Master Shell Scripting: From Basics to Real‑World Automation

Shell Scripting Introduction

The shell is a command interpreter that connects users with the operating system; a shell script is simply a sequence of Linux commands saved in a file to automate tasks and improve efficiency.

1.1 What Is Shell

Shell script overview

# Why introduce shell
# The previous article on Linux commands received many requests for a shell programming guide.
# After two weeks of preparation, this tutorial covers shell basics to practical use.

# What is shell
# If you are familiar with Linux commands, writing a shell script is just combining those commands.
# A shell script reduces manual work and boosts productivity.

# Official definition
# The shell prompts for input, passes it to the OS, and displays the result. It acts as a command interpreter.

# Common shells
# Bourne Shell (/usr/bin/sh), Bourne Again Shell (/bin/bash), C Shell (/usr/bin/csh), K Shell (/usr/bin/ksh), Root Shell (/sbin/sh)
# Bash is the most widely used due to its ease of use and free availability.

1.2 Shell Programming Guidelines

Shell script naming : Use English letters, case‑sensitive, end with .sh Avoid special symbols and spaces

The script should start with #!/bin/bash Variable names cannot start with digits or special symbols; underscores are allowed, hyphens are not.

1.3 First Shell Script – Hello World

Create a simple Hello World script

# Create HelloWorld.sh
# touch HelloWorld.sh
# vim HelloWorld.sh
# cat HelloWorld.sh
#!/bin/bash
# This is our first shell script
# by author rivers 2021.09
echo "hello world"
# Make it executable
# chmod o+x HelloWorld.sh
# Run the script
# ./HelloWorld.sh
# Output: hello world

2 Shell Environment Variables

2.1 Variable Basics

Environment variable introduction

# What is a variable
# A variable holds a value that can change, e.g., a=1, a=2, a=3.

# Three types of variables in shell
# System variables, environment variables, and user variables.
# Variable names must start with a letter, can contain underscores, but not digits at the start or hyphens.

# Simple variable example
# a=18
# echo $a   # prints 18

2.2 System Variables

# Common system variables
$0   # script name
$n   # nth argument
$*   # all arguments
$#   # number of arguments
$?   # exit status of last command
$$   # PID of the script

2.3 Environment Variables

Common environment variables

# PATH      – command search path
# HOME      – user home directory
# SHELL     – current shell type
# USER      – current username
# ID        – user ID
# PWD       – present working directory
# TERM      – terminal type
# HOSTNAME  – host name
# PS1       – command prompt definition
# HISTSIZE  – history size
# RANDOM    – random integer 0‑32767

2.4 User‑Defined Variables

Custom variables

# Example custom variables
a=rivers
Httpd_sort=httpd-2.4.6-97.tar
BACK_DIR=/data/backup/
IPaddress=10.0.0.1

3 Shell Control Flow Statements

3.1 if Conditional Statements

Basic if syntax

# Single‑branch if
if (condition); then
  command1
fi

# Double‑branch if‑else
if (condition); then
  command1
else
  command2
fi

# Multi‑branch if‑elif‑else
if (condition); then
  command1
elif (condition); then
  command2
else
  command3
fi

Common test operators

-f   # file exists
-d   # directory exists
-eq  # integer equal
-ne  # integer not equal
-lt  # less than
-gt  # greater than
-a   # logical AND
-o   # logical OR
-z   # empty string
-x   # executable permission
||   # OR (command level)
&&   # AND (command level)

3.2 for Loops

Loop over a list

# Syntax: for var in list; do ...; done
for i in $(seq 1 5); do
  echo $i
done

Check multiple hosts in a LAN

#!/bin/bash
Network=$1
for Host in $(seq 1 254); do
  ping -c 1 $Network.$Host > /dev/null && result=0 || result=1
  if [ "$result" == 0 ]; then
    echo -e "\033[32;1m$Network.$Host is up\033[0m"
    echo "$Network.$Host" >> /tmp/up.txt
  else
    echo -e "\033[31m$Network.$Host is down\033[0m"
    echo "$Network.$Host" >> /tmp/down.txt
  fi
done

3.3 while Loops

Basic while syntax

# while (condition); do ...; done
while true; do
  let N++
  if [ $N -eq 5 ]; then break; fi
  echo $N
done

Sum 1‑100 using while

#!/bin/bash
j=0
i=1
while ((i<=100)); do
  j=$((i + j))
  ((i++))
done
echo $j

3.4 case Statements

Multiple choice handling

# Syntax
case $var in
  pattern1) command;;
  pattern2) command;;
  *) default command;;
esac

Example: HTTP service control

#!/bin/bash
while true; do
  echo -e "
  start
  stop
  status
  quit"
  read -p "Enter your choice: " char
  case $char in
    start) systemctl start httpd && echo "httpd started" || echo "start failed";;
    stop)  systemctl stop httpd && echo "httpd stopped" || echo "stop failed";;
    status) systemctl status httpd;;
    quit) exit;;
    *) echo "Invalid option";;
  esac
done

3.5 select Menus

Menu‑driven selection

#!/bin/bash
PS3="Select a number: "
while true; do
  select option in http php mysql quit; do
    case $option in
      http) echo "Test Httpd"; break;;
      php)  echo "Test PHP"; break;;
      mysql) echo "Test MySQL"; break;;
      quit) exit;;
      *) echo "Input error, try again!"; break;;
    esac
done
done

4 Shell Functions and Arrays

4.1 Functions

Define reusable code blocks

# Function syntax
my_func() {
  local result=$((1+1))
  echo "This is a function."
  return $result
}
my_func
 echo $?

4.2 Arrays

Store ordered collections

# Define an array
IP=(10.0.0.1 10.0.0.2 10.0.0.3)
# Iterate using index
for ((i=0;i<${#IP[@]};i++)); do
  echo ${IP[$i]}
 done
# Iterate directly
for ip in "${IP[@]}"; do
  echo $ip
done

5 Practical Shell Scripts

5.1 System Backup Script

#!/bin/bash
# Auto backup Linux system files
SOURCE_DIR=($*)
TARGET_DIR=/data/backup
YEAR=$(date +%Y)
MONTH=$(date +%m)
DAY=$(date +%d)
WEEK=$(date +%u)
TIME=$(date +%H%M)
FILES=system_backup.tgz
if [ -z "$*" ]; then
  echo -e "Usage: $0 /boot /etc"
  exit 1
fi
if [ ! -d $TARGET_DIR/$YEAR/$MONTH/$DAY ]; then
  mkdir -p $TARGET_DIR/$YEAR/$MONTH/$DAY
fi
Full_Backup() {
  if [ "$WEEK" -eq 7 ]; then
    rm -rf $TARGET_DIR/snapshot
    cd $TARGET_DIR/$YEAR/$MONTH/$DAY && tar -g $TARGET_DIR/snapshot -czvf $FILES ${SOURCE_DIR[@]}
  fi
}
Add_Backup() {
  if [ "$WEEK" -ne 7 ]; then
    cd $TARGET_DIR/$YEAR/$MONTH/$DAY && tar -g $TARGET_DIR/snapshot -czvf ${TIME}_$FILES ${SOURCE_DIR[@]}
  fi
}
sleep 3
Full_Backup
Add_Backup

5.2 Server Information Collection

# Collect system info and optionally store in MySQL
ip_info=$(ifconfig | grep "Bcast" | tail -1 | awk '{print $2}' | cut -d: -f2)
cpu_model=$(awk -F: '/model name/ {print $2}' /proc/cpuinfo | tail -1 | sed 's/^ //')
cpu_cores=$(awk '/physical id/ {print $4}' /proc/cpuinfo | sort -u | wc -l)
hostname=$(hostname)
disk=$(fdisk -l | grep "Disk" | grep -v "identifier" | awk '{print $2,$3,$4}' | tr -d ',')
mem=$(free -m | awk '/Mem/ {print "Total",$2"M"}')
load=$(uptime | awk -F'load average:' '{print $2}' | cut -d, -f1)
read -p "Write data to database? (yes/no): " ans
if [[ $ans == yes || $ans == y || $ans == Y ]]; then
  mysql -uroot -p123456 -D audit -e "INSERT INTO audit_system VALUES('', '$ip_info', '$hostname', '$cpu_model X$cpu_cores', '$disk', '$mem', '$load', 'BeiJing_IDC');"
else
  echo "Exit without storing data."
  exit 0
fi

5.3 One‑Click LNMP Deployment (nginx, mysql, php)

#!/bin/bash
# Simplified LNMP installer (selection menu)
PS3="Select component to install: "
select comp in nginx mysql php quit; do
  case $comp in
    nginx) echo "Installing nginx..."; # download, configure, make, install steps omitted for brevity;;
    mysql) echo "Installing mysql..."; # download, configure, make, install steps omitted;;
    php)   echo "Installing php..."; # download, configure, make, install steps omitted;;
    quit) exit;;
    *) echo "Invalid choice";;
  esac
done