Master Linux Environment Variables: Types, Commands, and Configuration Files

What are environment variables?

Environment variables are key‑value pairs stored by the operating system that define the runtime environment for a user session. They are kept in a table in memory and are created for each user at login.

Classification of environment variables

By lifecycle

Permanent : Defined in startup scripts (e.g., /etc/profile, ~/.bash_profile) and persist across logins.

Temporary : Defined with export in the current shell; disappear when the shell exits.

By scope

System‑wide : Visible to all users (e.g., variables set in /etc/profile).

User‑specific : Visible only to the defining user (e.g., variables set in ~/.profile).

Common environment‑variable commands

echo

Print the value of a specific variable.

echo $HOME

env

List all environment variables for the current process.

env

export

Define a new environment variable or modify an existing one. The variable becomes part of the environment of subsequently launched processes.

export MY_VAR="value"

set

Display both shell variables and environment variables.

set

unset

Remove a variable from the environment.

unset MY_VAR

printenv

Show the value of a specific variable.

printenv PATH

Typical environment variables

HOME        # User's home directory
PWD         # Current working directory
SHELL       # Path to the current shell executable
HISTSIZE    # Number of commands stored in history
HOSTNAME    # System host name
LOGNAME     # Login name of the user
LANG / LANGUAGE # Locale settings

PATH

Colon‑separated list of directories searched for executable programs. A trailing dot ( .) represents the current directory.

# Default example
export PATH=/usr/local/bin:/usr/bin:/usr/local/sbin:/usr/sbin:$PATH

# Prepend a custom directory
export PATH=$HOME/bin:$PATH

LD_LIBRARY_PATH

Colon‑separated list of directories searched by the dynamic linker for shared libraries (C/C++).

export LD_LIBRARY_PATH=/opt/lib:/usr/local/lib:$LD_LIBRARY_PATH

C_INCLUDE_PATH & CPLUS_INCLUDE_PATH

Specify additional search paths for C and C++ header files.

export C_INCLUDE_PATH=/my/include
export CPLUS_INCLUDE_PATH=/my/include_cpp

CLASSPATH

Colon‑separated list of directories or JAR files that the Java Virtual Machine searches for class files.

export CLASSPATH=.:$HOME/lib/my.jar:$CLASSPATH

Configuration files for environment variables

/etc/profile

System‑wide file executed once for each user at the first login. It typically sources scripts in /etc/profile.d. After editing, apply changes with source /etc/profile.

/etc/profile.d

Directory containing scripts that /etc/profile runs at each startup. Placing a .sh file here is the recommended way to add system‑wide variables.

/etc/bashrc (or /etc/bash.bashrc on Ubuntu)

Executed for every interactive Bash session. Useful for system‑wide aliases and functions.

User login files (~/.bash_profile, ~/.bash_login, ~/.profile)

Only one of these files is read at login, in the order shown. They are the place to set user‑specific environment variables.

~/.bashrc

Read for each new interactive shell. Commonly used for per‑user aliases, functions, and variables that should be available in every terminal.

~/.bash_logout

Executed when a Bash login shell exits; useful for cleanup commands.

Execution order

1. /etc/profile
2. /etc/bashrc (or /etc/bash.bashrc)
3. /etc/profile.d/*
4. ~/.bash_profile → ~/.bash_login → ~/.profile (first existing file)
5. ~/.bashrc

Programmatic access to environment variables

Global variable environ

In C, extern char **environ; provides direct access to the environment array.

Command‑line arguments

Variables can be passed to a program via the command line, e.g., VAR=value ./myprog.

getenv()

The standard C library function getenv() retrieves the value of a named variable.

#include <stdio.h>
#include <stdlib.h>

int main(void) {
    char *pwd = getenv("PWD");
    if (pwd == NULL) {
        perror("getenv");
    } else {
        printf("Current directory: %s
", pwd);
    }
    return 0;
}