Linux Bash Scripting — From Beginner to Advanced

🐚 1. What is Bash?

Bash (Bourne Again Shell) is a Unix shell and command language used for:

Automating tasks
Server administration
DevOps pipelines
System monitoring / log processing

Check your shell:

echo $SHELL
bash --version

Run a script:

bash script.sh
# OR make executable
chmod +x script.sh
./script.sh

Shebang line (must be first line of every script):

#!/bin/bash

📘 2. Basic Script Structure

#!/bin/bash
# My first bash script

echo "Hello, $(whoami)! Today is $(date)."

File permissions

chmod +x script.sh
./script.sh

🔠 3. Variables

Declare & use variables

name="Alice"
echo "Hello $name"

Read input

read -p "Enter your name: " username
echo "Welcome, $username"

Command substitution

today=$(date +%A)
echo "Today is $today"

Environment variables

echo $HOME
echo $PATH
export API_KEY="12345"

🔄 4. Conditional Statements

If / Else

#!/bin/bash
num=10

if [ $num -gt 5 ]; then
  echo "Greater than 5"
elif [ $num -eq 5 ]; then
  echo "Equal to 5"
else
  echo "Less than 5"
fi

File tests

Test	Meaning
`-e file`	Exists
`-f file`	Regular file
`-d dir`	Directory
`-r file`	Readable
`-w file`	Writable
`-x file`	Executable
`-s file`	Not empty

if [ -f "/etc/passwd" ]; then
  echo "File exists"
fi

String tests

if [ "$a" = "$b" ]; then
  echo "Equal"
fi

if [ -z "$str" ]; then
  echo "Empty string"
fi

🔁 5. Loops

For loop

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

With range:

for i in {1..5}; do echo $i; done

With command output:

for user in $(cat /etc/passwd | cut -d: -f1); do
  echo "User: $user"
done

While loop

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

Until loop

x=1
until [ $x -gt 3 ]; do
  echo "x is $x"
  ((x++))
done

⚙️ 6. Case Statement

read -p "Enter choice (start|stop): " choice

case $choice in
  start)
    echo "Starting service..."
    ;;
  stop)
    echo "Stopping service..."
    ;;
  *)
    echo "Invalid choice"
    ;;
esac

🔢 7. Arithmetic Operations

Using `expr`

a=5; b=3
expr $a + $b

Using double parentheses

a=$((10 + 5))
b=$((a * 2))
echo $b

Increment / Decrement

((count++))
((count--))

📂 8. Arrays

fruits=("apple" "banana" "cherry")
echo ${fruits[1]}          # banana
echo ${fruits[@]}          # all elements
echo ${#fruits[@]}         # count

# Add element
fruits+=("mango")

Loop through array:

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

🧩 9. Functions

function greet() {
  echo "Hello, $1!"
}

greet "Alice"

Return value:

add() {
  return $(( $1 + $2 ))
}
add 5 7
echo $?   # shows 12

📜 10. Command-line Arguments

#!/bin/bash
echo "Script name: $0"
echo "First arg: $1"
echo "Second arg: $2"
echo "All args: $@"
echo "Count: $#"

Example:

./myscript.sh server1 server2

🧰 11. Input / Output Redirection

Symbol	Description
`>`	Redirect stdout (overwrite)
`>>`	Append
`<`	Input
`2>`	Redirect stderr
`&>`	Redirect both stdout + stderr

ls > files.txt
ls /notfound 2> errors.log

🧾 12. Piping and Grep

ps aux | grep nginx
cat /etc/passwd | grep root

🪶 13. String & Text Processing

Substring

str="HelloWorld"
echo ${str:0:5}   # Hello

Replace

echo ${str/World/Bash}  # HelloBash

Length

echo ${#str}

🕹️ 14. Script Exit & Status

echo "Running command..."
if command; then
  echo "Success"
else
  echo "Failed"
fi

echo "Exit code: $?"
exit 0

🧱 15. File Handling Example

#!/bin/bash
read -p "Enter filename: " file

if [ -f "$file" ]; then
  echo "File found!"
  cat "$file"
else
  echo "File not found!"
fi

⚠️ 16. Error Handling & Debugging

Debug mode

bash -x script.sh

Within script

set -x    # turn on debug
set +x    # turn off debug
set -e    # exit on error

Trap signals

trap "echo 'Script interrupted'; exit" SIGINT SIGTERM

🧮 17. Logging

LOGFILE="/var/log/myscript.log"
echo "$(date) - Script started" >> $LOGFILE

🧰 18. Cron Jobs (Automation)

Edit crontab:

crontab -e

Example entries:

0 * * * * /home/user/backup.sh
@daily /home/user/cleanup.sh

List:

crontab -l

🧬 19. Advanced Topics

Regex with `[[ ]]`

word="hello123"
if [[ $word =~ ^[a-z]+[0-9]+$ ]]; then
  echo "Matched pattern"
fi

Command substitution chain

files=$(ls | wc -l)
echo "Total files: $files"

Color output

RED='\033[0;31m'
NC='\033[0m'
echo -e "${RED}Error occurred${NC}"

Parallel execution

(command1 &) && (command2 &)
wait

🧠 20. Practical Mini Projects

✅ Check service status

#!/bin/bash
service="nginx"
if systemctl is-active --quiet $service; then
  echo "$service is running"
else
  echo "$service is stopped"
fi

✅ Ping Monitor

#!/bin/bash
for host in google.com 8.8.8.8; do
  ping -c1 $host &> /dev/null && echo "$host UP" || echo "$host DOWN"
done

✅ Log backup

#!/bin/bash
tar -czf /backup/logs_$(date +%F).tar.gz /var/log/*

🧩 21. Mermaid Diagram — Script Flow

flowchart TD
A[Start Script] --> B[Read Variables]
B --> C{Condition?}
C -->|True| D[Execute Commands]
C -->|False| E[Print Message]
D --> F[Loop / Function]
F --> G[Log / Output]
G --> H[End Script]

⚡ 22. Interview Quick Q&A

Q: How do you handle errors in Bash? A: Use set -e to stop on error or check $?. Combine with trap for clean exit.

Q: Difference between > and >>? A: > overwrites file; >> appends.

Q: How to pass arguments to a script? A: Via $1, $2, $@.

Q: How to check if a directory exists? A: [ -d "/path" ] && echo "exists"

Q: What does #!/bin/bash mean? A: It tells OS to use bash interpreter.

Q: How to debug bash scripts? A: bash -x script.sh or use set -x inside.

🧩 23. Cheatsheet Summary

Task	Command
Make script executable	`chmod +x script.sh`
Execute script	`./script.sh`
Print variable	`echo $var`
Arithmetic	`$((a+b))`
If condition	`if [ cond ]; then ... fi`
For loop	`for i in {1..5}; do ...; done`
Function	`func() { ... }`
Debug	`bash -x script.sh`
Redirect output	`cmd > file`
Append output	`cmd >> file`
Run in background	`cmd &`

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