Unix - Environment Variables vs Shell Variables in UNIX

Variables in UNIX are used to store data that can influence shell behavior and program execution. They are broadly classified into shell variables and environment variables, each serving a different purpose.

A shell variable is local to the current shell session. It is created using simple assignment and is not available to child processes. Shell variables are mainly used for temporary storage and internal shell operations.

An environment variable is a variable that is exported from the shell and becomes available to child processes. These variables define the execution environment for programs, such as search paths and user settings.

The key difference lies in scope. Shell variables are limited to the shell in which they are defined, whereas environment variables are inherited by all processes spawned from that shell. This makes environment variables essential for system-wide configurations.

Common environment variables include PATH, HOME, USER, and SHELL. For example, PATH tells the system where to search for executable files. Incorrect configuration of such variables can affect command execution.

Shell variables can be converted into environment variables using the export command. Without exporting, child processes cannot access the variable, even if it exists in the parent shell.

Understanding the difference between shell and environment variables is important for scripting, debugging, and system administration, as improper usage can lead to unexpected behavior.