more robust logic to get last command exit value

fixes #151

Author: Michele Mesiti

bash_kernel/exit_code_checker.py

@@ -0,0 +1,12 @@
+import re
+
+exit_code_format = "BASHKERNEL_RETCODE<<<{}>>>"
+exit_code_re = re.compile(exit_code_format.format("([0-9]+)"))
+exit_code_printf_format = exit_code_format.format("%d")
+exit_code_command_checker = f'{{ printf "{exit_code_printf_format}" $?; }} 2>/dev/null'
+
+
+def get_last_exit_code(bashwrapper):
+    output = bashwrapper.run_command(exit_code_command_checker)
+    matches = exit_code_re.findall(output)
+    return int(matches[0])

bash_kernel/kernel.py

@@ -16,6 +16,7 @@
 version_pat = re.compile(r'version (\d+(\.\d+)+)')
 
 from .display import (extract_contents, build_cmds)
+from .exit_code_checker import get_last_exit_code
 
 class IREPLWrapper(replwrap.REPLWrapper):
     """A subclass of REPLWrapper that gives incremental output
@@ -224,7 +225,7 @@ def do_execute(self, code, silent, store_history=True,
             return {'status': 'abort', 'execution_count': self.execution_count}
 
         try:
-            exitcode = int(self.bashwrapper.run_command('{ echo $?; } 2>/dev/null').rstrip().split("\r\n")[0])
+            exitcode =  get_last_exit_code(self.bashwrapper)
         except Exception as exc:
             exitcode = 1