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How do I set a Linux environment variable from within C/C++

Posted on 2004-08-07
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Last Modified: 2008-01-09
Hi,

I wish to set a Linux environment variable from within GNU C/C++.

I've tried "system("var=hello"); but this doesnt work - the variable seems to disappear when I check it in a shell script.
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Question by:Risky101
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6 Comments
 
LVL 45

Expert Comment

by:sunnycoder
ID: 11745516
man setenv

       #include <stdlib.h>

       int setenv(const char *name, const char *value, int overwrite);
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LVL 51

Expert Comment

by:ahoffmann
ID: 11748753
the variable seems to disappear when I check it in a shell script
you cannot set an variable from within your program which is still alive after this program exits
The only way to do this is to exit your program with any of the exec*() function, calling another program or shell
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LVL 8

Expert Comment

by:akshayxx
ID: 11750100
i would agree with ahoffmann,  env variables can be passed on to children processes, but not escalated up to parents., hence in the C program, whatever u set , will die once the program dies.. unless u exec some shell/program from there in.
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Author Comment

by:Risky101
ID: 11754023
the program 'read' (built into Bash) manages to set the environment variable properly, ie:

read yesno
if [ "$yesno" = "y" ];then
  # do something
fi

So far, I've tried:
system("yesno=\"n\");
setenv()
system ("export etc etc");
system ("declare etc etc");

Without exception, the variables disappear when the program exits. Is there any way, any at all, to set an environment variable, just like read does?

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LVL 51

Expert Comment

by:ahoffmann
ID: 11754193
>  the program 'read' (built into Bash) manages to set the environment variable properly, ie:
NO.

> Without exception, the variables disappear when the program exits.
yes, as already said.
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Accepted Solution

by:
jmcg earned 500 total points
ID: 11754319
Environment variables are process-based. A process can only change environment variables for itself and, by inheritance, for its children.

Any invocation of 'system' creates at least one additional process. Any changes that process makes to its environment are lost when it exits and most certainly cannot effect the environment variables of the parent.

A subprocess can produce, as an output, an instruction for changing the parent's environment variable. And as you've experienced with the 'read' built-in, the shell can change its own environment variables and pass them along to its children.

So, the usual way to have a program return a value into a shell environment variable looks something like this:

result=`external-command`
export result

You can also have the program write its new settings for environment variables into a file

external-command >new-env

where the new-env file ends up looking like:

env1=value1
export env1
env2=value2
export env2

and the shell can then use the "." command to read in the new-env file to make the settings effective

. new-env

======

None of this is particular helpful if the only thing you have control over is a C/C++ program. The shell that calls your program has to cooperate if its environment variables are to be changed.
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