Shell Scripting

Connecting to a linux terminal

We'll be using Docker Compose to mount the repository into an Ubuntu Image. Create a docker-compose.yaml file in the root of the repository and copy the contents below.

docker-compose.yaml

version: '3.9'
services:
  devel:
    image: ubuntu:20.04
    working_dir: /repository
    volumes:
      - './:/repository'
    command: bash

You'll probably notice that this looks very similar to the docker-compose.yaml file you wrote in lab 1. The main differences are that we are using version 20.04 of the ubuntu image, and running bash at startup.

We also changed the name to devel to better represent how we will be using the container.

You can run the devel service.

docker-compose run devel
Creating <folder_name>_devel_run ... done

Now, as you are connected to the ubuntu terminal, you should see your working directory is /repository and your files match those on the host machine.

pwd
/repository

ls
README.md  course-catalog.txt docker-compose.yaml

Shell Scripting

Why use shell scripting?

1. Automation: Perform routine tasks, like backups, updates, and system monitoring.

2. Efficiency: Reduce the time spent manually typing commands.

3. Reproducibility: Ensure that tasks are performed the same way each time.

4. Integration: Combine tools and commands into cohesive workflows.

Getting started

Let's practice some linux commands that we can use to navigate the file system.

Earlier, we used the pwd command to show our location in the filesystem. We could also use echo to show the PWD environment variable.

echo $PWD
/repository

Create a file named tmp.sh.

touch tmp.sh

Install the vim editor. Enter "yes" on the prompt that appears.

apt-get update && apt-get install vim

Open the file with the vim editor.

vim tmp.sh

Type "i" to enter insert mode, then write the following line into the file.

echo $HOME

Save and close the file. In the editor press escape (esc) and type: :wq

List the contents of the current directory:

ls -l

Now let's execute the file using the following method:

./tmp.sh

Did it work? Why or why not?

How about we change the permissions of the file 'tmp.sh' to make it executable and check again?

chmod +x tmp.sh

Execute the file:

./tmp.sh

Check the contents of 'tmp.sh'.

cat tmp.sh

Delete the tmp.sh file.

rm tmp.sh

List the contents of the current directory. It shouldn't show the tmp.sh file anymore as we deleted it.

ls
README.md  course-catalog.txt docker-compose.yaml

Commonly used shell commands

• ls - lists contents of a directory

• cat - displays contents of a file. It can also be used to combine the contents of 2 files into 1.

• echo - used to print the string or the value of the variable that follows it

• cd - change directory to a different one

• mkdir - create a directory

• rmdir - delete a directory

• touch - create a file

• grep - search for a file or specific text within a file

• man - manual for the commands. You should use this to learn more about the different commands.

• tee - used to redirect the output of a command to a file along with the standard output on the terminal.

Constructs in Scripting

0. Shebang:

Shebang is the first line of the script that defines the interpreter for the script that follows. For a shell script, we include the type of shell we would like to use for the provided script. Even though it is optional, adding a shebang improves the readability of a script. For example -

#!/bin/bash

1. Variables:

Declaring and assigning values to variables -

name = "Jack"

Accessing the variable requires you to use a $ before the variable name -

echo $name

2. Comments

To write comments you need to use # -

# This is a comment

3. Programming Constructs -

   i. Conditionals -

   if [ "$name" = "Jack" ]; then
       echo "Hi, Jack!"
   else
       echo "Who are you?"
   fi

   ii. Loops -

   # for loop
   
   for i in 1 2 3 4 5; do
       echo $i
   done
   
   # while loop
   
   n=1
   while [ $n -le 5 ]; do
       echo $n
       n=$((n + 1))
   done

   iii. Reading input

   read -p "Enter your name: " user
   echo "Hello, $user!"

   iv. Redirecting output

   ls > output.txt

   iv. Functions

   # Functions without arguments
   
   print(){
       echo "Hello World"
   }
   
   print
   
   # Functions with arguments
   
   hello(){
       echo "Good Morning $1!"
   }
   
   input() {
       read -p "Enter your name: " name
       echo $name
   }
   
   hello $(input)

Activity

Try writing a basic shell script to create a "Guess the number" game.

1. Initally, you need the script to pick a random number for the user to guess.

2. Then, the script should allow the user to enter a number.

3. You can provide feedback about a "hit" or a "miss".

4. You could also let the user know if their guess is "hot" or "cold" depending on how closer or farther they are from the number.

5. Finally, you can limit the number of guesses a user has in each iteration of the game.

Here is a sample solution for you to look at once you are done writing your solution.