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Bash Scripting

Lerning Resources

You need to learn Bash Scripting right now playlist - NetworkChuck (Video)

We start by creating a file with the .sh extension. For example script.sh. Then we write the script in it. For example:

Basic Script

#!/bin/bash

echo "Hello World"

We can run this by bash script.sh or ./script.sh. But the second will only work if the script is executable (permission to execute). We can make it executable by chmod +x script.sh. Now we can run it by ./script.sh.

Shebang

The first line of a bash script is called the shebang. It tells the system which interpreter to use to run the script. The shebang for bash is #!/bin/bash. The shebang for python is #!/usr/bin/env python. It vary from the langauge to language.

Variables

It is placeholder for a value. Just like any other programming language. We use $ to access the variable. Eg: $NAME. We can also use ${NAME}. The braces are used to make sure that the variable is not mistaken for a command.

#!/bin/bash

# Variable Declaration
NAME="John"

# Variable Usage

echo "My name is $NAME"
echo "My name is ${NAME}"

NOTE: We can create variables by NAME="John" through CLI, we can't use in the script beacuse it is not exported. We can export it by export NAME="John". Now we can use it in the script.

But here is one more catch. If we exit the terminal and open a new one, the variable will be gone. To make it permanent, we can add it to the .bashrc file. It is a hidden file in the home directory. We can open it by vi ~/.bashrc or any other editor. We can add the variable to the file. Eg: export NAME="John".

User Input

We can take input from the user using the read command.

#!/bin/bash

echo "Enter your name: "
read NAME
echo "Hello $NAME, nice to meet you!"

Arguments

We can pass arguments to the script. The arguments are stored in the $1, $2, $3 and so on. $0 is the name of the script.

Eg: bash script.sh arg1 arg2

#!/bin/bash

echo "First Argument: $1"

Arthemetic Operations

We can do arthemetic operations in bash. We use the (( )) to do arthemetic operations.

```bash
#!/bin/bash

echo $(( 5 + 5 ))

Arthemetic Operators

  • + - Addition
  • - - Subtraction
  • * - Multiplication
  • / - Division
  • % - Modulus

Conditional Statements

We can use the if statement to check for a condition. The syntax is:

#!/bin/bash

if [ "$1" == "John" ]
then
    echo "Hello John"
    exit 1
elif [ "$1" == "Doe" ]
then
    echo "Hello Doe"
else
    echo "I don't know you"
fi
if [$1 == "hello"],then echo "Hello World", fi

Comparison Operators

  • == - Equal to
  • > - Greater than
  • < - Less than
  • >= - Greater than or equal to
  • <= - Less than or equal to
  • != - Not equal to
  • -ge - Greater than or equal to
  • -le - Less than or equal to

Boolean Operators

  • -a - And
  • -o - Or
  • ! - Not

Loops

We can use loops to repeat a set of commands. There are two types of loops in bash. for and while. The the body is enclosed in do and done.

For Loop

#!/bin/bash

for i in 1 2 3 4 5
do
    echo $i
done

While Loop

#!/bin/bash

i=1
while [ $i -le 5 ]
do
    echo $i
    (( i++ ))
done

Break and Continue

We can use break and continue in loops. break will break the loop and continue will skip the current iteration.

#!/bin/bash

for i in 1 2 3 4 5
do
    if [ $i -eq 3 ]
    then
        continue
    fi
    echo $i
done

Functions

We can create functions in bash. The syntax is:

#!/bin/bash

function sayHello() {
    echo "Hello World"
}

sayHello
  • exit 1 - Exit the script with an error (non-zero exit code).
  • $RANDOM gives a random number between 0 and 32767.
  • $SHELL gives the path of the shell.
  • $USER gives the username of the user.
  • $HOSTNAME gives the hostname of the machine.

jq

jq is a command-line JSON processor. It is used to parse JSON. It is used to extract data from JSON. It is used to transform JSON. It is used to generate JSON.

Installation

sudo apt install jq

Usage

Format in JSON.

echo '{"name": "John", "age": 30}' | jq

Will print out the the specified key.

echo '{"name": "John", "age": 30}' | jq '.name'