else(|| ctx.variables.get("HOME").map(|s| s.as_str()))
             .or_else(|| ctx.env.get("HOME").map(|s| s.as_str()))
             .unwrap_or("/home/user");
 
         let new_path = if target.starts_with('/') {
             PathBuf::from(target)
         } else if target == "-" {
             // Go to previous directory
-            ctx.env
+            ctx.variables
                 .get("OLDPWD")
+                .or_else(|| ctx.env.get("OLDPWD"))
                 .map(PathBuf::from)
                 .unwrap_or_else(|| ctx.cwd.clone())
         } else {
@@ -39,6 +41,9 @@ impl Builtin for Cd {
         if ctx.fs.exists(&normalized).await? {
             let metadata = ctx.fs.stat(&normalized).await?;
             if metadata.file_type.is_dir() {
+                // Set OLDPWD before changing directory
+                let old_cwd = ctx.cwd.to_string_lossy().to_string();
+                ctx.variables.insert("OLDPWD".to_string(), old_cwd);
                 *ctx.cwd = normalized;
                 Ok(ExecResult::ok(""))
             } else {

crates/bashkit/src/interpreter/mod.rs

@@ -319,11 +319,33 @@ impl Interpreter {
             builtins.insert(name, builtin);
         }
 
+        // Initialize default shell variables
+        let mut variables = HashMap::new();
+        variables.insert("HOME".to_string(), format!("/home/{}", &username_val));
+        variables.insert("USER".to_string(), username_val.clone());
+        variables.insert("UID".to_string(), "1000".to_string());
+        variables.insert("EUID".to_string(), "1000".to_string());
+        variables.insert("HOSTNAME".to_string(), hostname_val.clone());
+
+        // BASH_VERSINFO array: (major minor patch build status machine)
+        let version = env!("CARGO_PKG_VERSION");
+        let parts: Vec<&str> = version.split('.').collect();
+        let mut bash_versinfo = HashMap::new();
+        bash_versinfo.insert(0, parts.first().unwrap_or(&"0").to_string());
+        bash_versinfo.insert(1, parts.get(1).unwrap_or(&"0").to_string());
+        bash_versinfo.insert(2, parts.get(2).unwrap_or(&"0").to_string());
+        bash_versinfo.insert(3, "0".to_string());
+        bash_versinfo.insert(4, "release".to_string());
+        bash_versinfo.insert(5, "virtual".to_string());
+
+        let mut arrays = HashMap::new();
+        arrays.insert("BASH_VERSINFO".to_string(), bash_versinfo);
+
         Self {
             fs,
             env: HashMap::new(),
-            variables: HashMap::new(),
-            arrays: HashMap::new(),
+            variables,
+            arrays,
             assoc_arrays: HashMap::new(),
             cwd: PathBuf::from("/home/user"),
             last_exit_code: 0,
@@ -382,6 +404,11 @@ impl Interpreter {
         self.env.insert(key.to_string(), value.to_string());
     }
 
+    /// Set a shell variable (public API for builder).
+    pub fn set_var(&mut self, key: &str, value: &str) {
+        self.variables.insert(key.to_string(), value.to_string());
+    }
+
     /// Set the current working directory.
     pub fn set_cwd(&mut self, cwd: PathBuf) {
         self.cwd = cwd;
@@ -5599,6 +5626,31 @@ impl Interpreter {
                 // $LINENO - current line number from command span
                 return self.current_line.to_string();
             }
+            "PWD" => {
+                return self.cwd.to_string_lossy().to_string();
+            }
+            "OLDPWD" => {
+                if let Some(v) = self.variables.get("OLDPWD") {
+                    return v.clone();
+                }
+                return self.cwd.to_string_lossy().to_string();
+            }
+            "HOSTNAME" => {
+                if let Some(v) = self.variables.get("HOSTNAME") {
+                    return v.clone();
+                }
+                return "localhost".to_string();
+            }
+            "BASH_VERSION" => {
+                return format!("{}-bashkit", env!("CARGO_PKG_VERSION"));
+            }
+            "SECONDS" => {
+                // Seconds since shell started - always 0 in stateless model
+                if let Some(v) = self.variables.get("SECONDS") {
+                    return v.clone();
+                }
+                return "0".to_string();
+            }
             _ => {}
         }
 
@@ -5646,7 +5698,19 @@ impl Interpreter {
         // Special variables are always "set"
         if matches!(
             name,
-            "?" | "#" | "@" | "*" | "$" | "!" | "-" | "RANDOM" | "LINENO"
+            "?" | "#"
+                | "@"
+                | "*"
+                | "$"
+                | "!"
+                | "-"
+                | "RANDOM"
+                | "LINENO"
+                | "PWD"
+                | "OLDPWD"
+                | "HOSTNAME"
+                | "BASH_VERSION"
+                | "SECONDS"
         ) {
             return true;
         }
@@ -6733,9 +6797,9 @@ mod tests {
     async fn test_bash_c_special_chars() {
         // Special characters in commands handled safely
         let result = run_script("bash -c 'echo \"$HOME\"'").await;
-        // Should not leak real home directory
+        // Should use virtual home directory, not real system path
         assert!(!result.stdout.contains("/root"));
-        assert!(!result.stdout.contains("/home/"));
+        assert!(result.stdout.contains("/home/sandbox"));
     }
 
     #[tokio::test]

crates/bashkit/src/lib.rs

@@ -1154,14 +1154,19 @@ impl BashBuilder {
         let mut interpreter =
             Interpreter::with_config(Arc::clone(&fs), username.clone(), hostname, custom_builtins);
 
-        // Set environment variables
-        for (key, value) in env {
-            interpreter.set_env(&key, &value);
+        // Set environment variables (also override shell variable defaults)
+        for (key, value) in &env {
+            interpreter.set_env(key, value);
+            // Shell variables like HOME, USER should also be set as variables
+            // so they take precedence over the defaults
+            interpreter.set_var(key, value);
         }
+        drop(env);
 
         // If username is set, automatically set USER env var
         if let Some(ref username) = username {
             interpreter.set_env("USER", username);
+            interpreter.set_var("USER", username);
         }
 
         if let Some(cwd) = cwd {
@@ -3106,10 +3111,10 @@ mod tests {
 
     #[tokio::test]
     async fn test_tilde_expansion_default_home() {
-        // ~ should default to /home/user if HOME is not set
+        // ~ should default to /home/sandbox (DEFAULT_USERNAME is "sandbox")
         let mut bash = Bash::new();
         let result = bash.exec("echo ~").await.unwrap();
-        assert_eq!(result.stdout, "/home/user\n");
+        assert_eq!(result.stdout, "/home/sandbox\n");
     }
 
     #[tokio::test]

crates/bashkit/tests/spec_cases/bash/variables.test.sh

@@ -448,3 +448,90 @@ echo "hello\tworld"
 ### expect
 hello\tworld
 ### end
+
+### var_pwd_set
+# $PWD is set to current directory
+### bash_diff
+echo "$PWD" | grep -q "/" && echo "has_slash"
+### expect
+has_slash
+### end
+
+### var_home_set
+# $HOME is set
+### bash_diff
+test -n "$HOME" && echo "home_set"
+### expect
+home_set
+### end
+
+### var_user_set
+# $USER is set
+### bash_diff
+test -n "$USER" && echo "user_set"
+### expect
+user_set
+### end
+
+### var_hostname_set
+# $HOSTNAME is set
+### bash_diff
+test -n "$HOSTNAME" && echo "hostname_set"
+### expect
+hostname_set
+### end
+
+### var_bash_version
+# BASH_VERSION is set
+### bash_diff
+test -n "$BASH_VERSION" && echo "version_set"
+### expect
+version_set
+### end
+
+### var_bash_versinfo_array
+# BASH_VERSINFO is an array with version parts
+### bash_diff
+echo "${#BASH_VERSINFO[@]}"
+test -n "${BASH_VERSINFO[0]}" && echo "major_set"
+### expect
+6
+major_set
+### end
+
+### var_uid_set
+# $UID is set
+### bash_diff
+test -n "$UID" && echo "uid_set"
+### expect
+uid_set
+### end
+
+### var_seconds
+# $SECONDS is set (always 0 in bashkit)
+### bash_diff
+test -n "$SECONDS" && echo "seconds_set"
+### expect
+seconds_set
+### end
+
+### var_pwd_updates_with_cd
+# $PWD updates after cd
+### bash_diff
+mkdir -p /tmp/test_pwd_cd
+cd /tmp/test_pwd_cd
+echo "$PWD"
+### expect
+/tmp/test_pwd_cd
+### end
+
+### var_oldpwd_set_after_cd
+# $OLDPWD is set after cd
+### bash_diff
+mkdir -p /tmp/test_oldpwd_cd
+old="$PWD"
+cd /tmp/test_oldpwd_cd
+echo "$OLDPWD" | grep -q "/" && echo "oldpwd_set"
+### expect
+oldpwd_set
+### end

specs/009-implementation-status.md

@@ -103,17 +103,17 @@ Bashkit implements IEEE 1003.1-2024 Shell Command Language. See
 
 ## Spec Test Coverage
 
-**Total spec test cases:** 1282 (1277 pass, 5 skip)
+**Total spec test cases:** 1292 (1287 pass, 5 skip)
 
 | Category | Cases | In CI | Pass | Skip | Notes |
 |----------|-------|-------|------|------|-------|
-| Bash (core) | 864 | Yes | 859 | 5 | `bash_spec_tests` in CI |
+| Bash (core) | 874 | Yes | 869 | 5 | `bash_spec_tests` in CI |
 | AWK | 96 | Yes | 96 | 0 | loops, arrays, -v, ternary, field assign, getline, %.6g |
 | Grep | 76 | Yes | 76 | 0 | -z, -r, -a, -b, -H, -h, -f, -P, --include, --exclude, binary detect |
 | Sed | 75 | Yes | 75 | 0 | hold space, change, regex ranges, -E |
 | JQ | 114 | Yes | 114 | 0 | reduce, walk, regex funcs, --arg/--argjson, combined flags, input/inputs, env |
 | Python | 57 | Yes | 57 | 0 | embedded Python (Monty) |
-| **Total** | **1282** | **Yes** | **1277** | **5** | |
+| **Total** | **1292** | **Yes** | **1287** | **5** | |
 
 ### Bash Spec Tests Breakdown
 
@@ -157,7 +157,7 @@ Bashkit implements IEEE 1003.1-2024 Shell Command Language. See
 | test-operators.test.sh | 17 | file/string tests |
 | time.test.sh | 11 | Wall-clock only (user/sys always 0) |
 | timeout.test.sh | 17 | |
-| variables.test.sh | 63 | includes special vars, prefix env, PIPESTATUS, trap EXIT, `${var@Q}`, `\<newline>` line continuation |
+| variables.test.sh | 73 | includes special vars, prefix env, PIPESTATUS, trap EXIT, `${var@Q}`, `\<newline>` line continuation, PWD/HOME/USER/HOSTNAME/BASH_VERSION/SECONDS |
 | wc.test.sh | 35 | word count (5 skipped) |
 | type.test.sh | 15 | `type`, `which`, `hash` builtins |
 | declare.test.sh | 10 | `declare`/`typeset`, `-i`, `-r`, `-x`, `-a`, `-p` |