3 - BASH Scripting

What are we learning?

By the end of this lesson you will have:

• Created and edited text files inside a Linux BASH shell

• Created and run basic BASH scripts

• Reviewed some basic programming constructs

The Lesson

Resources:    BASH Scripting for Beginners    |    BASH Syntax Cheat Sheet    |    Official BASH Reference

Aliases:

Since BASH is a shell to the kernel and a scripting language, it can be very powerful

• Let's create an alias to update our system so that we don't have to type sudo apt update and sudo apt upgrade every time we want to check for updates.

  • We're going to edit/create a file called .bash_aliases

    • You can use vim or nano

  • We create an alias with alias <name>='<command>'

    • Example:  alias update='sudo apt update'

  • We'll create one for upgrading as well:  alias upgrade='sudo apt -y upgrade'

• The alias will take place when we exit and restart the terminal or we can use source .bash_aliases

  • Now to upgrade the computer we only need to run update && upgrade

Scripts:

Let's make our own executable program using BASH

• Right now we can run the himom.sh script using bash himom.sh but wouldn't it be great if we could just run himom.sh?

  • Where does the bash executable live?  Run:  which bash

  • Edit your himom.sh script and add a "she-bang" at the top to express how to run it
    #!/usr/bin/bash
    echo Hi Mom!

  • Save and exit the editor

  • Try to run the script:  himom.sh   (it won't run)

  • The file is still just a text file and is not listed as "executable". What proof? Check with ls -l

  • To make the file executable, use the chmod (change mode) tool:  chmod +x himom.sh

  • Great! Now it'll run, right?  himom.sh   ...  nope. Command not found?

  • Binaries (programs) should be in /bin or /usr/bin

The ADD script:

• Create/edit the file add.sh and add the she-bang

  • At first, let's just make the program add 9 + 5:  echo 9 + 5   (save and exit, make the file executable and run it)

    • Why didn't it do the math?

  • Edit the file again - this time, we'll tell BASH that "9 + 5" is an expression:  echo $[9 + 5]  (save, exit, execute)

  • Great! But that is a boring program... Let's make it add any two numbers...

  • Edit the file and let's change the expression to this:   echo $[$1 + $2]  (save, exit)

    • Run the program like this:  ./add.sh 9 33

    • If everything goes well, it should output 42

  • BASH is a programming (scripting) language, so it has all the constructs you're used to, including if-statements!

    • Let's modify / rewrite the add.sh script so that it checks if we've provided enough arguments.

#!/usr/bin/bash

if [[ $# == 0 ]] then

echo "Null"

elif [[ $# == 1 ]] then

  echo $1

elif [[ $# == 2 ]] then

echo $[ $1 + $2 ]

else

  echo "Too many arguments"

fi

• Test your program!

Your Tasks

1. Dog Treats Create the script treats.sh such that it is executable and it solves the following problem:

Barley the dog loves treats. At the end of the day he is either happy or sad depending on the number and size of treats he receives throughout the day. The treats come in three sizes: small, medium, and large. His happiness score can be measured using the following formula: 1 x S + 2 x M + 3 x L

where S is the number of small treats, M is the number of medium treats and L is the number of large treats.

If Barley’s happiness score is 10 or greater, then he is happy. Otherwise, he is sad. Determine whether Barley is happy or sad at the end of the day.

Input Specification

Your program will take three command-line arguments (no more, no less). Assume correct input. The first argument contains the number of small treats, S, the second is the number of medium treats, M, and the third is the number of large treats, L, that Barley receives in a day.

Output Specification

If Barley’s happiness score is 10 or greater, output happy. Otherwise, output sad.

Samples:

~$ ./treats.sh 0 0 0

sad

~$ ./treats.sh 3 1 0

sad

~$ ./treats.sh 3 2 1

happy