Master Shell Scripting: Core Commands, Variables, Loops, and Functions Explained

Shell Script Introduction

In the digital era, Linux is the preferred OS for developers and system administrators due to its efficiency, stability, and open‑source nature. Shell scripting bridges users and the system kernel, enabling flexible command execution and automation.

Why Learn Shell Programming

Shell scripts are essential for Linux/UNIX system management and automation. Every competent Linux sysadmin or DevOps engineer should master writing and reading shell scripts to improve efficiency, handle complex tasks, and advance their career.

Prerequisites for Learning Shell

Proficiency with the vim editor and SSH terminal.

Familiarity with at least 80 common Linux commands.

Understanding of Linux regular expressions and the three "grep, sed, awk" utilities.

How to Study Shell Effectively

Practice repeatedly: write, test, and refine scripts.

Reflect on each exercise: think, then code.

Iterate and repeat to build deep understanding.

Shell Basics

What Is a Shell Variable?

A shell variable stores a string; the shell does not interpret its type. For arithmetic, you must use commands like let, expr, or double parentheses.

Local variables are visible only in the current shell.

Environment variables are inherited by child processes.

Variable names must start with a letter or underscore, followed by letters, digits, or underscores. They are case‑sensitive; environment variables are conventionally uppercase. No spaces around the equals sign when assigning.

Variable Operations

Delete a variable: unset var Make a variable read‑only:

readonly var

Variable Expansion

You can reference variables in many ways to obtain their value, length, substrings, or perform replacements.

Substring Extraction

name="1234567"
# Extract two characters starting at position 2
echo ${name:2:2}   # 34

Length of a Variable

name="1234567"
# Four ways to get length
echo $name | wc -L
expr length "$name"
awk '{print length}' <<< "$name"
echo ${#name}

Pattern Deletion

url="www.baidu.com"
# Remove prefix greedily
echo ${url#www.}   # baidu.com
# Remove suffix
echo ${url%.*}    # www.baidu

Pattern Replacement

url="www.baidu.com"
# Replace first occurrence
echo ${url/w/W}   # Www.baidu.com
# Replace all occurrences
echo ${url//w/W}  # WWW.baidu.com

Arithmetic in Shell

Using expr

expr 1 + 1   # 2
expr 10 - 5  # 5
expr 10 \* 2 # 20

Using Double Parentheses

echo $((10 + 5))   # 15

Using $[]

echo $[10 * 5]   # 50

Using let

let a=5+3
echo $a   # 8

Using bc for Decimal Arithmetic

echo "10.5 + 2.3" | bc   # 12.8

Using awk for Arithmetic

awk 'BEGIN{print 10.5+2.3}'   # 12.8

Control Structures

If Statements

# Single branch
if [ -f /etc/hosts ]; then
  echo "File exists"
fi

# Two branches
if [ -f /etc/hosts ]; then
  echo "Exists"
else
  echo "Missing"
fi

# Multiple branches
if [ $num -gt $max ]; then
  echo "Greater"
elif [ $num -lt $max ]; then
  echo "Less"
else
  echo "Equal"
fi

Case Statement

case $1 in
  Shell) echo "Shell..." ;;
  MySQL) echo "MySQL..." ;;
  Docker) echo "Docker..." ;;
  *) echo "Usage: $0 [Shell|MySQL|Docker]" ;;
esac

For Loop

# Iterate over a list
for i in 1 2 3; do
  echo $i
done

# Iterate over a range
for i in {1..5}; do
  echo $i
done

While Loop

# Infinite loop
while true; do
  echo "Running"
  sleep 1
done

# Read file line by line
while read line; do
  echo $line
done < /etc/hosts

Functions

# Define a function
my_func() {
  echo "Function body"
}

# Call the function
my_func

Functions can accept positional parameters ( $1, $2, …) and return a status with return. Use local to create variables scoped to the function.

Arrays

Indexed (Numeric) Arrays

# Declare and assign
array[0]=shell
array[1]=mysql
array[2]=docker

# Access elements
echo ${array[1]}   # mysql

# All values
echo ${array[@]}   # shell mysql docker

# All indices
echo ${!array[@]}  # 0 1 2

Associative Arrays

# Declare associative array
declare -A colors
colors[red]="#FF0000"
colors[green]="#00FF00"

# Access
echo ${colors[red]}   # #FF0000

# Iterate
for key in "${!colors[@]}"; do
  echo "$key => ${colors[$key]}"
done

Practical Examples

YUM Repository Switch Based on OS Version

#!/bin/sh
backup="mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup"
os_version=$(cat /etc/redhat-release | awk '{print $(NF-1)}')
if [ ${os_version%%.*} -eq 7 ]; then
  $backup
  yum -y install wget
  wget -O /etc/yum.repos.d/CentOS-Base.repo https://mirrors.aliyun.com/repo/Centos-7.repo
elif [ ${os_version%%.*} -eq 6 ]; then
  $backup
  wget -O /etc/yum.repos.d/CentOS-Base.repo https://mirrors.aliyun.com/repo/Centos-6.repo
fi

Interactive Menu for System Info

#!/bin/sh
menu(){
cat <<EOF
1) Show memory (f)
2) Show load (w)
3) Show disk (d)
4) Show login info (l)
5) Show menu (m)
6) Exit (e)
EOF
}
menu
while true; do
  read -p "Choose option: " opt
  case $opt in
    1|f) free -h ;;
    2|w) uptime ;;
    3|d) df -h ;;
    4|l) last ;;
    5|m) menu ;;
    6|e) exit ;;
    *) echo "Invalid option" ;;
  esac
done

Jump Server Menu

#!/bin/sh
web01=10.0.0.7
web02=10.0.0.8
menu(){
cat <<EOF
1) web01  $web01
2) web02  $web02
3) Exit
EOF
}
menu
while true; do
  read -p "Select server: " sel
  case $sel in
    1|web01) ssh root@$web01 ;;
    2|web02) ssh root@$web02 ;;
    3|exit) exit ;;
    *) echo "Invalid" ;;
  esac
done