In 2025–2026, mastering shell script if else remains essential for writing smart, efficient Bash scripts. Conditional logic lets your scripts make decisions, validate input, check files, compare values, and handle different scenarios — all without running every line unnecessarily.

This comprehensive shell script if else guide covers syntax, real-world examples, arithmetic evaluation, file/string tests, elif chains, nested conditions, user input validation, and best practices. All examples are Bash-specific (/bin/bash) — some features like [[ ]], (( )), and =~ regex won’t work in strict POSIX sh/dash.

Key Takeaways – Shell Script if else Essentials

  • it executes code blocks only when conditions are true (or false).
  • Always use spaces inside [ condition ] — [ “$var” = “value” ] (correct), [“$var”=”value”] (wrong).
  • Use (( )) for clean arithmetic: (( n % 2 == 0 )) — safer and more readable than [ ].
  • elif handles multiple conditions cleanly; avoid deep nesting when possible.
  • Common tests: -f (file exists), -r (readable), -z (empty string), -eq/-gt (numeric), =~ (regex in [[ ]]).
  • Nested shell script if else solves complex logic; combine with logical operators &&/||.
  • Validate user input with shell script if else + regex/pattern matching for robust scripts.
  • Run scripts with bash script.sh to use Bash-only features like [[ ]] and (( )).

How Does Shell Script if else Work?

The basic syntax is:

				
					if [ condition ]
then
    # commands if true
else
    # commands if false
fi
  • if starts the block.
  • [ condition ] is the test (must have spaces around brackets).
  • then runs if condition is true.
  • else runs if false (optional).
  • fi ends the block (if backwards).

Condition examples:

  • Numeric: [ “$a” -eq “$b” ]
  • String: [ “$str” = “hello” ]
  • File: [ -f “/path/file.txt” ]

How to Use Shell Script if else (Practical Examples)

1. Check if Two Numbers Are Equal

				
					#!/bin/bash
a=5
b=5

if [ "$a" -eq "$b" ]
then
    echo "Both numbers are equal"
else
    echo "Numbers are different"
fi

Output: Both numbers are equal

2. Find Which Number Is Greater

				
					#!/bin/bash
x=12
y=8

if [ "$x" -gt "$y" ]
then
    echo "$x is greater than $y"
else
    echo "$y is greater than $x"
fi

Output: 12 is greater than 8

3. Check if Number Is Even (Using Arithmetic (( )))

				
					#!/bin/bash
num=10

if (( num % 2 == 0 ))
then
    echo "$num is even"
else
    echo "$num is odd"
fi

Output: 10 is even

Why (( ))? Cleaner syntax, no quotes needed, C-style operators (==, <, >, %).

4. Simple Password Prompt

				
					#!/bin/bash
echo "Enter password:"
read -s pass  # -s hides input

if [ "$pass" = "secret123" ]
then
    echo "Access granted!"
else
    echo "Access denied. Try again."
fi

5. Shell Script if else with elif – Grade Calculator

				
					#!/bin/bash
score=87

if [ "$score" -ge 90 ]
then
    echo "Grade: A"
elif [ "$score" -ge 80 ]
then
    echo "Grade: B"
elif [ "$score" -ge 70 ]
then
    echo "Grade: C"
else
    echo "Grade: F"
fi

Output: Grade: B

6. Nested Shell Script if else – File Existence + Permissions

				
					#!/bin/bash
file="report.pdf"

if [ -f "$file" ]
then
    if [ -r "$file" ]
    then
        echo "File exists and is readable"
    else
        echo "File exists but is not readable"
    fi
else
    echo "File does not exist"
fi

7. Validate User Input (Numbers Only)

				
					#!/bin/bash
echo "Enter a number:"
read input

if [[ "$input" =~ ^[0-9]+$ ]]
then
    echo "$input is a valid number"
else
    echo "Error: Please enter only digits"
fi

Shell Script if else – Best Practices (2025–2026)

  • Always quote variables: [ “$var” = “value” ] prevents errors when empty.
  • Use [[ ]] instead of [ ] for safer string/pattern matching (Bash-only).
  • Prefer (( )) for math — no word-splitting issues.
  • Use elif chains instead of deep nesting for readability.
  • Test scripts with bash -n script.sh (syntax check) and shellcheck script.sh.
  • Run with #!/bin/bash shebang to enable Bash features.

Shell Script if else – FAQ (2025–2026)

  1. How to write basic syntax? 
    Use if [ condition ]; then … else … fi — core of shell script if else.
  2. How to use it for comparisons?
    -eq, -ne, -gt, -lt, -ge, -le for numbers; =, != for strings.
  3. What is elif ?
    elif [ condition ]; then … — handles multiple conditions cleanly in shell script if else.
  4. How to do arithmetic? 
    Use (( expression )) — example: (( num % 2 == 0 )) for even/odd checks.
  5. How to check file existence?
    [ -f “$file” ] — very common in this logic.
  6. What is the difference between [ ] and [[ ]]?
    [[ ]] is Bash-only, safer for strings/patterns, supports &&, ||, =~ regex.

Summary

You’ve mastered shell script if else: basic syntax, comparisons, arithmetic, file tests, elif, nesting, input validation, and best practices. Use shell script if else to make your Bash scripts intelligent and reliable.

Practice these examples — they’re the foundation of real-world automation, DevOps scripts, and system tools.

Recommended Resources

  • Bash Conditional Expressions (Official GNU Bash Manual)
  • ShellCheck – Lint your shell scripts online
  • Advanced Bash-Scripting Guide (tlpd.org)
  • Bash Pitfalls (wiki.bash-hackers.org)
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