⚙ Automate Linux with Shell Scripting 🐧👨‍💻

👉 Quick Intro 🔥

Shell is the intermediate or medium of communication between applications/users and Hardware
To automate Linux commands, along with some condition-based or repetitive tasks, requires writing a few lines of code, which is termed as Shell Scripting

👉 Types of shell ⚙

Type	Year
Bourne shell (sh)	1977
C shell (csh)	1970
KornShell (ksh)	1980
Bourne Again Shell (bash)	1989
Z shell (zsh)	1990

Most preferred is bash in Linux and zsh in mac
You can see your shell type using $0

👉 How to create shell script ❓

Create a file with any name but the extension should be .sh
Open the file and write the below code in it
```
  #!/bin/bash

  echo "Hello World"
```
First line that you see is called the shebang line, It's like set of instructions that computer reads at beginning of script to know which interpreter should be used to execute script
To print anything on the terminal the echo command is used
After saving the file, you need to give execute permission to file to execute it
Now you can execute the script by ./filename.sh

👉 Variables

It is like an object that can store different types of data like, integer, float, string, and boolean
To declare the variable see the below code

var=value

OR

declare var=value

OR

declare -i var=value

-i for integer variable

To display variables there are two syntax

echo $variable

echo ${variable}

  #!/bin/bash

  # string
  name="John"
  # integer
  age=27
  # float
  height=5.3
  # boolean
  isEligible=true

  # you can display value of variables by two ways
  echo "$name"
  echo "$age"
  echo "${height}"
  echo "${isEligible}"

Few rules to declare a variable & their names 📍
- No space around =
- Variable name must start with a letter or underscore
- Number can be used anywhere else but not at the start
- DO NOT USE special characters like @, #, %

Case sensitive

Input 🧑‍💻
- To take input from the user we use read
```
  #!/bin/bash

  echo "Enter name"
  read name
  echo "Your name = ${name}"
```
- In the above code, first it will print "Enter name" on a terminal and then will wait for the user to enter a name, after entering the name press "Enter", On the next line "Your name = xyz" will be present
- You can take all types of data as an input
- You can combine display message for input and taking input into single line as below
```
  #!/bin/bash

  read -p "Enter name = " name
  echo "Your name = ${name}"
```

Output 🖥️
- To display anything on the terminal we have two command echo and printf
- echo we have already seen it
- Let's see printf
  
  printf <output-with-format> <variable>
```
  #!/bin/bash

  name="John"
  age=27
  height=5.321

  printf "Name = %s\n" "$name"
  printf "Age = %d\n" "$age"
  printf "Height = %f\n" "$height"
```
- \n is for a new line
- By default decimal number will print 6 digits after the dot, to limit it you can use the below types
```
  #!/bin/bash

  pi=3.14159
  printf "%10.2f\n" "$pi"
```

👉 Comments

Code comments are like little notes that programmers write in their code to explain what it does. It helps them and other people understand the code better.

It is ignored by the Interpreter.

👉 Constants

Constants are variables whose value can not be changed once declared

readonly var=value

declare -r var=value

  #!/bin/bash

  readonly name="John"
  echo "Your name = ${name}"
  # below line will give error as name is readonly
  name="Paul"
  echo "Your name = ${name}"

👉 Command Line Arguments

CLA are one type of variables which we pass to the script while executing it

./file.sh var1 var2 var3 ...
We can pass n number of CLA

To use these variables in script we can use the below notations

  #!/bin/bash

  echo "File name = $0"
  echo "First argument = $1"
  echo "Second argument = $2"
  echo "Third argument = ${3}"
  echo "All arguments = $*"
  echo "All arguments = $@"
  echo "Count of arguments = $#"

echo "Count of arguments = $#"

shift is used to remove the first few CLA

  #!/bin/bash

  # remove first CLA
  shift 1
  echo "$*"

  # remove starting 3 CLA
  shift 3
  echo "$*"

👉 Array

An array is a set of variables
Can contain different types of data

It has different syntax for different operations

  #!/bin/bash

  # array contains four types of data
  arr=(91 "john" true 3.14)

  # display all array items
  echo "Array = ${arr[*]}"
  echo "Array = ${arr[@]}"

Array Operations ⚙

Indexing starts from 0

Element at index, all elements, array length, slicing

  #!/bin/bash

  arr=(91 "john" true 3.14)

  echo "Element at index 2 = ${arr[2]}"
  echo "Array elements = ${arr[*]}"
  echo "Array elements = ${arr[@]}"
  echo "Length of array = ${#arr[*]}"
  echo "Length of array = ${#arr[@]}"
  echo "2 elements from 1 = ${arr[@]:1:2}"

Add elements to an array, Remove elements, or Delete array

  #!/bin/bash

  arr=(91 "john" true 3.14)
  echo "Array = ${arr[*]}"

  # add 4 and Hi at end of array
  arr+=(4 "Hi")
  echo "Array = ${arr[*]}"

  unset arr[2]
  echo "Removed element at 2 = ${arr[*]}"

  unset arr
  echo "Remove all elements = ${arr[*]}"

Copy one array to another, concatenate two arrays

  #!/bin/bash

  arr=(91 "john" true 3.14)

  # * or @ both will work
  new_arr=(${arr[*]})
  echo "New Array = ${new_arr[*]}"

  concatenated_arr=(${arr[@]} ${new_arr[@]})
  echo "Concatenated Array = ${concatenated_arr[*]}"

👉 Arithmetic Operators ±

To evaluate arithmetic operations there are three ways

$((...))

$[...]

let result=...

  #!/bin/bash

  a=2
  b=4

  echo "Addition = $(( $a + $b ))"
  echo "Division = $[ $a / $b ]"
  let c=$a**$b
  echo "Exponent = $c"

You can see for division only 0 is printed not the entire decimal number
To work with decimal numbers we need an external calculator i.e bc

bc can be used for all calculations not only decimals

  #!/bin/bash

  echo "3.8 + 4.2" | bc
  bc <<< "1.5 * 3.3"
  echo "scale=3; 2/5" | bc
  bc <<< "scale=3; 4/2"

To increment/decrement integer numbers you can use the below ways

let c+=1

let c++

let c-=1

let c--

👉 String Operations & Operators

Suppose you have a string variable named str

String has six main operations

  #!/bin/bash

  message="Hello John, Good Morning"

  echo "Length = ${#message}"
  echo "UPPERCASE = ${message^^}"
  echo "lowercase = ${message,,}"
  echo "Name = ${message:6:4}"
  echo "Replace John with Paul = ${message/John/Paul}"
  echo "Replace o with O = ${message//o/O}"

String has four main operators

These operators will be used with if to check the conditions

== => Check if two strings are equal

!= => Check if two strings are not equal

-z => Check if a string is zero length or null

-n => Check if a string is non-zero length

👉 Integer Operators 🔢

These operators will be used with if to check conditions

-eq => Equal

-ne => Not Equal

-gt => Greater than

-lt => Less than

-ge => Greater than or equal to

-le => Less than or equal to

👉 File Operators 🗃

These operators will be used with if to check the conditions

-e => Tests if the file exists.

-f => Tests if the file is a regular file (not a directory or a device file).

-d => Tests if the file is a directory.

-s => Tests if the file is not zero size (i.e., it has some content).

-r => Tests if the file has read permission.

-w => Tests if the file has write permission.

-x => Tests if the file has execute permission.

👉 If Else ⤳

There are three variants of if else

If

Code inside "if" is executed if the condition is true otherwise it is skipped

  #!/bin/bash
  name1="John"
  name2="John"

  if [[ $name1 == $name2 ]]
  then 
        echo "Both names are equal"
  fi

  # -z is used to check if string is empty or null
  # below code will be skipped because name is not empty
  if [[ -z $name1 ]]
  then
      echo "name is empty"
  fi

If else

code inside "if" is executed if the condition is true otherwise code inside "else" is executed

  #!/bin/bash

  file_path="/home/ubuntu/demo.txt"

  # check if file exists
  if [[ -e "$file_path" ]]
  then
      echo "File exists."
  else
      echo "File does not exist."
  fi

  # check if file is directory
  if [[ -d "$file_path" ]]
  then
      echo "Directory exists."
  else
      echo "Directory does not exist."
  fi

  # check if file has write permission
  if [[ -w "$file_path" ]]
  then
      echo "File has write permission."
  else
      echo "File does not have write permission."
  fi

If elif else

it has multiple conditions with code for each condition

  #!/bin/bash

  num1=10
  num2=20

  # print largest number
  if [[ $num1 -gt $num2 ]]
  then
      echo "$num1 is greater"
  elif [[ $num1 -lt $num2 ]]
  then
      echo "$num2 is greater"
  elif [[ $num1 -eq $num2 ]]
  then
      echo "Both are equal"
  fi

👉 Loops ↻

A loop is a block of code that keeps on executing until the provided condition becomes false.
There are main three types of loops, For, While and Until

For
- There are three parts in for loop
- Initialization - Initialize variable value with some data
- Condition - Run for loop until the condition is true
- Updation - Increase or decrease the value of an initialized variable

    for (( Initialization ; Condition ; Updation ))
    do
        ...
    done

    #!/bin/bash

    # print 1 to 10
    for(( i=1 ; i<=10 ; i++ ))
    do
        echo $i
    done

To run for loop infinite times don't give condition

  #!/bin/bash

  # print 1 to infinity till system breaks
  for(( i=1 ; ; i++ ))
  do
      echo $i
  done

For in

This type of for loop is used to iterate over lists or arrays

  #!/bin/bash

  # list
  for i in "John" "Paul" 123
  do
      echo $i
  done

  # array
  arr=(91 "john" true 3.14)

  for i in ${arr[*]}
  do
      echo $i
  done

While

while loop will keep on executing till the condition become false

  Initialization
  while [[ condition ]]
  do
      ...
      Updation
  done

  #!/bin/bash

  i=1
  while [[ $i -le 10 ]]
  do
      echo $i
      let i++
  done

To execute while loop infinite times instead of condition give true

  #!/bin/bash

  i=1
  while [[ true ]]
  do
      echo $i
      let i++
  done

Until
- Until loop works exact opposite of the while loop
- It will keep on executing till the condition becomes true
- Everything else is the same as while loop
- For infinite until loop we give false instead of condition

👉 Break and Continue 💔

If you want to stop the loop if some particular condition or state is reached then break is used

  #!/bin/bash

  arr=(91 "john" true 3.14)

  # break loop after printing john
  for i in ${arr[*]}
  do
      echo $i
      if [[ $i == "john" ]]
      then
          break
      fi
  done

If you want to skip a particular iteration of the loop continue is used

  #!/bin/bash

  arr=(91 "john" true 3.14)

  # break loop after printing john
  for i in ${arr[*]}
  do
      if [[ $i == "john" ]]
      then
          continue
      fi
      echo $i
  done

👉 Case ┋

If we have multiple elif conditions then case is most suitable to use

  case expression in
    pattern1)
      # commands to execute if expression matches pattern1
      ;;
    pattern2)
      # commands to execute if expression matches pattern2
      ;;
    pattern3)
      # commands to execute if expression matches pattern3
      ;;
    *)
      # default case if nothing matches
      ;;
  esac

expression is the variable you want to test against multiple patterns, expression variable can be of any type
pattern1), pattern2), etc.: These are the patterns you want to match against the expression. Each pattern should end with a closing parenthesis.
;; => This is used to terminate each pattern block. If a pattern matches, the commands for that pattern are executed, and then the case statement exits. The ;; serves as a break.

*) => This is a default case if no pattern matches.

  #!/bin/bash

  read -p "Enter extension: " extension

  case "$extension" in
    "txt")
      echo "Text file."
      ;;
    "jpg" | "jpeg" | "png")
      echo "Image file."
      ;;
    "sh")
      echo "Shell script."
      ;;
    *)
      echo "Unknown file type."
      ;;
  esac

👉 Function ⚙

functions are small codes, which can be called with a single function name n numbers of times as per requirement

  # preferred way
  function functionName() {
      ...
  }

  OR

  function functionName {
      ...
  }

  OR

  functionName() {
      ...
  }

To pass arguments to function see below code

  #!/bin/bash

  # pass arguments to function
  function functionName() {
      echo "Name = $1"
      echo "Age = $2"
  }

  functionName "John" 22

All Command Line Arguments can't be used in functions unless we pass them as parameter
To use functions from different scripts, source can be used
At the top of a file do source ~/anotherFile.sh

To declare local variable in function see below code

#!/bin/bash

function functionName() {
    # this variable is only accessible in function
    local name="Paul"
    echo $name
    echo "Name = $1"
    echo "Age = $2"
}

functionName "John" 22

👉 Logger 🚩

To log anything from bash you can use logger
```
  #!/bin/bash

  logger "This is log"
```
You can find this log at the bottom of /var/logs/syslog

👉 Conclusion ✅

This much is more than enough for DevOps

Happy Learning !!! 👋

⚙ Automate Linux with Shell Scripting 🐧👨‍💻

👉 Quick Intro 🔥

👉 Types of shell ⚙

👉 How to create shell script ❓

👉 Variables

Few rules to declare a variable & their names 📍

Input 🧑‍💻

Output 🖥️

👉 Comments

👉 Constants

👉 Command Line Arguments

👉 Array

Array Operations ⚙

👉 Arithmetic Operators ±

👉 String Operations & Operators

👉 Integer Operators 🔢

👉 File Operators 🗃

👉 If Else ⤳

If

If else

If elif else

👉 Loops ↻

For

For in

While

Until

👉 Break and Continue 💔

👉 Case ┋

👉 Function ⚙

👉 Logger 🚩

👉 Conclusion ✅

Comments

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Command Palette

👉 Quick Intro 🔥

👉 Types of shell ⚙

👉 How to create shell script ❓

👉 Variables

Few rules to declare a variable & their names 📍

Input 🧑‍💻

Output 🖥️

👉 Comments

👉 Constants

👉 Command Line Arguments

👉 Array

Array Operations ⚙

👉 Arithmetic Operators ±

👉 String Operations & Operators

👉 Integer Operators 🔢

👉 File Operators 🗃

👉 If Else ⤳

If

If else

If elif else

👉 Loops ↻

For

For in

While

Until

👉 Break and Continue 💔

👉 Case ┋

👉 Function ⚙

👉 Logger 🚩

👉 Conclusion ✅

Comments

More from this blog