Holy cow, I wrote a book!
The command interpreter cmd.exe has a concept
known as the error level,
which is the exit code of the program most recently run.
You can test the error level with the
IF ERRORLEVEL command:
IF ERRORLEVEL 1 ECHO error level is 1 or more
The IF ERRORLEVEL n test succeeds
if the error level is n or more.
This was presumably because there were programs that
expressed different degrees of failure with higher and
higher exit codes.
For example, the diff program has three exit codes:
0 means the files are the same;
1 means the files are different;
2 means that something terrible happened.
There are also programs that use an exit code of zero
to mean success and anything else to mean failure.
IF ERRORLEVEL n
In addition to this internal state,
you can, if you wish, create an environment variable
with the name ERRORLEVEL,
in the same way that you can create an environment variable
But, as with FRED,
that variable won't have any effect on the error level.
rem this next command sets the error level to zero
CMD /C EXIT 0
if ERRORLEVEL 1 echo Does this print?
The message is not printed because the ERRORLEVEL
environment variable has no effect on the error level.
It's just a variable whose name happens to coincide with a command
"Hey, when I tried to withdraw the money, I got an insufficient funds error.
What am I doing wrong?"
Now, it does happen to be the case that if command extensions are
enabled and you say
%ERRORLEVEL%, then the command processor first looks
for an environment variable called ERRORLEVEL,
and if it can't find one, then it replaces %ERRORLEVEL%
with the current value of the internal error level value.
It's a fallback step,
in the same way that your neighbor is a fallback delivery location if
you aren't home.
If you file a change-of-address form for yourself, that doesn't affect
packages sent to your neighbor.
The same behavior can be seen with %CD%:
If you did not explicitly set an environment variable called CD,
then %CD% expands to the command processor's current
But you can't change directories by saying set CD=C:\Windows.
I can think of a few reasons why this feature may have been added.
ECHO error level is %ERRORLEVEL%>logfile
IF %ERRORLEVEL% EQU 1 echo Different!
But I'm digressing.
My point for today is that the error level is not the same
as the ERRORLEVEL environment variable.