Shell Scripting

Much of the notes are taken from Classic Shell Scripting book.

Intro

What is Unix? Unix is a family of operating systems with a command line interface, initially developed at Bell Labs in 1970. It served as a backbone for many of the modern OSes like Linux or MacOS. Due to the diversity of Unix systems and their tools (grep might have different flags between systems), the POSIX standards were developed to ensure that software tools that conformed to POSIX could run on any of the POSIX compliant OSes - leading to standardized tools. Hence, shell scripts became portable (so you have bash which is a POSIX shell)!

What is the shell? As opposed to an OS, the shell is a user interface to the OS. Typically this is via a command line interface. Shells like sh, bash or zsh for example are interpreter programs that can intepret your commands and communicate with the OS. The typical interaction pattern is REPL or Read-Eval-Print-Loop.

A way to describe bash for example, is that it is a Unix shell.

Even more terms...

I actually get more confused when I start hearing about GNU/Linux, so here is a helpful analogy of the key differences:

• Unix: the original family of OSes
• POSIX: the standard that describes how Unix-like systems should behave so tools can become standardized for all of them
• Linux: an OS kernel that when combined with GNU tools makes a fully usable operating system
• GNU: stands for GNU is not Unix and is an OS developed and subsequently merged with Linux kernel for a full OS - this is why it is often grouped as GNU/Linux

Scripting

The shell scripting language is just like Python, Perl and Ruby - it's a high level language where you can express complex operations clearly and easily, and you can write a powerful useful script in short amount of time. They are not compiled but interpreted so a compiled program interpreter will read the script and translate it into an internal form before executing it.

Principles

• Lines of text are the universal format in Unix - not binary.
• Write programs to read from stdin and write to stdout, with error messages to stderr. This makes programs easy to use as data filters which act as components in larger pipelines or scripts.
• Avoid messages mixed into stdout of program (at least by default).

The #! Shebang

The special first lines in the script start with #! to specify to the kernel the full path to the interpreter to use to run the program, as well as a single option to pass to the interpreter.

Typically shell scripts start like this:

#! /bin/sh

Which will run bin/sh scriptname under the hood.

Detail

The sh references a command interpreter (shell) which is NOT an operating system.

#! /bin/csh -f

Or invoke a standalone awk program with,

#! /bin/awk - f

Commands and Args

With shell there are:

1. built-in commands - e.g. cd
2. shell functions - self contained chunks of code written in shell language that act like regular commands
3. external commands - commands the shell runs by creating a separate process

With commands, short options can start with a dash -c, while long options can start with one dash -p1 or two --backup. Semicolons ; separate multiple commands on the same line so the shell will execute them sequentially

Variables

Define variables with = and no intervening spaces.

myvar=this_is_a_long_string
echo $myvar

first=isaac  middle=bashevis last=singer # multiple assignments allowed per line
fullnam="$first $middle $last"

echo

Echo prints each of its arguments to stdout separated by a single space and terminated with newline.