pres.md

author:
- Aline Abler
title: Bash Workshop

Overview

Table of Contents

What will we do today?

• Theory
  • What is bash?
  • The basics
• Exercises

What is bash?

Shells

Bash is a not a programming language.

• It is a shell
• It is an interface between you and your computer
• It allows you to access the operating system’s services (i.e. run programs)
• It is not designed as a programming language, but can be used as such

Bash is not the only shell there is.

• sh, ksh, zsh, fish, dash...
• Most commands work in any shell, but some don’t
• Usually this doesn’t matter too much

Bash scripts are not programming

• Glue existing programs together to create new ones
• It’s possible to write entire programs, but please don’t

The basics

How to bash

Look, a bash script

# !/bin/bash

echo 'Hello World'

echo Bash is awesome

# Now some fun stuff:
sudo zypper update
notify-send 'Update complete'
feh --bg-fill 'pictures/fancy_wallpaper.jpg'
youtube-dl -o 'Video.%(ext)s' 'https://www.youtube.com/watch?v=lAIGb1lfpBw'

What bash can do

• Automate anything you can do from a console
• Let several separate programs work together

Strings

All about strings

\bigtext{Everything is a string}

Strings and word splitting

• A string is a sequence of characters that is treated as a unit
• Commands are strings, too
• Strings are split at every space, tab, or newline unless they're in quotes

Meaning of strings

echo Hello World

• echo, Hello and World are single strings
• The first string becomes the command, all following become arguments

echo 'Hello World'

• Here, Hello World is just one string

Repeat after me

\bigtext{Every word is a single argument unless you use quotes.}

Commands

All about commands

\bigtext{Everything that does something is a command}

Why is this so important?

• if and while are commands
• [[ is a command

Example

• wrong:

    [[1==3]]

• Bash's answer:

    bash: [[1==3]]: command not found

• correct:

    [[ 1 == 3 ]]

Repeat after me

\bigtext{If there's brackets, you probably need spaces.}

All about return values

All about return values

\bigtext{Every command returns a value}

Return values

• Every command returns a number, its return value
• 0 means success
  Everything else means there was an error
• Some commands also print to stdout - that's what you see.

Return values

• What you run

    echo 'Hello World'

• What you see

    Hello World

• What bash sees

    0

Why is that important?

• &&, ||, if and while all act based on the return value of something
• They disregard that command’s actual output

Example

if ls -l foo
then
    echo 'File foo exists'
else
    echo 'File foo does not exist'
fi

Expansion

Bash doesn’t only run commands

• Tilde expansion
  ~/files becomes /home/alinea/files
• Variable expansion
  $BROWSER becomes Firefox
• Arithmetic expansion
  $(( 1 + 4 )) becomes 5
• Command substitution
  $( pwd ) becomes /home/alinea/scripts
• Pathname expansion (or globbing)
  files/qui* becomes files/quicknotes files/quiz

Bash doesn’t only run commands

• Expansion happens before any command is run

• Double quotes (") don't prevent expansion, but single quotes (') do.

    $ echo "$HOME" '$HOME'
    /home/alinea $HOME

• Expansion happens before word splitting

The issue with word splitting

var='Unimportant File.odt'
rm $var  # Variable expansion

becomes

rm Unimportant File.odt