Variables

Introduction

In Linux, a variable is a named pointer to a chunk of data. There are two main types of variables in the shell:

1. Local Variables: Only available within the current shell session or script.
2. Environment Variables: Available system-wide and inherited by child processes (like Node.js or Python applications running on the server).

Defining Local Variables

To define a local variable, you simply assign a value to a name. Do not put spaces around the equals sign.

greeting="Hello World"
count=42

# To use a variable, prefix it with a dollar sign ($)
echo $greeting

Environment Variables

Environment variables are critical for configuring applications. For example, you might use an environment variable to pass a database password to a Node.js app without hardcoding it in the source code.

To create an environment variable, use the export command:

export DB_PASSWORD="supersecretpassword"

Now, any application started from this shell can access DB_PASSWORD.

Viewing Variables

To see all environment variables currently set on your system, use the env or printenv command.

Some common, highly important environment variables include:

• USER: The current logged-in user.
• HOME: The path to the current user's home directory.
• PATH: A colon-separated list of directories where the shell looks for executable programs. If you type node, the shell checks every directory in the $PATH to find the node binary.

echo $PATH
# Output: /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

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