Tuesday, 26 November 2013

if errorlevel 1 vs if %errorlevel% neq 0

The problem with having used batch files since the days of DOS is that certain constructs are just way too embedded in your brain. For example the way I check whether a command in a batch file has failed is by using the special “errorlevel” version of the “if” statement:-

ExternalProgram.exe
if errorlevel 1 exit /b 1

This style of “if” says that if the exit code from the last run program run was greater than or equal to 1 the script should terminate, with an exit code of 1. The general convention for exit codes is that 0 means success and anything else just means “something else happened” [1] and it’s entirely dependent on the program. For example the ROBOCOPY utility has a variety of different codes that may or may not be an error depending on what you asked it to do. The MSI installer (MSIEXEC.EXE) is another program that has a few exit codes that you soon get to know if you’re trying to script a deployment process, e.g.

msiexec /x <GUID>
if %errorlevel% equ 1605 goto :not_installed

This form of “if” statement (with a variable called errorlevel) is the newer form that was introduced in Windows NT (I think) and it allows you to do an equality comparison with a single exit code, which was less than intuitive before. This form is also required when you have anti-social processes that return negative exit codes [2]. In fact the earlier form should probably be considered defunct (if only my muscle memory would let go) and the newer form used by default instead:-

ExternalProgram.exe
if %errorlevel% neq 0 exit /b %errorlevel%

If you can’t remember what the operators are use “C:\> HELP IF” to list them [3].

[1] C & C++ programmers will of course already be used to using the macros EXIT_SUCCESS and EXIT_FAILURE from <stdlib.h>. I don’t think .Net has an equivalent and so I often end up creating a class called ExitCode with the same two constants.

[2] Yes SCHTASKS (the command line tool for managing scheduled tasks) I’m looking at you. The .Net runtime can also chuck out negative exit codes if something really serious goes wrong, e.g. the process fails to start with an out of memory or assembly loading problem.

[3] They’re different from PowerShell too which is a shame.

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