Prefer /bin/bash and validate SHELL points to bash before using it (#81)

Co-authored-by: swissspidy <841956+swissspidy@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Pascal Birchler <pascal.birchler@gmail.com>
Co-authored-by: Pascal Birchler <pascalb@google.com>

Author: 3 people

features/shell.feature

@@ -61,6 +61,38 @@ Feature: WordPress REPL
       """
     And STDERR should be empty
 
+  Scenario: Use SHELL environment variable as fallback for bash
+    Given a WP install
+
+    And a session file:
+      """
+      return true;
+      """
+
+    # SHELL pointing to bash should work (when bash is available).
+    When I try `SHELL=/bin/bash wp shell --basic < session`
+    Then STDOUT should contain:
+      """
+      bool(true)
+      """
+    And STDERR should be empty
+
+    # SHELL pointing to non-bash binary should be ignored and fall back to /bin/bash.
+    When I try `SHELL=/bin/sh wp shell --basic < session`
+    Then STDOUT should contain:
+      """
+      bool(true)
+      """
+    And STDERR should be empty
+
+    # SHELL pointing to invalid path should be ignored and fall back to /bin/bash.
+    When I try `SHELL=/nonsense/path wp shell --basic < session`
+    Then STDOUT should contain:
+      """
+      bool(true)
+      """
+    And STDERR should be empty
+
   Scenario: Input starting with dash
     Given a WP install
     And a session file:

src/WP_CLI/Shell/REPL.php

@@ -122,8 +122,15 @@ private static function create_prompt_cmd( $prompt, $history_path ) {
 		$history_path = escapeshellarg( $history_path );
 		if ( getenv( 'WP_CLI_CUSTOM_SHELL' ) ) {
 			$shell_binary = (string) getenv( 'WP_CLI_CUSTOM_SHELL' );
-		} else {
+		} elseif ( is_file( '/bin/bash' ) && is_readable( '/bin/bash' ) ) {
+			// Prefer /bin/bash when available since we use bash-specific commands.
 			$shell_binary = '/bin/bash';
+		} elseif ( getenv( 'SHELL' ) && self::is_bash_shell( (string) getenv( 'SHELL' ) ) ) {
+			// Only use SHELL as fallback if it's bash (we use bash-specific commands).
+			$shell_binary = (string) getenv( 'SHELL' );
+		} else {
+			// Final fallback for systems without /bin/bash.
+			$shell_binary = 'bash';
 		}
 
 		if ( ! is_file( $shell_binary ) || ! is_readable( $shell_binary ) ) {
@@ -144,6 +151,21 @@ private static function create_prompt_cmd( $prompt, $history_path ) {
 		return "{$shell_binary} -c " . escapeshellarg( $cmd );
 	}
 
+	/**
+	 * Check if a shell binary is bash or bash-compatible.
+	 *
+	 * @param string $shell_path Path to the shell binary.
+	 * @return bool True if the shell is bash, false otherwise.
+	 */
+	private static function is_bash_shell( $shell_path ) {
+		if ( ! is_file( $shell_path ) || ! is_readable( $shell_path ) ) {
+			return false;
+		}
+		// Check if the basename is exactly 'bash' or starts with 'bash' followed by a version/variant.
+		$basename = basename( $shell_path );
+		return 'bash' === $basename || 0 === strpos( $basename, 'bash-' );
+	}
+
 	private function set_history_file() {
 		$data = getcwd() . get_current_user();