How to use while loop in bash

Mastering the 'while' Loop in Bash Scripting

Learn how to effectively use 'while' loops in Bash for repetitive tasks, conditional execution, and processing data streams. This guide covers basic syntax, common use cases, and best practices.

The while loop is a fundamental control flow statement in Bash scripting, allowing you to execute a block of commands repeatedly as long as a specified condition remains true. It's incredibly versatile, useful for tasks ranging from simple counters to processing lines from a file or monitoring system states. Understanding how to wield the while loop effectively is crucial for writing robust and efficient Bash scripts.

Basic Syntax and Execution Flow

The basic structure of a while loop in Bash is straightforward. It begins with the while keyword, followed by a condition. If the condition evaluates to true (an exit status of 0), the commands within the do...done block are executed. This process repeats until the condition evaluates to false (a non-zero exit status).

while [ condition ]
do
  # Commands to execute repeatedly
done

Basic syntax of a Bash 'while' loop

flowchart TD
    A[Start] --> B{Condition True?}
    B -- Yes --> C[Execute Commands]
    C --> B
    B -- No --> D[End Loop]
    D --> E[Continue Script]

Flowchart illustrating the execution of a 'while' loop

Common Use Cases for 'while' Loops

Bash while loops shine in several scenarios. Here are some of the most common and practical applications:

1. Simple Counter Loop

One of the most basic uses is to create a counter that iterates a specific number of times. This is often achieved by incrementing a variable within the loop and checking its value against a limit.

#!/bin/bash

i=1
while [ $i -le 5 ]
do
  echo "Iteration number: $i"
  ((i++))
done

echo "Loop finished."

Example of a 'while' loop as a simple counter

2. Reading Lines from a File

The while read construct is extremely powerful for processing text files line by line. It's often combined with input redirection (< filename) or a pipe (|).

#!/bin/bash

# Create a sample file
echo -e "Apple\nBanana\nCherry" > fruits.txt

echo "Processing fruits.txt:"
while IFS= read -r line
do
  echo "Fruit: $line"
done < fruits.txt

rm fruits.txt # Clean up

Reading a file line by line using 'while read'

3. Infinite Loops and Controlled Exits

An infinite loop can be created by providing a condition that is always true. While seemingly dangerous, this is useful for services or daemons that need to run continuously, often with a sleep command to prevent excessive resource consumption. You can exit such loops using break or exit.

#!/bin/bash

count=0
while true # Condition is always true
do
  echo "Running... (Press Ctrl+C to stop)"
  ((count++))
  if [ $count -ge 3 ]; then
    echo "Reached 3 iterations, breaking out."
    break # Exit the loop
  fi
  sleep 1 # Wait for 1 second
done

echo "Script finished."

An infinite loop with a controlled exit using 'break'

4. Processing Command Output

Similar to reading from files, while loops can process the output of other commands using pipes. This is a very common pattern in shell scripting.

#!/bin/bash

echo "Listing files in current directory:"
ls -l | while IFS= read -r line
do
  # Skip the total line from ls -l output
  if [[ "$line" == total* ]]; then
    continue # Skip to the next iteration
  fi
  echo "Found: $line"
done

Processing 'ls -l' output with a 'while' loop