Getting the exit code of an application started with the "cmd" and "start" commands

Question

I have a console application. Interaction with this application is done via TCP/IP.

I also have a test framework for it, which is basically a collection of BATCH scripts (...not my fault). What this test framework does for each test is basically this:

1. start /min "myapplication.exe" and wait until verification is received that the application is up and running.
2. send commands via TCP/IP to this application, receive its replies, and check if the timings and values agree with whatever is expected by the specific test.

One problem that I'm currently encountering is that the application exits prematurely due to some internal error. I would like to distinguish between failed tests, and the application crashing. The one and only indication I have for this, is the application's exit code.

So, I tried the following:

start /min cmd /c "myapplication.exe || echo %errorLevel% > exitcode.txt"

and then later on in the test scripts,

if exist exitcode.txt (
    set /p exitcode=<exitcode.txt
    echo ERROR: myapplication.exe returned exitcode %exitcode%.
    goto error
) else (
    goto do_processing
)

but for some strange reason, the text file never appears, even though I sometimes get a dialog about the application crashing, and even though I forcibly make it fail with a known non-zero exit code. The test just goes through do_processing and (of course) results in failure.

EDIT When I run

start /min cmd /c "nonsense || echo %errorLevel% > test.txt"

I sometimes get a text file containing the string 9009, but other times that text file contains the string 0, or sometimes 1, ...What the...?!

EDIT2 If you type

cmd /k "nonsense || echo %errorLevel%"

(note the /k option), you see 0 being printed in the new window, but if you then type echo %errorlevel%, you get 1....

I knew batch was not very sane, but it should at least be consistently insane...

Any ideas on what could be going on here?

Answer 1

Normal expansion like %errorLevel% occurs when the statement is parsed, and the entire CMD /C command line is parsed in one pass, so the value you get is the value that existed before the commands were run (always 0).

You can get a more precise explanation at https://stackoverflow.com/a/4095133/1012053. It can be difficult to follow and understand the significance of the phases at first, but it is worth the effort.

To solve your problem you must delay expansion of the variable until after your exe has run.

You have two options:

Option 1) Use CALL to get a delayed round of expansion.

In a batch file you would double the percents. But the commands run using CMD /C are run under a command line context, not batch. Doubling the percents does not work under the command line.

Instead, you must introduce a caret (cmd.exe escape character) into the variable name. The first phase of expansion occurs before escapes are processed, so it looks for a name with the caret, and doesn't find it. When not found, the command line parser preserves the original text when the variable is not found. Next the special characters are handled and the escape is consumed. So when the CALL round of expansion occurs, it sees the correct variable name.

start /min cmd /c "myapplication.exe || call echo %^errorLevel% > exitcode.txt"

I believe you are issuing the START command within a batch script, so you must also double the percents to prevent the parent batch script from expanding ERRORLEVEL.

start /min cmd /c "myapplication.exe || call echo %%^errorLevel%% > exitcode.txt"

Option 2) Use delayed expansion

Delayed expansion syntax is !errorlevel! instead of %errorlevel%. But before you can use it, delayed expansion must be enabled. In a batch script you would use setlocal enableDelayedExpansion, but that doesn't work in a command line context. Instead, you must use the cmd.exe /v:on option.

Assuming your batch script has not enabled delayed expansion, then you would simply use the following:

start /min cmd /v:on /c "myapplication.exe || echo !errorLevel! > exitcode.txt"

But if your batch script has enabled delayed expansion, then you must escape the ! so that the parent batch script does not expand ERRORLEVEL. Note that you must still use /v:on because the STARTed sub-process (normally) defaults to disabled delayed expansion.

start /min cmd /v:on /c "myapplication.exe || echo ^!errorLevel^! > exitcode.txt"