49.2 Environment variables

source bash-setup.sh

As defined by the POSIX (Portable Operating System Interface) standard, operating systems store a set of values called environment variables that processes can access for getting information about the system, or configuration values for instance. Each process can set new environment variables or modify current ones, and pass them to child processes.

Each environment variable has a name. They are thus stored as name/value pairs, and retrieve through their names.

Here is an example in Bash for setting an environment variable:

export MY_VAR=some_value

Still in Bash, we retrieve the value of the environment variable as any other Bash variable:

echo "$MY_VAR"

some_value

Each programming language proposes functions to retrieve and set environment variables.

The following Python program prints the same environment variable:

cat >print_env_var.py <<EOF
import os
print(os.environ['MY_VAR'])
EOF

bat print_env_var.py

import os

print(os.environ['MY_VAR'])

Here is its effect:

python print_env_var.py

some_value

We can do the same in Rust:

cat >print_env_var.rs <<EOF
fn main() {
  if let Ok(s) = std::env::var("MY_VAR") {
    println!("{s}");
  }
}
EOF
rustc -o print_env_var print_env_var.rs

bat print_env_var.rs

fn main() {

  if let Ok(s) = std::env::var("MY_VAR") {

    println!("{s}");

  }

}

The effect is the same:

./print_env_var

some_value