# Understanding Environmental Variables in Linux

Environmental variables in Linux are key-value pairs stored in the system’s environment, used to pass information into processes launched from the shell. These dynamic values can significantly affect the behavior of running processes.

• Key-Value Structure: Variables are defined by a key (name) and value, like PATH=/usr/bin:/bin, where PATH is the key and /usr/bin:/bin is the value.
• Purpose and Usage: They store system settings and preferences such as executable file directories (PATH), user’s home directory (HOME), and default editor (EDITOR).
• Scope and Accessibility: Can be system-wide (in /etc/environment) or user-specific (in .bashrc or .bash_profile).
• Dynamic Configuration: Enables dynamic system configuration without changing executable binaries, useful in scripts and batch processes.
• Inheritance: Child processes inherit environmental variables from their parent, allowing settings to pass down.

# Viewing Environmental Variables

• printenv: Displays all environment variables. Example: printenv.
• For a specific variable: echo $VARIABLE_NAME. Example: echo $PATH.

# Setting Environmental Variables

• Temporary Assignment: For the current session. Example: export VARNAME=value.
• Permanent Assignment: Add to profile file and reload. Example: Add export VARNAME=value to .bashrc and run source .bashrc.

# Modifying Environmental Variables

• Append or modify existing variables. Example: export PATH=$PATH:/new/path.

# Unsetting Environmental Variables

• unset: Removes a variable from the current session. Example: unset VARNAME.

# Important Points

• Distinguish between environment and shell variables.
• Persistence requires writing to shell’s profile scripts like .bashrc.
• System-Wide vs User-Specific: Set in /etc/environment or .bashrc.
• Common Variables: PATH, HOME, USER, PWD, OLDPWD.

# Example: Adding a Directory to the PATH

1. View the current PATH: echo $PATH.
2. Append a new directory: export PATH=$PATH:/usr/local/newdir.
3. Verify the change: echo $PATH.
4. For permanence, add to .bashrc and reload with source .bashrc.

# Conclusion

Mastering environmental variables in Linux enhances system customization and control. They provide a flexible way to configure system settings and manage process communication, vital for both system administrators and users. Proper management of these variables is key to leveraging Linux’s full capabilities.