Raju Ginni

wordpress tutorials seo hosting etc

You are here: Home / Linux sysadmin tutorials linux system administrator / If-Else Statements in Shell Scripting

If-Else Statements in Shell Scripting

The if statement allows the script to make decisions based on conditions. It can be used with elif (else-if) and else to handle multiple conditions.

🚀 Key Takeaways

✅ Use if [ condition ] for decision-making
✅ Use elif for multiple conditions
✅ Use else for a default action
✅ Test files, numbers, and strings with built-in operators
✅ $? helps check command success/failure

1️⃣ Basic If-Else Syntax

bash
if [ condition ]; then
# Code executes if condition is true
elif [ another_condition ]; then
# Code executes if elif condition is true
else
# Code executes if no condition is true
fi

2️⃣ Example: Checking a Number

bash
#!/bin/bash

echo -n "Enter a number: "
read num

if [ $num -gt 10 ]; then
echo "The number is greater than 10."
elif [ $num -eq 10 ]; then
echo "The number is exactly 10."
else
echo "The number is less than 10."
fi

📌 Operators Used:

  • -eq → Equal to

  • -ne → Not equal to

  • -gt → Greater than

  • -lt → Less than

  • -ge → Greater than or equal to

  • -le → Less than or equal to

3️⃣ Example: Checking if a File Exists

bash
#!/bin/bash

echo -n "Enter a filename: "
read file

if [ -f "$file" ]; then
echo "The file '$file' exists."
else
echo "The file '$file' does not exist."
fi

📌 File Test Operators:

  • -f filename → Checks if the file exists

  • -d directory → Checks if the directory exists

  • -r filename → Checks if the file is readable

  • -w filename → Checks if the file is writable

  • -x filename → Checks if the file is executable

4️⃣ Example: Checking User Permissions

bash
#!/bin/bash

echo -n "Enter filename: "
read file

if [ ! -e "$file" ]; then
echo "File does not exist."
elif [ -r "$file" ] && [ -w "$file" ]; then
echo "You have read and write permissions for $file."
elif [ -r "$file" ]; then
echo "You have only read permission for $file."
elif [ -w "$file" ]; then
echo "You have only write permission for $file."
else
echo "You do not have permission for $file."
fi

📌 ! -e checks if the file does not exist.

5️⃣ Example: Nested If Conditions

bash
#!/bin/bash

echo -n "Enter a number: "
read num

if [ $num -gt 0 ]; then
if [ $num -lt 10 ]; then
echo "The number is positive and less than 10."
else
echo "The number is positive and greater than or equal to 10."
fi
else
echo "The number is zero or negative."
fi

6️⃣ Example: Checking if a User is Root

bash
#!/bin/bash

if [ "$USER" == "root" ]; then
echo "You are the root user."
else
echo "You are not the root user."
fi

📌 String Comparison Operators:

  • = → Equal to

  • != → Not equal to

  • -z string → Checks if string is empty

  • -n string → Checks if string is not empty

7️⃣ Example: Checking Internet Connectivity

bash
#!/bin/bash

ping -c 1 google.com &> /dev/null

if [ $? -eq 0 ]; then
echo "Internet is working!"
else
echo "No internet connection."
fi

📌 $? stores the exit status of the last executed command (0 = success, non-zero = failure).