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Understand Shell Script

· 7 min read

Summary:​

Repository on GitHub

Goal:​

My intention is to give to you an idea of how you can create your own shell script commands to automate some tasks and show to you some references (https://www.gnu.org/savannah-checkouts/gnu/bash/manual/bash.html) (https://www.shellscript.sh).

Shell script has Variables, Repetition Structures, Conditional Statements and commands UNIX so you can create scripts to use as entrypoint.sh on docker container or just to automate one boring manual task in your S.O.

e.g: This blog is deployed with this Shell Script that generate a build, commit, push to gh-pages branch in my repo.

You can use CLI commands from others softwares installed in your machine, like GIT, Docker, node, npm...

If you understand the power of shell script your imagination will be the limit.

Without further ado...

1. Introduction​

What is Shell Script?

  • A shell script is a text file that contains a sequence of commands for a UNIX-based operating system. It is called a shell script because it combines a sequence of commands, that would otherwise have to be typed into the keyboard one at a time, into a single script.

A Unix shell is both a command interface and a programming language

In our case the computer program is example.sh file


1.1 Create a file called example.sh using terminal, follow:

touch example.sh
chmod +x example.sh
  • touch: will create the file
  • chmod +x: will give permission to execute file script

You can use touch and chmod commands inside your shell script file too


1.2 Open file example.sh in your preferred IDE

#!/bin/sh

echo "Start" # this command will output in terminal Start string
  • #!/bin/sh tells Unix that the file is to be executed by /bin/sh.

1.3 Running file example.sh in terminal

sh example.sh

1.4 Create a new file using shell script

#!/bin/sh
echo "Start"
touch new_file.txt # create new file called new_file.txt
echo "First Line">> new_file.txt # append in new_file.txt the text First Line

2. Variables​

A variable is a character string to which we assign a value. The value assigned could be a number, text, filename, device, or any other type of data.

2.1 Declare Variable​

#!/bin/sh

VARIABLE_MY_NAME="Vinicius"
VARIABLE_TEXT="Hi, my name is "
echo "$VARIABLE_TEXT $VARIABLE_MY_NAME"

Terminal Output:

Hi, my name is Vinicius

2.2 Read Input Variable​

#!/bin/sh

echo "What is your name?"
read VARIABLE_MY_NAME

VARIABLE_TEXT="Hi,"

echo "$VARIABLE_TEXT $VARIABLE_MY_NAME"

Terminal Output:

What is your name?
> Vinicius

Hi, Vinicius

3. Loops​

Loops will enable you to execute one or more commands repeatedly

3.1 For​

#!/bin/sh
for i in 1 2 3 4 5
do
  echo "Looping ... number $i"
done

3.2 While​

#!/bin/sh
while :
do
  echo "Please type something in (^C to quit)"
  read INPUT_STRING
  echo "You typed: $INPUT_STRING"
done

4. Comparison​

Sometimes you need to check if one variable is equal or not to another or greater/less...
In this section you can check ways to compare numbers and strings variables.

4.1 Numbers Comparison​

# is equal to:
if [ "$a" -eq "$b" ]

# is not equal to:
if [ "$a" -ne "$b" ]

# is greater than:
if [ "$a" -gt "$b" ]

# is greater than or equal to:
if [ "$a" -ge "$b" ]

# is less than: -lt
if [ "$a" -lt "$b" ]

# is less than or equal to:
if [ "$a" -le "$b" ]

# is less than (within double parentheses):
(("$a" < "$b"))

# is less than or equal to (within double parentheses):
(("$a" <= "$b"))

# is greater than (within double parentheses):
(("$a" > "$b"))

# is greater than or equal to (within double parentheses):
(("$a" >= "$b"))

4.2 String Comparison​

# is equal to: 
if [ "$a" = "$b" ]

# is equal to:
if [ "$a" == "$b" ]

# is not equal to:
if [ "$a" != "$b" ]

# is less than, in ASCII alphabetical order:
if [[ "$a" < "$b" ]]

# is greater than, in ASCII alphabetical order:
if [[ "$a" > "$b" ]]

# string is null, that is, has zero length:
if [ -z "$String" ]
then
  echo "\$String is null."
else
  echo "\$String is NOT null."
fi

# string is not null:
if [ -n "$String" ]
then
  echo "\$String is not null."
else
  echo "\$String is null."
fi

5. Final example.sh​

Copy the script below and run it on your machine and after use, edit to your needs

Repository on GitHub

#!bin/sh

script() {
    echo "Start" # Output Start in terminal

    # Receive variable by user input
    echo "Put your name"      # Output Put your name in terminal
    read name                 # Wait/Receive input variable name by user
    echo Your name is ${name} # Output Your name is variable name

    # Execute commands of another programs (like docker, git...)
    docker -v # docker version
    git --version # git version


    rm Dockerfile # Remove old Dockerfile
    touch Dockerfile # Create Dockerfile
    echo "FROM alpine:3.14" >>Dockerfile # Input echo string inside file
    echo 'ENTRYPOINT ["echo", "Container running 👍 ( ͡° ͜ʖ ͡°)"]' >>Dockerfile # Input echo string inside file
    echo 'CMD ["'${name}'"] && /dev/null' >>Dockerfile # Input echo string inside file

    # Build and Execute Docker Container
    imageName="${name}-docker-image" # docker image name variable
    docker build -t ${imageName} .   # build image
    docker images                    # list images

    # Generate utils variables
    timestamp=$(date +%s)                                 # create variable timestamp to attach in container name
    containerName="${name}-docker-container-${timestamp}" # variable container name to assign in run command

    # Running command and see logs
    echo "-------"
    echo "Start: " && docker run -d --name ${containerName} ${imageName} # running container
    echo "-------"
    echo "Logs: " && docker logs ${containerName} # output logs container
    echo "-------"
    # Remove all Containers
    containers=$(docker ps -a --format "{{.Names}}" | grep "${name}-docker-container") # list all containers created in this script
    docker rm -f ${containers}                                                         # remove all containers created in this script
    docker ps -a                                                                       # list all containers

    # Remove all Images
    imgs=$(docker images --format "{{.Repository}}" | grep "${imageName}") # imgs created repository
    docker rmi -f ${imgs} # Remove images in imgs variable
}

script | tee ex.log # fill log file called ex.log

Steps:

  1. output "start" in terminal
  2. output "Put your name" in terminal
  3. wait user input variable name
  4. output "Your name is ..." in terminal
  5. check docker version
  6. check git version
  7. remove old Dockerfile
  8. create new Dockerfile empty
  9. write in file Dockerfile echo string to create a valid docker image
  10. build image
  11. run container
  12. output logs container
  13. remove all containers created (normally only one)
  14. remove all images created (normally only one)

Use SOURCES to study and deepen your knowledge

Sources: