4.3 Shebang

A “shebang” is a special line at the top of a script that tells your shell which program to use to execute the file.

The format of a shebang is:

#! interpreter [optional-arg]

For example, if your script is a Python script and you want to use Python 3, your shebang might look like this:

#!/usr/bin/python3

This tells the system to use the Python 3 interpreter located at /usr/bin/python3 to run the script.

4.4 Bourne Shell

To get hand-wavy about it, I want to explain the difference between the 3 shells you’re likely to encounter:

  • sh: The Bourne shell. This is the original Unix shell and is POSIX-compliant. It’s very basic and doesn’t have many quality-of-life features.
  • bash: The Bourne Again shell. This is the most popular shell on Linux. It builds on sh, but also has a lot of extra features.
  • zsh: The Z shell. This is the most popular shell on macOS. Like bash, it does what sh can do, but also has a lot of extra features.

Both zsh and bash are “sh-compatible” shells, meaning they can run .sh scripts, but they also have extra features that generally make them more pleasant to use. For your purposes, the differences between zsh and bash are not super significant. Everything we do in this course will work in both shells.

4.6 Environment Variables

You can view all of the environment variables that are currently set in your shell with the env command.

To set a variable in your shell, use the export command:

export NAME="Lane"

You can then use the variable in your shell, just as before:

echo $NAME
# Lane

The interesting part is that programs and scripts you run in your shell can also use that variable:

4.7 PATH

IMPORTANT: There are environment variables that are sort of “built-in” to your shell. By “built-in” I just mean that different programs and parts of your system know about them and use them. The PATH variable is one of those.

PATH variables

Take a look at your current PATH variable:

echo $PATH

You should see a giant list of directories separated by colons (:). Each of those directories is a place where your shell will look for executables. For example, with a PATH like this:

/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin

Your shell will look for executables in the following directories:

  • /usr/local/bin
  • /usr/bin
  • /bin
  • /usr/sbin
  • /sbin

Change Your PATH

A common problem you’ll run into in the future is that you install a new program on your machine, but when you try to run it from your terminal, you get an error like:

$ my-new-program
-bash: my-new-program: command not found

Nine times out of ten, it’s because the program is installed in a directory that’s not in your PATH variable. Oftentimes when you install a program using the CLI, it will print a message during the installation process that tells you where the command was installed. Don’t let your eyes glaze over when your terminal prints important messages! Sometimes you just gotta rtfm.

To add a directory to your PATH without overwriting all of the existing directories, use the export command and reference the existing PATH variable:

export PATH="$PATH:/path/to/new"

The $PATH part is a reference to the existing PATH variable. The : separates the existing directories from the new directory (/path/to/new) that you’re adding.