Conditional Statements | Shell Script

Conditional statements, or if-then blocks, are the most important part of any script. They are the "decision-makers" that allow your script to run, skip, or change code based on a specific condition.

You will use conditional logic for all sorts of common tasks:

• Checking if a file or directory exists before trying to read or write to it.
• Validating user input to see if it's a valid number or string.
• Running a command only if a previous command was successful.
• Checking if a user has "root" permissions.

A Simple 'if' Script

This script asks for a number and tells you if it's greater than 10.

Command:

#!/bin/bash

read -p "Enter a number: " NUMBER

# The [ ... ] is a test.
# -gt means "greater than".
if [ "$NUMBER" -gt 10 ]; then
    echo "Your number ($NUMBER) is greater than 10."
fi

echo "Script finished."

Output:

"if" Checks for Exit Status 0

This is the most important concept to understand: if does not test for "true" or "false".

• Instead, the if statement runs a command.
• If the command succeeds (returns an exit status of 0), the then block is executed.
• If the command fails (returns a non-zero exit status), the then block is skipped.

if grep -q "ERROR" /var/log/syslog; then    echo "An error was found in the log!"    # send an email, restart a service, etc.fi

Here, the if statement runs the grep -q "ERROR" ... command. If grep finds the string "ERROR" (success, exit code 0), the then block runs.

How to Build Logic

You can stack if statements to build complex logic.

1. The if

• What it does: Runs code only if the condition is true.

Syntax:

if [ condition ]; then    
  # code to run...
fi

2. The if...else...fi

• What it does: Provides an alternative block of code to run if the condition is false.

Syntax:

if [ condition ]; then
    # code to run if true...
else
    # code to run if false...
fi

Example (File check):

if [ -f "/etc/hosts" ]; then
    echo "The /etc/hosts file exists."
else
    echo "Error: /etc/hosts file not found."
fi

3. The if...elif...else...fi

• What it does: Lets you chain multiple "else if" conditions. This is the cleanest way to check for multiple, mutually exclusive options.

Syntax:

if [ condition1 ]; then
    # code for condition1...
elif [ condition2 ]; then
    # code for condition2...
elif [ condition3 ]; then
    # code for condition3...
else
    # code to run if nothing matches...
fi

Example :

#!/bin/bash

# Get the current hour (00-23)
HOUR=$(date +%H)

if [ "$HOUR" -lt 12 ]; then
    echo "Good morning!"
elif [ "$HOUR" -lt 18 ]; then
    echo "Good afternoon!"
else
    echo "Good evening!"
fi

The if Statement Sheet

You must use the correct operator for what you are testing (files, strings, or numbers).

1. File Test Operators (Use with [[ -f $file ]])

2. String Test Operators (Use with [[ "$str1" == "$str2" ]])

3. Integer Test Operators (Use with [ "$num" -eq 10 ])

Example:

if [[ "$USER" == "root" && "$num" -gt 10 ]]; then    echo "You are root and the number is greater than 10."fi