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Re: one shell command doesn't return result

From: Eli Zaretskii
Subject: Re: one shell command doesn't return result
Date: Sun, 15 Jun 2008 21:22:39 +0300

> Date: Sun, 15 Jun 2008 16:34:16 +0200
> From: "address@hidden" <address@hidden>
> I would like to make a simple calculation in my makefile, but I have a 
> strange problem.
> On windows, I use the DOS command "set /A 3*100+13/2", it works on a shell 
> and returns "306" but not in my makefile. I have make a simple makefile to 
> test it :
> --------------------------------------------------
> # command to make a mathematical operation
> CMD_MATH:=$(shell set /A 3*100+13/2)
> # command to list files
> CMD_FILES:=$(shell dir /b/a-d/-W)
> all::
>       @echo CMD_MATH:$(CMD_MATH)
>       @echo CMD_FILES:$(CMD_FILES)
> --------------------------------------------------
> It returns me :
> --------------------------------------------------
> CMD_FILES:Makefile Makefile~
> --------------------------------------------------
> One command returns correctly the result but not the command "set /A" which 
> returns nothing.
> I have GNU Make 3.81

This is expected behavior.  "set /?" has this small print among its

    If SET /A is executed from the command line outside of a command
    script, then it displays the final value of the expression.

Note the "outside of a command script" condition: this is the cause of
your surprise.  When Make uses cmd.exe as the shell, it invokes all
the commands through temporary batch files (a.k.a. command scripts);
you can see that if you invoke Make with the --debug=j switch.  And
when "set /a" is invoked from a batch file, it does not print the

The following trick will do what you want:

  CMD_MATH:=$(shell cmd.exe /c "set /A 3*100+13/2")

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