Loops in Bash

# CHAPTER 6

Loops in Bash #

1. Introduction #

2. Learning Objectives #

• Construct a for loop to iterate over lists of words, files, or numbers.

• Construct a while loop to execute code continuously while a condition remains true.

• Construct an until loop to execute code continuously until a condition becomes true.

• Utilize the break command to immediately escape a running loop.

• Utilize the continue command to skip the current iteration and jump to the next one.

3. The for Loop (Iterating over Lists) #

Syntax 1: Iterating over words

#!/bin/bash

# Loop through a predefined list of servers
for SERVER in Web1 Web2 Database1
do
    echo "Connecting to $SERVER..."
    # The code here runs 3 times
done

Syntax 2: Iterating over numbers (C-style) If you want to count from 1 to 5, you can use the traditional programming syntax utilizing double parentheses.

#!/bin/bash

for (( i=1; i<=5; i++ ))
do
    echo "Creating backup number: $i"
done

4. The while Loop (Condition-based) #

#!/bin/bash

COUNTER=1

# This loop runs as long as COUNTER is less than or equal to 3
while [ $COUNTER -le 3 ]
do
    echo "Processing batch $COUNTER"
    # MUST INCREMENT THE COUNTER! Otherwise, it is an infinite loop.
    let COUNTER++
done

5. Loop Control: break and continue #

1. The break Command (The Escape Hatch): If a catastrophic error occurs, break instantly kills the loop and escapes.

#!/bin/bash
for (( i=1; i<=10; i++ ))
do
    if [ $i -eq 5 ]; then
        echo "Critical Error at Step 5. Aborting loop!"
        break
    fi
    echo "Processing step $i"
done
# Output will only reach Step 4, then exit.

2. The continue Command (The Skip Button): If you hit a minor error, you don't want to abort the whole loop. You just want to skip that specific item and move to the next one.

#!/bin/bash
for SERVER in ServerA ServerB ServerC
do
    if [ "$SERVER" == "ServerB" ]; then
        echo "ServerB is offline. Skipping."
        continue
    fi
    echo "Backing up $SERVER..."
done
# Output: Backs up ServerA, skips B, backs up ServerC.

6. The Infinite Loop (Monitoring) #

#!/bin/bash

while true
do
    echo "Checking server health..."
    # Pause for 5 seconds so you don't melt the CPU
    sleep 5
done

7. Diagrams/Visual Suggestions #

8. Best Practices #

• Always sleep inside Infinite Loops: If you write a while true loop and forget to include the sleep command, the script will execute the loop millions of times per second. This will instantly consume 100% of your CPU core, locking up the server and potentially requiring a hard reboot.

9. Common Mistakes #

• Forgetting to increment the While Loop counter: If you write a while [ $X -lt 10 ] loop, but you forget to add let X++ at the bottom of the loop, the value of $X will never change. The condition will always remain true, and you will accidentally create an infinite loop that crashes your terminal.

10. Mini Project: Automated File Generator #

1. 1. nano onboarding.sh

1. 2. Write the code:

#!/bin/bash

# Define the list of new hires
NEW_HIRES="Alice Bob Charlie Diana Eve"

echo "Starting automated workspace creation..."

for EMPLOYEE in $NEW_HIRES
do
    echo "Creating directory for $EMPLOYEE..."
    mkdir -p "/tmp/workspace/$EMPLOYEE"
    
    echo "Generating welcome document..."
    echo "Welcome to the company, $EMPLOYEE!" > "/tmp/workspace/$EMPLOYEE/welcome.txt"
done

echo "Onboarding complete. Verifying directories:"
ls -l /tmp/workspace/

1. 3. chmod +x onboarding.sh

1. 4. Run it: ./onboarding.sh. You just performed 10 manual administrative actions in 0.1 seconds.

11. Practice Exercises #

1. 1. Contrast the operational use case of a for loop against a while loop. If you need to ping 50 specific IP addresses from a text file, which loop is structurally superior?

1. 2. Explain the functional difference between the break command and the continue command within the context of iterating through a list of servers.

12. MCQs with Answers #

13. Interview Questions #

• Q: A junior engineer writes a while [ $COUNTER -le 10 ] loop, but the script never stops running and eventually crashes the terminal. Explain the mechanical oversight that causes a while loop to become infinite, and how you would fix the code.

• Q: Explain the C-style for loop syntax for (( i=0; i<10; i++ )). Specifically, break down what the i++ component accomplishes and why it is necessary.

• Q: You are writing an automation script that loops through an array of databases to perform backups. If the database named 'MasterDB' is encountered, the script should abort the entire backup process immediately to prevent data corruption. Walk me through the control flow logic required to implement this failsafe.

14. FAQs #

15. Summary #

16. Next Chapter Recommendation #