my-docs/Linux/Bash Script/10-While.md

Bash while Loop Guide

A clear, concise, and visually pleasant reference for using while loops in Bash.

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1. Overview

The while loop in Bash repeatedly executes a block of commands as long as a condition evaluates to true. It is a fundamental control structure for iteration when the number of repetitions is not known in advance.

General form

while [ condition ]
do
  command1
  command2
  ...
done

• The condition is evaluated before each iteration. If it returns a zero (truthy) exit status, the loop body runs. Otherwise, the loop exits.
• Square brackets ([ ... ]) are a synonym for the test command; spacing matters.

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2. Examples

2.1 Simple counter

Counts from 1 to 5 and prints the current value.

counter=1

while [ $counter -le 5 ]
do
  echo "Counter: $counter"
  ((counter++))
done

What it does:

• Initializes counter to 1.
• The loop continues while counter is less than or equal to 5.
• Prints and increments the counter each iteration.

2.2 Waiting for a file to appear

Polls for the existence of /tmp/myfile.txt once per second.

while [ ! -f /tmp/myfile.txt ]
do
  echo "Waiting for file..."
  sleep 1
done

echo "File exists!"

What it does:

• Checks if the file does not exist (! -f).
• Sleeps for one second between checks to avoid busy-waiting.
• When the file appears, exits the loop and prints confirmation.

2.3 Interactive prompt with exit condition

Keeps prompting the user until they type q.

while true
do
  read -p "Type 'q' to quit: " input
  if [ "$input" = "q" ]; then
    break
  fi
done

What it does:

• Uses an infinite loop (while true).
• Prompts the user each time.
• Exits the loop when the input equals q using break.

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3. Tips & Best Practices

• Quote variables to avoid word splitting and issues when they are empty: [ "$foo" = "bar" ].
• Use arithmetic expressions with (( ... )) when dealing with numbers (((counter++)) is more concise than counter=$((counter + 1))).
• Avoid busy loops: if waiting on external state, insert a sleep to reduce CPU usage.
• Use set -u and set -e in scripts to catch undefined variables and stop on errors, but be careful when using them with conditional tests.
• Test conditions explicitly: e.g., use -f, -d, -r, -s, etc., for file-related checks.

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4. Common Pitfalls

• Missing spaces inside [ ] — while [$counter -le 5] is invalid; it must be [ $counter -le 5 ].
• Forgetting to initialize loop variables, leading to unexpected behavior when using them in tests.
• Using read without handling empty input; consider providing a default or validating.
• Infinite loops without an exit strategy (e.g., missing break or condition never false).

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5. Variations

• Until loop: opposite of while — runs until a condition becomes true.

  until [ -f /tmp/myfile.txt ]
  do
    echo "Waiting..."
    sleep 1
  

done

• Command-based condition: you can use any command whose exit status matters.

while ping -c1 example.com >/dev/null 2>&1
do
  echo "Host is reachable"
  sleep 5
done

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6. Summary

Use while loops when the number of iterations depends on dynamic conditions (user input, file existence, external services). Combine with proper quoting, sleeping, and exit strategies to write robust and readable scripts.