execve(2) and friends allow you to specify a new processes's environment variables. How Java may choose to expose that I don't know. Assuming they do, that is a

execve(2) and friends allow you to specify a new processes's environment variables. How Java may choose to expose that I don't know. Assuming they do, that is all a shell needs. You do not strictly need to call setenv(3) to change a variable that gets evaluated with $, etc. You do need to pass the modified environment to a child process.