If, metaphorically speaking, a shell pipeline is kind of like an incantation or minor spell that you come up with on the spot to solve some problem, then scripts are a lot like the pages of a spellbook where you keep the incantations that you've found really useful.

If you're familiar with a general-purpose programming language like Python, C, Ruby, or JavaScript, then maybe you've noticed by now that Bash has some familar features. That's because the shell is a programming language.

Just about any sequence of shell commands you'd type at the prompt can be stashed in a simple text file and executed as a program.

What's more, Bash has many of the features you'd expect of a programming language:

Between these constructs and the features provided by the standard utilities, shell scripts can be used to solve all sorts of problems. For example:

• Here's a script adapted from Lady Ada's PiTFT installation guide which does the tedious work of configuring a touchscreen for the Pi.
• Here's one for cross-compiling kernels for the Raspberry Pi.
• raspi-config runs the first time a Pi boots and covers a wide range of configuration options.

A Sample Script

Let's assemble a very simple filter script that turns all the non-space characters in its input into little stars. First, open stars.sh in Nano:

Next, type or paste the following lines and hit Ctrl-x to save the file.

To break this down:

• #!/bin/bash is what's known as a shebang or a hashbang. #! are special characters that tell the kernel "this file should be run by feeding it to the following program". (You'll also commonly see this written as #!/usr/bin/env bash, which is sometimes considered a more portable way to invoke an interpreter.)
• sed is the stream editor. It acts as a filter, taking a stream of text in one side and transforming it by running one or more commands.
• s/\S/★/g is a command that says, more or less, "substitute strings matching this regular expression (\S) with this replacement string (★), globally". The \S will match everything that's not whitespace in its input.
• \s (lowercase) is the metacharacter for space characters; think of the uppercase version as inverting this.
• /dev/stdin is a special file that contains the standard input to our script.

Now make the file executable, and give it a try...

Hit Enter to submit text, and Ctrl-d on a line by itself to end input. You could also, just as easily, pipe the output of some other command to ./stars.sh.

Further Reading

• Advanced Bash-Scripting Guide, by Mendel Cooper.
• Bash Reference Manual