Master Linux Environment Variables: View, Set, and Use Them in Bash

1. Overview

Linux is a multi‑user operating system. Each logged‑in user gets a dedicated environment defined by a set of variables called environment variables. Users can modify their own environment variables to suit their needs. Environment variables are closely tied to the shell and are set via shell commands; they are accessible to all programs run by the current user. In Bash, variables can be accessed by their names.

2. Display

You can view all environment variables with the env or printenv commands.

To display the value of a specific variable, use echo, e.g.:

echo $HOME
/home/admin

3. Set a New Environment Variable

export NAME=" Example "
echo $NAME
Example

4. Use unset to Remove an Environment Variable

unset NAME
echo $NAME

5. Use readonly to Set a Read‑Only Variable

export NAME=" Example "
readonly NAME

unset NAME
-bash: unset: NAME: cannot unset: readonly variable
NAME=" New " # This will also fail to modify the variable
-bash: TEST: readonly variable

6. Access and Set Environment Variables in C Programs

C programs can use three functions to work with environment variables:

getenv() – retrieves the value of a variable; returns NULL if the variable does not exist.

setenv() – sets a variable in the program.

unsetenv() – removes a specific variable.

There is also a global pointer environ that points to the list of all environment variables. The following program prints every environment variable:

#include <stdio.h>
extern char **environ;
int main() {
    char **var;
    for (var = environ; *var != NULL; ++var)
        printf("%s
", *var);
    return 0;
}

7. Modify System Configuration Files

To make environment variable changes persistent, edit the relevant configuration files such as /etc/profile for the root user or ~/.bash_profile for regular users (create the file if it does not exist). After editing, log out and back in or run source <file> to apply the changes.

8. Common Environment Variables

PATH – directories the shell searches for commands. HOME – current user's home directory. TERM – terminal type. UID – numeric user identifier. PWD – current working directory (updates with cd ). HISTSIZE – number of commands stored in history. LOGNAME – login name of the current user. HOSTNAME – name of the host machine. SHELL – type of the user's shell. LANGUAGE – language‑related settings for localization. MAIL – directory where the user's mail is stored. PS1 – primary command‑line prompt ("#" for root, "$" for normal users). PS2 – secondary prompt (default ">").