Scripting¶

Why use scripts?¶

Scripts allow you to run you (bash) commands in an easy and reproducible manner.

Exercises¶

In these exercises, we’ll be using the book ‘Introduction to Bash scripting for developers’, as this book fits this course well, is free and open source and allows you to continue studying after this course

Exercise 1: Bash structure¶

• Read the text at chapter 2: ‘Bash structure’
• Create a file called greeter.sh with a shebang following that chapter

touch greeter.sh

#!/bin/bash

#!/usr/bin/env bash

Exercise 2: Hello world¶

• Read the text at chapter 3: ‘Hello world’
• Modify your Bash script called greeter.sh in such a way that you can run (‘execute’) it. When it runs, it should show ‘Hello World!’ on the screen. Run the script to verify

#!/bin/bash
echo "Hello World!"

chmod +x greeter.sh

./greeter.sh

Exercise 3: Bash variables¶

• Read the text at chapter 4: ‘Bash Variables’
• Modify your Bash script called greeter.sh in such a way that it uses two variables:
  • greeting, which should have value Hello
  • name, which should have value World When it runs, it should (still) show ‘Hello World!’ on the screen. Run the script to verify

#!/bin/bash
greeting="Hello"
name="World"

echo "$greeting $name!"

./greeter.sh

Exercise 4: counting files¶

Bash can store the output of other tools in variables.

Write a script that counts and shows the number of files in our current folder:

• Create an executable script called count_files.sh.
• In the script, create a variable called n_files and initialize it like this:

n_files=$(ls | wc --lines)

• Running the script should show: ‘You have [n_files] files’, where [n_files] is the number of files

• Run the script to verify

#!/bin/bash
n_files=$(ls | wc --lines)

echo "You have ${n_files} files"

./count_files.sh

Exercise 5: user input¶

• Read the text at chapter 5: ‘User input’
• Modify your Bash script called greeter.sh in such a way that the script asks for a name using the text ‘Who to greet?’. If the name World is typed it, it should (again) show ‘Hello World!’ on the screen. Run the script to verify

#!/bin/bash

echo "Who to greet?"
read name

greeting="Hello"

echo "$greeting $name!"

./greeter.sh

Exercise 6: conditional expressions¶

• Read the text at chapter 9: ‘Conditional expressions’
• We will modify your Bash script called greeter.sh in such a way that when the name ‘Bond’ it chosen, the program shows It is Bond. James Bond!, else it shows the regular greeting. Which conditional will you need?

[[ ${string1} == ${string2} ]]

Exercise 7: conditionals¶

• Read the chapters ‘If statement’ and ‘If Else statement’ of chapter 10: ‘Conditionals’, do not read ‘Switch case statements’
• Modify your Bash script called greeter.sh in such a way that when the name ‘Bond’ it chosen, the program shows It is Bond. James Bond!, else it shows the regular greeting. Run the script to verify

#!/bin/bash

echo "Who to greet?"
read name

greeting="Hello"

if [[ "${name}" == "Bond" ]] ; then
  echo "It is Bond. James Bond!"
else
  echo "$greeting $name!"
fi

./greeter.sh

Exercise 8: For loops¶

• Read only the ‘For loops’ section of chapter 11: ‘Bash loops’
• Create a new Bash script called greet_names.sh. The script should:
  • Ask who to greet, with the text Who shall I greet?. The expected input are names separated by spaces, e.g. Anna Berndt Cindy
  • Greet all of these people in the form Hello [name]!, using a for loop to go through the names
  • Run the script to verify

#!/bin/bash

echo "Who shall I greet?"
read names

for name in ${names}
do
    echo "Hello ${name}!"
done

./greet_names.sh

Exercise 9: For loops with Bash commands¶

Instead of iterating over names, we can iterate over something useful instead, such as the output of a Bash command.

• Create a new Bash script called show_files.sh. The script should:
  • Store the output of ls in a variable called filenames
  • Per filename, show I found a file called [filename]!, where [filename] is the name of the file
  • Run the script to verify

#!/bin/bash

filenames=$(ls)

for filename in ${filenames}
do
    echo "I found a file called {filename}!"
done

./show_files.sh

Conclusions¶

if [[ "${name}" == "Bond" ]] ; then
    echo "It is Bond. James Bond!"
else
    echo "$greeting $name!"
fi

for name in ${names}
do
    echo "Hello ${name}!"
done

Next session¶