Loop Control

Loop control is essential for managing iteration flow in BASH scripts. The break, continue, and exit commands allow precise control over loop execution, enabling scripts to respond dynamically to conditions. This section explores their usage and best practices.

Break: Exit Loops Prematurely

The break command terminates the innermost loop immediately. It is useful for exiting loops when a condition is met, avoiding unnecessary iterations.

Example: A for loop that breaks when a specific item is found.

#!/bin/bash
fruits=("apple" "banana" "cherry" "date")
for fruit in "${fruits[@]}"; do
    if [[ "$fruit" == "cherry" ]]; then
        echo "Found cherry!"
        break
    fi
    echo "Processing $fruit"
done

Output:

Processing apple
Processing banana
Found cherry!

Notes:
- break can take an argument (e.g., break 2) to exit a nested loop at a specified level.
- Avoid overusing break in favor of refining loop conditions for clarity.

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Continue: Skip Current Iteration

The continue command skips the current iteration of a loop and proceeds to the next one. It is ideal for filtering out unwanted entries during iteration.

Example: A while loop that skips even numbers.

#!/bin/bash
count=1
while [[ $count -le 10 ]]; do
    if (( count % 2 == 0 )); then
        echo "Skipping $count"
        continue
    fi
    echo "Processing $count"
    ((count++))
done

Output:

Processing 1
Skipping 2
Processing 3
Skipping 4
...

Notes:
- continue works with for, while, and until loops.
- Use it to refine data processing without terminating the loop.

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Exit: Terminate Script Execution

The exit command terminates the entire script, not just the loop. It is useful for handling critical errors or signaling script completion.

Example: A loop that exits on a critical error.

#!/bin/bash
for i in {1..5}; do
    if [[ $i -eq 3 ]]; then
        echo "Critical error at $i!"
        exit 1
    fi
    echo "Step $i completed"
done

Output:

Step 1 completed
Step 2 completed
Critical error at 3!

Notes:
- exit can take an argument (e.g., exit 0 for success, exit 1 for failure).
- Use exit sparingly in loops to avoid unintended script termination.

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Key takeaways

• Use break to exit loops when a condition is met.
• Use continue to skip specific iterations during processing.
• Use exit to terminate the script, but be cautious of its global effect.
• Prioritize clarity by refining loop conditions over frequent use of control commands.
• Understand the difference between loop-specific control and script-level termination.