Shell Scripting Basics

Objective

Write your first shell scripts with variables, user input, and proper structure including shebang line, exit codes, and basic I/O.

Tools & Technologies

bash
variables
read
echo
exit codes
shebang

Key Commands

#!/bin/bash

chmod +x script.sh

./script.sh

$? for exit code

read -p 'Input: ' var

Architecture Overview

flowchart TD EDIT[Write script.sh] --> SHEBANG[Add shebang\n#!/bin/bash] SHEBANG --> PERM[chmod +x script.sh] PERM --> RUN[./script.sh] RUN --> EXEC[Shell executes\nline by line] EXEC --> EXIT[Exit code $?\n0=success 1+=error] style SHEBANG fill:#1a1a2e,stroke:#00d4ff,color:#e0e0e0 style RUN fill:#1a1a2e,stroke:#00ff88,color:#e0e0e0

Step-by-Step Process

Script Structure

Every script starts with a shebang line. Set executable permission before running.

#!/bin/bash
# This is a comment
# Script: hello.sh

echo 'Hello, World!'

NAME='Taki'
echo "Hello, $NAME"
echo 'No expansion: $NAME'  # single quotes = literal

Variables

Variables store values. No spaces around =. Use $VAR or ${VAR} to expand.

AGE=25
GREETING='Hello'
FILE=/etc/hosts

echo $AGE
echo ${GREETING}, user!
echo "File: $FILE"

# Command substitution
DATE=$(date +%Y-%m-%d)
FILES=$(ls | wc -l)
echo "Today: $DATE, Files: $FILES"

User Input

Use read to get input from the user during script execution.

#!/bin/bash
read -p 'Enter your name: ' NAME
read -sp 'Enter password: ' PASS  # silent
echo
echo "Welcome, $NAME!"

# Read with timeout
read -t 5 -p 'Continue? [y/n]: ' CHOICE

Exit Codes

Commands return exit codes: 0 = success, non-zero = failure. Check $? after each command.

#!/bin/bash
ping -c1 8.8.8.8 > /dev/null 2>&1
if [ $? -eq 0 ]; then
  echo 'Network OK'
else
  echo 'Network FAIL'
  exit 1
fi

# Better pattern:
if ping -c1 8.8.8.8 > /dev/null 2>&1; then
  echo 'OK'
fi

Challenges & Solutions

Forgetting chmod +x makes script 'Permission denied'
Variable assignment with spaces causes 'command not found' error

Key Takeaways

Use #!/usr/bin/env bash for portability across systems
Always quote variable expansions: "$VAR" prevents word splitting bugs