While Loop in Bash

The while loop in Bash is used to execute a block of code repeatedly as long as a specified condition evaluates to true. It's commonly used for processing files, waiting for events, or iterating over commands.

Syntax

while [condition]; do
    # Commands to execute
done

Key Points:

[condition] is a command or test expression. As long as it evaluates to true (exit status 0), the loop continues.
The do block contains the commands to execute repeatedly.
The done keyword marks the end of the loop.

Basic Example: Counter

count=1
while [ $count -le 5 ]; do
    echo "Count: $count"
    ((count++)) # Increment count
done

Output:

Count: 1
Count: 2
Count: 3
Count: 4
Count: 5

Example: Reading a File Line by Line

The while loop can process files line by line using read:

while read -r line; do
    echo "Line: $line"
done < input.txt

Explanation:

read -r line: Reads a single line from the file.
< input.txt: Redirects the file input.txt as input to the loop.

Example: Waiting for a Condition

The while loop can pause execution until a specific condition is met:

status="pending"
while [ "$status" != "done" ]; do
    echo "Waiting for completion..."
    sleep 2 # Pause for 2 seconds
    status="done" # Simulate status change
done
echo "Task completed!"

Example: Infinite Loop

An infinite loop runs until explicitly terminated. Use with caution!

while true; do
    echo "Press Ctrl+C to stop."
    sleep 1
done

Explanation:

true: Always evaluates to true.
Use Ctrl+C to stop the script manually.

Example: Nested While Loops

You can nest while loops for complex tasks, like printing a matrix:

rows=3
cols=3
row=1

while [ $row -le $rows ]; do
    col=1
    while [ $col -le $cols ]; do
        echo -n "$row,$col "
        ((col++))
    done
    echo
    ((row++))
done

Output:

1,1 1,2 1,3 
2,1 2,2 2,3 
3,1 3,2 3,3

Example: Using Command Output in While Loop

You can use the output of a command in a while loop:

ls *.txt | while read -r file; do
    echo "Processing file: $file"
done

Explanation:

ls *.txt: Lists all .txt files.
while read -r file: Processes each filename from the command output.

Breaking and Continuing a While Loop

break: Exits the loop immediately.
continue: Skips the rest of the current iteration and starts the next.

Example:

count=1
while [ $count -le 5 ]; do
    if [ $count -eq 3 ]; then
        echo "Skipping $count"
        ((count++))
        continue
    fi
    if [ $count -eq 5 ]; then
        echo "Breaking at $count"
        break
    fi
    echo "Count: $count"
    ((count++))
done

Output:

Count: 1
Count: 2
Skipping 3
Count: 4
Breaking at 5

Common Errors to Avoid

Infinite Loops: Ensure the loop condition eventually becomes false. Example:

count=1
while [ $count -le 5 ]; do
    echo $count
    # Missing ((count++)) causes an infinite loop
done

Incorrect Test Syntax: Use square brackets [ ] or double brackets [[ ]] properly.
```
while [[ $count -lt 5 ]]; do
    # Correct usage
done
```
Redirection Misuse: Be cautious when using redirection (<, |). Test the loop with smaller input files to ensure correctness.

Conclusion

The while loop is a versatile construct in Bash, suitable for repetitive tasks and real-time monitoring. By mastering its syntax and usage, you can handle complex scripting challenges efficiently.

Let me know if you need further examples or details for your post!

Souy Soeng

While Loop in Bash