The source command (or .) executes a script in the current shell, making changes like variables or functions persist. 1. Use source to apply environment changes immediately without restarting the shell, such as after editing .bashrc. 2. It ensures variables, aliases, and functions defined in a script affect the current session, unlike normal script execution which runs in a subshell. 3. Common use cases include activating virtual environments or loading project-specific settings. 4. source and . are functionally identical, with source being Bash-specific and . being POSIX-standard. 5. Exercise caution when sourcing untrusted files as they can alter or break your current shell session.

The source command (or its shorthand equivalent ., a single dot) in Unix-like operating systems is used to read and execute commands from a file in the current shell environment. This means that instead of running a script in a new subshell, the commands inside the file are run directly in your current terminal session.

That’s important because any changes made by the sourced script—like setting environment variables, defining functions, or changing directories—are applied to your current shell. That wouldn’t happen if you just ran the script normally.

Why Use source or .?

You use it when you want to apply changes to your current shell session.

A common example is modifying environment variables like PATH, or setting up configuration options that only take effect in the current session. For instance, after editing your .bashrc or .zshrc, rather than restarting your terminal, you can:

source ~/.bashrc

Or equivalently:

. ~/.bashrc

This reloads the file and applies your new settings immediately.

When Should You Use It Instead of Running a Script Normally?

When you need variable or function definitions to persist in your current shell.

If you have a script that defines a function or exports a variable, running it normally won't affect your current shell session. Only the subshell where the script runs will see those changes.

For example, consider a file named setup.sh containing:

export NAME="Alice"

• If you run:

  ./setup.sh
  echo $NAME

  The output will be blank because the export happened in a subshell.

• But if you run:

  source setup.sh
  echo $NAME

  It will print Alice.

So using source makes exported variables, aliases, and functions available in your current shell.

Where Else Do You See source Used?

In configuration files and development workflows.

Many developers use source to manage virtual environments or load local settings. For example, Python's virtualenv uses:

source venv/bin/activate

This loads the virtual environment settings into your current shell, adjusting paths so that python and pip point to the right versions.

Another common case: storing sensitive credentials or project-specific settings in a file that shouldn’t be committed to version control. You might create a .env file and source it before running your app.

What’s the Difference Between source and .?

There isn’t one — they do exactly the same thing.

source is a Bash built-in command. The dot (.) is the POSIX-standard way of doing the same thing. So:

source config.sh

and

. config.sh

are completely interchangeable in most shells, including bash and zsh.

But note:

• On some older or minimal shells, source may not exist. In those cases, use ..
• In csh/tcsh, source is the standard command, and . works too.

A Few Things to Keep in Mind

• Sourcing a file gives it full access to your current shell — so be careful with what you source.
• If a script exits with an error when sourced, it affects your current shell session.
• Always double-check the file path when sourcing — especially if you're working in different directories.

Some habits that help:

• Use absolute paths or well-defined relative paths for consistency.
• Add a check at the top of your script to prevent unintended sourcing:

  [ "${BASH_SOURCE[0]}" != "${0}" ] || { echo "Don't source this script directly"; exit 1; }

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  So basically, use source or . when you want a script’s effects to stay in your current shell — like setting variables, functions, or environment changes. Otherwise, just run the script normally.

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