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Introduction to Shell Scripting

What is Shell Scripting?

Shell scripting is writing a series of commands in a text file to automate tasks in a Linux/Unix shell (like bash). Think of it like writing a recipe: Instead of typing commands one-by-one, you save them in a .sh file, and run them all together.

Example:

#!/bin/bash

echo "Hello, Connections!"
date

Run it:

  1. Save as script.sh
  2. Make it executable: chmod +x script.sh
  3. Run: ./script.sh

Who invented Shell Scripting?

Shell scripting wasn't "invented" by one person like a tool; it evolved with Unix.

Here's the key timeline:

  • Ken Thompson (1971): Created the first Unix shell called sh (Bourne shell).
  • Stephen Bourne (1979): Wrote the improved Bourne Shell (sh), which made scripting possible.

So, Stephen Bourne is mostly credited for popularizing shell scripting.

Later came other shells: bash (Bourne Again Shell), zsh, ksh, etc.

Why is Shell Scripting important for DevOps?

Shell scripting is super important in DevOps because it helps automate stuff - which is what DevOps is all about.

Here's why it matters:

  1. Automation: Run builds, deployments, tests, cleanups all with one script.
  2. CI/CD Pipelines: Shell scripts are often used in tools like Jenkins, GitHub Actions, GitLab CI to automate pipelines.
  3. Server Management: Start/stop services, backup logs, check disk space easily scripted.
  4. No Extra Tools Needed: Works out-of-the-box on any Linux system (which most servers run).
  5. Glue Language: Connects tools like Docker, Git, Kubernetes, etc.

In DevOps, if you can't automate, you can't scale. Shell scripts = your automation superpower.

How do I write and run my first shell script?

Step 1: Create the script file

Open terminal and type:

nano hello.sh

Step 2: Write this code

#!/bin/bash

echo "Hello, Connections!"
date

Save & Exit (in nano): Press CTRL+X, then Y, then Enter.

Step 3: Make it executable

chmod +x hello.sh

Step 4: Run the script

Run Output
./hello.sh Hello, Connections!
Sat Jun 14 16:05:00 IST 2025

What is #!/bin/bash and explain the syntax of the previous script?

Let's break down the script line-by-line:

#!/bin/bash

echo "Hello, Connections!"
date

Line 1: #!/bin/bash

  • Called shebang.
  • Tells the system: "Use the Bash shell to run this script."
  • If you skip it, the system might run your script with the wrong shell (like sh, dash, etc.).

Line 2: echo "Hello, Connections!"

  • echo = prints text on the screen.
  • "Hello, Connections!" = string being printed.
  • Output: Hello, Prashant! (assuming "Connections!" was a typo or example for a name)

Line 3: date

  • A built-in command that shows the current date and time.
  • Output: e.g., Sat Jun 14 16:10:45 IST 2025

What are variables in Shell Scripting and how to use them?

Variables store data like names, numbers, paths in the script to use later.

Example Script with Variables:

#!/bin/bash

name="Prashant"
age=21

echo "My name is $name"
echo "I am $age years old"

Syntax Rules:

  • No spaces around =.
    • ✅ name="Prashant Gohel"
    • ❌ name = "Prashant Gohel"
  • To use a variable: add $ before the name → $name.

Output:

My name is Prashant Gohel
I am 21 years old

You can even take input like this:

read username
echo "Welcome, $username!"

What are comments in Shell Scripting?

Comments are lines that explain the code - they're ignored by the shell.

Syntax:

# This is a comment

Where to use:

  • Above commands to explain.
  • In complex logic.
  • To disable code (temporarily).

Example:

#!/bin/bash

# This script greets the user

name="Prashant Gohel" # Storing name

echo "Hello, $name" # Printing greeting

Use comments wisely not for obvious stuff, but for clarity and logic.

What is if-else in Shell Scripting?

if-else is used to make decisions like "if this is true, do that, else do something else."

Basic Syntax:

if [ condition ]
then
    # commands if true
else
    # commands if false
fi
```fi` = "if" reversed → marks the end of `if` block.

**Example:**

```bash
#!/bin/bash

echo "Enter your age:"
read age

if [ "$age" -ge 18 ]
then
    echo "You are an adult."
else
    echo "You are a minor."
fi

Breakdown:

  • read age → takes user input.
  • [ "$age" -ge 18 ] → checks if age >= 18.
  • -ge = greater than or equal.

Common operators:

Operator Meaning
-eq equal to
-ne not equal to
-lt less than
-le less or equal
-gt greater than
-ge greater or equal

What is elif in Shell Scripting?

elif stands for "else if" and is used when you have multiple conditions to check, not just if and else.

Syntax:

if [ condition1 ]
then
    # do this
elif [ condition2 ]
then
    # do that
else
    # do something else
fi

Example:

#!/bin/bash

echo "Enter your marks:"
read marks

if [ "$marks" -ge 90 ]
then
    echo "Grade: A+"
elif [ "$marks" -ge 75 ]
then
    echo "Grade: A"
elif [ "$marks" -ge 60 ]
then
    echo "Grade: B"
else
    echo "Grade: C or below"
fi

Use elif to avoid writing too many nested ifs. Cleaner and readable.

What are loops in Shell Scripting?

Loops are used to repeat a block of code multiple times – very useful for automation and repetition.

Types of Loops in Shell Scripting

  1. for loop Runs for a fixed list or range of values.

    Input:

    #!/bin/bash

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

    Output:

    Number: 1
Number: 2
...

  2. while loop Runs while a condition is true.

    Input:

    #!/bin/bash

count=1

while [ "$count" -le 5 ]
do
    echo "Count is: $count"
    count=$((count + 1))
done

    Output:

    Count is: 1
Count is: 2
...

  3. until loop Runs until the condition becomes true (opposite of while).

    Input:

    #!/bin/bash

x=1
until [ "$x" -gt 5 ]
do
    echo "x is: $x"
    x=$((x + 1))
done

    Output:

    x is: 1
x is: 2
...

Summary Table:

Loop Runs When... Use Case
for For each value in list Known items/ranges
while Condition is true Repeat till false
until Condition is false Repeat till true

What are functions in Shell Scripting?

Functions are reusable blocks of code. You define them once and call them anytime.

Syntax:

function_name() {
    # code here
}
# or
function_name {
    # code here
}

Example:

Input:

#!/bin/bash

greet() {
    echo "Hello, Connections!"
}

greet # Calling the function

Output:

Hello, Connections!

Functions with arguments

Input:

say_hello() {
    echo "Hello, $1!"
}

say_hello "Prashant"

Output:

Hello, Prashant!

What are arrays in Shell Scripting?

Arrays let you store multiple values in a single variable — super handy when dealing with lists!

  1. Declare an Array

    fruits=("apple" "banana" "cherry")

  2. Access elements

    echo ${fruits[0]} # apple
echo ${fruits[1]} # banana

  3. Length of Array

    echo ${#fruits[@]} # 3

  4. Loop Through Array

    for fruit in "${fruits[@]}"
do
    echo "$fruit"
done

    Output:

    apple
banana
cherry

  5. Add elements

    fruits+=("orange")

  6. Remove elements: You can't remove directly, but you can unset:

    unset fruits[1] # removes banana

Use arrays when working with multiple items like file names, user inputs, package lists, etc.

What are Strings in Shell Scripting?

A string is just a sequence of characters — like text, names, paths, etc.

name="Prashant"
echo "Hello, $name"

Output:

Hello, Prashant

String with spaces:

Always use quotes if your string has spaces:

greet="Hello World"
echo "$greet"

String Operations

  • Length 
    echo ${#name} # 8
  • Concatenation 
    full="$name is learning Shell"
echo "$full"
  • Compare Strings 
    if [ "$name" == "Prashant" ]; then
    echo "Match!"
fi

What is Substring in Shell Scripting?

A substring is a part of a string. You can extract it using ${variable:position:length}.

Syntax Example: ${string:start:length}

str="DevOpsRocks"
echo ${str:0:6} # DevOps
echo ${str:6:5} # Rocks

Breakdown:

  • str: variable name
  • 0: starting index (0-based)
  • 6: number of characters to extract

Extract till end:

echo ${str:6} # From index 6 to end → Rocks

Negative indexing isn’t supported directly: Want last 2 chars? Use:

echo ${str: -2} # ks (space after `:` is required!)

What is file handling in Shell Scripting?

File handling lets you read, write, or modify files directly from a script — super useful in DevOps automation.

  1. Reading a File Line by Line

    #!/bin/bash
filename="file.txt"
while read line
do
    echo "Line: $line"
done < "$filename"

    Example output for file.txt containing "Hello", "World", "Tanvir":

    Line: Hello
Line: World
Line: Prashant

  2. Writing to a File

    echo "This is a new line" > output.txt
# Overwrites the file

  3. Appending to a File

    echo "Another line" >> output.txt
# Adds to the end without deleting existing content.

  4. Checking if File Exists

    if [ -f "file.txt" ]; then
    echo "File exists"
else
    echo "File not found"
fi

What are some useful keyboard shortcuts in Linux terminal?

Here are some powerful shortcuts to speed up terminal work:

Shortcut Action Description
Ctrl + K Delete from cursor to end of the line
Ctrl + U Delete from cursor to start of the line
Ctrl + W Delete the word before the cursor
Ctrl + R Search previous commands in history
Ctrl + L Clear the terminal screen (same as clear command)
Ctrl + S Pause output to the screen
Ctrl + Q Resume output (if paused with Ctrl + S)
Ctrl + C Terminate the running command
Ctrl + Z Suspend current command and send to background

These are very helpful when writing shell scripts or debugging — they save a lot of time!