Environment Variables

Environment variables are global system variables accessible by all the processes/users running under the Operating System (OS), such as Windows, macOS and Linux.

Environment variables are useful to store system-wide values, for examples,

• PATH: the most frequently-used environment variable, which stores a list of directories to search for executable programs.
• OS: the operating system.
• COMPUTENAME, USERNAME: stores the computer and current user name.
• SystemRoot: the system root directory.
• (Windows) HOMEDRIVE, HOMEPATH: Current user's home directory.

When you launch an executable program (with file extension of ".exe", ".bat" or ".com") from the CMD shell, Windows searches for the executable program in the current working directory, followed by all the directories listed in the PATH environment variable. If the program cannot be found in these directories, you will get the an error.

Most of the Unixes and macOS use the so-called Bash Shell in the "Terminal". When you launch an executable program (with file permission of executable) in a Bash shell, the system searches the program in ALL the directories listed in the PATH. If the program cannot be found, you will get the an error.

There are some differences among other OS:

• Windows searches the current directory (.) before searching the PATH entries. (Unixes/macOS does not search the current directory, unless you include it in the PATH explicitly.)
• Windows uses semicolon (;) as the path separator; while Unixes/macOS uses colon (:).
• If your directory name contains special characters such as space (strongly not recommended), enclosed it with double quotes.
• Environment Variables in Windows are NOT case-sensitive (because the legacy DOS is NOT case-sensitive) while in macOS/Unixes they are case-sensitive.
  • Global environment variables (available to ALL processes) are named in uppercase, with words joined with underscore (_), e.g., JAVA_HOME.
  • Local variables (available to the current process only) are in lowercase.
  • In Windows they are typically named in uppercase, with words joined with underscore (_), e.g., JAVA_HOME.

Language Categorization

Programming languages are divided into groups depending on their characteristics.

• Low level Language VS High Level Language
• Interpreted-Compiled