Using Loops and Conditions in Bash Scripts: Structure Your Scripts for Complex Tasks

Bash scripting is a powerful tool for automating tasks, managing systems, and making your life easier. One of the key features of Bash scripting is its ability to use loops and conditions. These features allow you to write scripts that can repeat tasks, make decisions, and adapt to different situations. In this article, we'll explore how to use loops and conditions in Bash scripts, and you'll learn how to structure your scripts for complex tasks.

Understanding Loops and Conditions

Let's imagine you're cleaning your room. You might need to repeat the same action, like picking up toys, several times. That's what a loop does in a script. It helps you repeat commands without writing them again and again.

Now, imagine you need to decide whether to wear a jacket or not based on the weather. That's what a condition does. It allows your script to make choices based on certain rules.

Loops in Bash: Repeating Actions

1. for Loops: Repeating for a Fixed Number of Times

Imagine you want to send a message to each of your five friends. You could use a for loop to send the message five times.

Basic Syntax:

for VAR in LIST; do
    COMMANDS
done

Example: Printing Numbers 1 to 5

#!/bin/bash
for i in {1..5}; do
    echo "Number: $i"
done

Explanation:

• for i in {1..5}: This loop will run five times, with the variable i taking on the values 1, 2, 3, 4, and 5.
• echo "Number: $i": Inside the loop, the echo command prints the current value of i.

This code will output:

Number: 1
Number: 2
Number: 3
Number: 4
Number: 5

Example: Iterating Over Files in a Directory

#!/bin/bash
for file in /path/to/directory/*; do
    echo "Processing file: $file"
done

Explanation:

• for file in /path/to/directory/*: This loop goes through each file in the specified directory.
• echo "Processing file: $file": For each file, it prints a message indicating the file being processed.

2. while Loops: Repeating Until a Condition is Met

Imagine you want to keep asking for a number until the user enters a number greater than 10. You could use a while loop for this.

Basic Syntax:

while CONDITION; do
    COMMANDS
done

Example: Counting Down from 5 to 1

#!/bin/bash
i=5
while [ $i -gt 0 ]; do
    echo "Countdown: $i"
    i=$((i - 1))
done

Explanation:

• i=5: This sets the initial value of the counter i to 5.
• while [ $i -gt 0 ]; do: This loop will continue as long as the value of i is greater than 0.
• echo "Countdown: $i": Inside the loop, it prints the current value of i.
• i=$((i - 1)): This line subtracts 1 from the value of i in each iteration.

This code will output:

Countdown: 5
Countdown: 4
Countdown: 3
Countdown: 2
Countdown: 1

Example: Reading a File Line by Line

#!/bin/bash
file="example.txt"
while read -r line; do
    echo "Line: $line"
done < "$file"

Explanation:

• file="example.txt": This sets the filename to example.txt.
• while read -r line: This loop reads each line from the file.
• echo "Line: $line": It prints each line read from the file.

Conditions in Bash: Making Decisions

Imagine you're playing a game where you need to check if you've won or lost. You could use an if-then-else statement to decide what to do based on the outcome of the game.

Basic Syntax:

if CONDITION; then
    COMMANDS
elif ANOTHER_CONDITION; then
    COMMANDS
else
    COMMANDS
fi

Example: Checking if a Number is Positive, Negative, or Zero

#!/bin/bash
num=-3
if [ $num -gt 0 ]; then
    echo "$num is positive"
elif [ $num -lt 0 ]; then
    echo "$num is negative"
else
    echo "$num is zero"
fi

Explanation:

• if [ $num -gt 0 ]; then: This checks if the value of num is greater than 0.
• elif [ $num -lt 0 ]; then: If the previous condition is false, this checks if num is less than 0.
• else: If both previous conditions are false, this part is executed.

This code will output:

-3 is negative

Example: Checking for File Existence

#!/bin/bash
file="example.txt"
if [ -e "$file" ]; then
    echo "$file exists"
else
    echo "$file does not exist"
fi

Explanation:

• file="example.txt": This sets the filename to example.txt.
• if [ -e "$file" ]; then: This checks if the file exists.

Combining Loops and Conditions: Powerful Combinations

You can combine loops and conditions to create complex and useful scripts.

Example: Checking File Sizes in a Directory

#!/bin/bash
for file in /path/to/directory/*; do
    if [ -f "$file" ]; then
        size=$(stat -c%s "$file")
        if [ $size -gt 1000000 ]; then
            echo "Large file: $file"
        else
            echo "Small file: $file"
        fi
    fi
done

Explanation:

• for file in /path/to/directory/*: This loops through all files in the specified directory.
• if [ -f "$file" ]; then: This checks if the current item is a regular file.
• size=$(stat -c%s "$file"): This gets the size of the file in bytes.
• if [ $size -gt 1000000 ]; then: This checks if the file size is greater than 1MB.

This script will print messages indicating whether each file is large or small.

Example: Pinging Multiple Hosts

#!/bin/bash
hosts=("google.com" "github.com" "nonexistent.host")

for host in "${hosts[@]}"; do
    if ping -c 1 "$host" &> /dev/null; then
        echo "$host is reachable"
    else
        echo "$host is unreachable"
    fi
done

Explanation:

• hosts=("google.com" "github.com" "nonexistent.host"): This defines an array of hostnames.
• for host in "${hosts[@]}"; do: This loop iterates through each host in the array.
• if ping -c 1 "$host" &> /dev/null; then: This checks if the host can be pinged successfully.
• else: This part is executed if the host is unreachable.

This script will try to ping each host and print whether it's reachable or not.

Conclusion

By understanding and using loops and conditions in your Bash scripts, you gain the ability to automate complex tasks, manage systems more effectively, and streamline your work. Experiment with the examples provided, and you'll soon become a proficient Bash scripter. Happy scripting!