Loops in Shell Scripting

# CHAPTER 6

Loops in Shell Scripting #

1. Introduction #

2. Learning Objectives #

• Construct a for loop to iterate systematically over a predefined list of strings or files.

• Construct a while loop to execute tasks continuously as long as a condition remains True.

• Construct an until loop to execute tasks continuously until a condition becomes True.

• Utilize the break command to instantly abort and escape a running loop.

• Utilize the continue command to skip a localized error and proceed to the next iteration.

3. The for Loop (List Iteration) #

Syntax:

#!/bin/sh

# The loop will run exactly 3 times.
for SERVER in Web1 Web2 DB1
do
    echo "Connecting via SSH to $SERVER..."
    # Imagine SSH connection code here
done

*Note the structure:* The list is provided after in. The variable SERVER dynamically changes to hold the current list item on each pass. The execution block is sandwiched between do and done.

4. The while Loop (Condition-Based) #

#!/bin/sh

COUNTER=1

# Run the loop AS LONG AS the counter is Less Than or Equal To (-le) 3
while [ $COUNTER -le 3 ]
do
    echo "Executing batch process number $COUNTER"
    # CRITICAL: You must increment the counter, or it will run infinitely!
    COUNTER=$(expr $COUNTER + 1)
done

5. The until Loop (Reverse Condition) #

#!/bin/sh

# Imagine a script waiting for a service to crash
STATUS="running"

# The loop runs AS LONG AS STATUS is NOT "crashed"
until [ "$STATUS" = "crashed" ]
do
    echo "Service is operating normally..."
    # A real script would check the service here. 
    # We will manually trigger the break for this example.
    STATUS="crashed"
done
echo "ALERT: Service has crashed. Loop exited."

6. Loop Control (break and continue) #

1. The break Statement (The Escape Hatch): If a catastrophic failure occurs during a loop, break instantly kills the entire loop and drops execution to the code beneath the done statement.

#!/bin/sh
for FILE in file1 file2 file3
do
    if [ "$FILE" = "file2" ]; then
        echo "Corrupt file detected! Aborting entire process."
        break
    fi
    echo "Processing $FILE..."
done

2. The continue Statement (The Skip Button): If a minor, non-fatal error occurs, you don't want to stop the whole script. continue skips the rest of the current pass and instantly jumps to the next item in the list.

#!/bin/sh
for USER in Alice Bob Charlie
do
    if [ "$USER" = "Bob" ]; then
        echo "Bob's account is locked. Skipping."
        continue
    fi
    echo "Updating password for $USER..."
done
# Output: Updates Alice, skips Bob, updates Charlie.

7. Diagrams/Visual Suggestions #

8. Best Practices #

• Use for loops for Directory Parsing: You can utilize wildcards in a for loop to automatically generate a list of files in a directory. For example, for FILE in *.txt will instantly locate every .txt file in the current working directory and loop through them one by one.

9. Common Mistakes #

• The Infinite While Loop: The most common and devastating error in shell scripting is writing a while [ $X -lt 10 ] loop, but forgetting to write X=$(expr $X + 1) inside the loop block. If the variable never increases, the condition never evaluates to False. The script will consume 100% of the CPU core indefinitely until the administrator forcibly kills the process.

10. Mini Project: Automated File Provisioning #

1. 1. nano provision.sh

1. 2. Write the code:

#!/bin/sh

# The list of developer environments to create
DEVS="frontend backend database devops"

echo "Initiating bulk workspace provisioning..."

for ENV in $DEVS
do
    echo "Provisioning directory for $ENV team..."
    mkdir -p "/tmp/workspaces/$ENV"
    
    # Create a default configuration file inside the new folder
    touch "/tmp/workspaces/$ENV/config.ini"
done

echo "Provisioning complete. Verifying structure:"
ls -l /tmp/workspaces/

1. 3. chmod +x provision.sh and run it. You just performed 8 administrative file-system commands dynamically using only 4 lines of core logic.

11. Practice Exercises #

1. 1. Contrast the structural design of a for loop against a while loop. In a scenario where you must parse exactly 50 distinct IP addresses listed in a text file, which looping architecture is structurally superior?

1. 2. Explain the functional difference between the break command and the continue command. What distinct operational outcomes occur when they are triggered?

12. MCQs with Answers #

13. Interview Questions #

• Q: A junior administrator writes a script to wait for a database server to come online. They use a while loop that pings the server repeatedly. Explain why it is an absolute architectural necessity to include the sleep command inside this specific looping structure.

• Q: Explain the logical inversion between the while loop and the until loop. Walk me through a specific administrative monitoring scenario where using until is more readable and intuitive than using while.

• Q: You are utilizing a for loop to read a list of file names from a variable (FILES="doc1 doc2 doc3"). A user accidentally creates a file named "my document". Walk me through the exact "Word Splitting" error that will occur when the loop attempts to process that specific filename.

14. FAQs #

15. Summary #

16. Next Chapter Recommendation #