Set and Get environmental shell variable using c++

Environment variables in the Bash shell can be accessed or set using a C++ program. This is facilitated by the getnenv() and putenv() functions defined in the C/C++ stdlib.h library. Environmental variable expansion is a great feature of a Linux shell as it enables programmers and users to rely on the environment settings of each user separately. C++ getenv() will read all exported environmental variables and putenv() will set existing or create new variables.

In this tutorial, you will see an example C++ script that uses getnenv() and putenv() functions to get and set Bash shell variables, respectively. Check out the script below to see how it is done, and then copy it to your own system and adapt it as needed.

In this tutorial you will learn:

• How to set Bash environment variables in C++ using putenv()
• How to get Bash environment variables in C++ using getnenv()
• Example C++ script that can get and set Bash variables

Software Requirements and Linux Command Line Conventions

Category	Requirements, Conventions or Software Version Used
System	Any Linux distro
Software	g++ or any C++ compiler
Other	Privileged access to your Linux system as root or via the sudo command.
Conventions	# – requires given linux commands to be executed with root privileges either directly as a root user or by use of sudo command $ – requires given linux commands to be executed as a regular non-privileged user

Set and Get environmental shell variable using c++

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1. First, copy the following code to a new file. This example shows how to get and print two environment variables, and how to set one.

   #include <stdlib.h> 
   #include <iostream>
    
   int main() { 
    
   // get and print shell environmental variable home 
   std::cout << "SHELL = " << getenv("SHELL") << std::endl; 
   std::cout << "MYENV = " << getenv("MYENV") << std::endl; 
    
   //set new shell environmental variable using putenv 
   char mypath[]="TEMP=/my/new/temp/path/"; 
   putenv( mypath ); 
    
   std::cout << "TEMP = " << getenv("TEMP") << std::endl; 
    
   return 0; 
    
   }

   Save as shell_env.cpp

2. Now let’s try to export new shell environment variable MYENV:

   $ export MYENV=linuxconfig.org
   

3. Compile your C++ program with g++ or any other compiler you want:

   $ g++ shell_env.cpp -o shell_env
   

4. Next, let’s run our compiled script:

   $ ./shell_env
   

Your output should be:

SHELL = /bin/bash
MYENV = linuxconfig.org
TEMP = /my/new/temp/path/

As you can see, our script was able to retrieve a preset environment variable SHELL, as well as an environment variable MYENV that we manually set ourselves. Lastly, we were able to set the TEMP environment variable within the C++ program itself.

Closing Thoughts

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In this tutorial, we saw how to set and get Bash environment variables using a C++ program on a Linux system. As seen here, the getnenv() and putenv() functions allow us to get and set environment variables in Bash, respectively. Relying on environment variables is a viable way to make your C++ script cross compatible with many different systems which could have different configurations from one another.