Mastering If-Else Statements in Shell Scripting: A Comprehensive Guide

1. Introduction to Conditional Statements in Shell Scripting <a name=”introduction”></a>

Conditional statements are fundamental in programming, allowing scripts to make decisions based on certain conditions. In shell scripting, the if-else statement is one of the most commonly used control structures.

• Why Use If-Else?
  • Execute different code blocks based on conditions.
  • Validate user inputs.
  • Check file/directory existence.
  • Control script flow dynamically.

2. Basic Syntax of If-Else in Shell Script <a name=”basic-syntax”></a>

The general structure of an if-else statement in Bash is:

sh

if [ condition ]; then  

    # Code to execute if condition is true  

else  

    # Code to execute if condition is false  

fi 

Key Notes:

• [ ] or [[ ]] are used for conditions.
• then must be on a new line or separated by ;.
• fi closes the if block.

3. Types of If-Else Conditions <a name=”types-of-conditions”></a>

3.1 Simple If Statement <a name=”simple-if”></a>

Executes code only if the condition is true.

sh

if [ “$var” -eq 10 ]; then  

    echo “Variable is 10.”  

fi 

3.2 If-Else Statement <a name=”if-else”></a>

Executes one block if true, another if false.

sh

if [ -f “file.txt” ]; then  

    echo “File exists.”  

else  

    echo “File not found.”  

fi 

3.3 If-Elif-Else Statement <a name=”if-elif-else”></a>

Handles multiple conditions.

sh

if [ “$age” -lt 18 ]; then  

    echo “Child.”  

elif [ “$age” -lt 60 ]; then  

    echo “Adult.”  

else  

    echo “Senior.”  

fi 

3.4 Nested If-Else <a name=”nested-if-else”></a>

If-else inside another if-else.

sh

if [ -d “folder” ]; then  

    if [ -w “folder” ]; then  

        echo “Folder exists and is writable.”  

    fi  

fi 

4. Conditional Operators in Shell Script <a name=”conditional-operators”></a>

4.1 Numeric Comparisons

Operator	Description	Example
-eq	Equal to	[ “$a” -eq “$b” ]
-ne	Not equal	[ “$a” -ne “$b” ]
-gt	Greater than	[ “$a” -gt “$b” ]
-lt	Less than	[ “$a” -lt “$b” ]
-ge	Greater or equal	[ “$a” -ge “$b” ]
-le	Less or equal	[ “$a” -le “$b” ]

4.2 String Comparisons

Operator	Description	Example
=	Equal strings	[ “$str1” = “$str2” ]
!=	Not equal strings	[ “$str1” != “$str2” ]
-z	Empty string	[ -z “$str” ]
-n	Non-empty string	[ -n “$str” ]

4.3 File and Directory Checks

Operator	Description	Example
-e	File exists	[ -e “file.txt” ]
-f	Regular file (not dir)	[ -f “file.txt” ]
-d	Directory exists	[ -d “/path” ]
-r	Readable	[ -r “file.txt” ]
-w	Writable	[ -w “file.txt” ]
-x	Executable	[ -x “script.sh” ]

5. Advanced Usage of If-Else <a name=”advanced-usage”></a>

5.1 Using Logical Operators (AND, OR)

sh

# AND (&&)  

if [ “$age” -gt 18 ] && [ “$age” -lt 60 ]; then  

    echo “Valid working age.”  

fi  

# OR (||)  

if [ “$user” = “admin” ] || [ “$user” = “root” ]; then  

    echo “Access granted.”  

fi 

5.2 Ternary-Like Operations

sh

[ “$status” = “success” ] && echo “Done” || echo “Failed” 

5.3 Case Statements vs. If-Else

For multiple conditions, case is cleaner:

sh

case “$OS” in  

    “Linux”) echo “Linux OS” ;;  

    “Windows”) echo “Windows OS” ;;  

    *) echo “Unknown OS” ;;  

esac 

6. Common Mistakes and Debugging <a name=”common-mistakes”></a>

• Missing spaces inside [ ] → [ “$a”==”$b” ] ❌ (correct: [ “$a” = “$b” ])
• Unquoted variables → [ -z $var ] (fails if $var is empty)
• Using = instead of -eq for numbers → [ “$a” = “$b” ] (string comparison)

7. Best Practices for Writing If-Else Statements <a name=”best-practices”></a>

✔ Use [[ ]] for advanced conditions (supports &&, || without -a, -o).
✔ Quote variables to avoid word splitting errors.
✔ Use comments for complex conditions.
✔ Prefer case for multiple fixed patterns.

8. Practical Examples and Use Cases <a name=”practical-examples”></a>

Example 1: User Input Validation

sh

read -p “Enter age: ” age  

if [ “$age” -lt 0 ]; then  

    echo “Invalid age!”  

elif [ “$age” -lt 18 ]; then  

    echo “Minor.”  

else  

    echo “Adult.”  

fi 

Example 2: File Backup Check

sh

if [ -f “/backup/data.tar.gz” ]; then  

    echo “Backup exists.”  

else  

    echo “Backup missing. Running backup script…”  

    tar -czf /backup/data.tar.gz /data  

fi 

9. Conclusion <a name=”conclusion”></a>

Mastering if-else in shell scripting is crucial for writing dynamic and efficient scripts. By understanding different conditions, operators, and best practices, you can automate tasks effectively.