While Loops

While loops execute commands repeatedly as long as a condition is true. They're perfect when you don't know in advance how many iterations you'll need.

Basic While Syntax

while [ condition ]; do
    commands
done

The loop continues as long as the condition evaluates to true (exit status 0).

Simple Counter Example

#!/bin/bash
COUNT=1

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

Output:

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

While with Arithmetic

Using (( )) for cleaner syntax:

#!/bin/bash
i=0

while (( i < 5 )); do
    echo "i = $i"
    (( i++ ))
done

Exercise: While Loop Counter

Create a simple while loop:

Reading Files Line by Line

The most common use of while loops:

#!/bin/bash
while read -r line; do
    echo "Line: $line"
done < input.txt

Processing CSV Data

#!/bin/bash
while IFS=',' read -r name age city; do
    echo "Name: $name, Age: $age, City: $city"
done < data.csv

Reading from Command Output

Process command output line by line:

#!/bin/bash
# Using process substitution
while read -r file; do
    echo "Processing: $file"
done < <(find . -name "*.txt")

# Using pipe (runs in subshell!)
find . -name "*.txt" | while read -r file; do
    echo "Processing: $file"
done

Important: With pipes, the while loop runs in a subshell, so variables set inside won't persist.

Exercise: Read from Pipe

Read lines from a command:

Infinite While Loops

Loop forever until explicitly broken:

#!/bin/bash
while true; do
    echo "Running..."
    sleep 1

    # Break condition
    if [ -f "stop.flag" ]; then
        echo "Stop flag detected"
        break
    fi
done

Or using : (always true):

while :; do
    echo "Still running..."
    sleep 1
done

While with Multiple Conditions

Combine conditions with logical operators:

#!/bin/bash
count=0
max=10
found=false

while [ $count -lt $max ] && [ "$found" = "false" ]; do
    count=$((count + 1))
    echo "Checking $count..."

    if [ $count -eq 5 ]; then
        found=true
    fi
done

Menu Loop Pattern

Interactive menus:

#!/bin/bash
while true; do
    echo "1) Option One"
    echo "2) Option Two"
    echo "3) Exit"
    read -p "Choose: " choice

    case $choice in
        1) echo "You chose One" ;;
        2) echo "You chose Two" ;;
        3) echo "Goodbye!"; break ;;
        *) echo "Invalid option" ;;
    esac
done

While with Read and Timeout

Add timeout to prevent hanging:

#!/bin/bash
while read -t 5 -r input; do
    echo "You entered: $input"
done
echo "Timed out or EOF"

Retry Loop Pattern

Retry operations with backoff:

#!/bin/bash
MAX_RETRIES=5
RETRY=0

while [ $RETRY -lt $MAX_RETRIES ]; do
    if some_command; then
        echo "Success!"
        break
    else
        RETRY=$((RETRY + 1))
        echo "Attempt $RETRY failed, retrying..."
        sleep $((RETRY * 2))    # Exponential backoff
    fi
done

if [ $RETRY -eq $MAX_RETRIES ]; then
    echo "All retries failed"
    exit 1
fi

Monitoring Loop

Watch for changes:

#!/bin/bash
LAST_SIZE=0

while true; do
    CURRENT_SIZE=$(wc -c < logfile.txt)

    if [ "$CURRENT_SIZE" -gt "$LAST_SIZE" ]; then
        echo "File grew: $LAST_SIZE -> $CURRENT_SIZE bytes"
        LAST_SIZE=$CURRENT_SIZE
    fi

    sleep 5
done

Nested While Loops

Loops within loops:

#!/bin/bash
outer=1

while [ $outer -le 3 ]; do
    inner=1
    while [ $inner -le 3 ]; do
        echo "outer=$outer, inner=$inner"
        inner=$((inner + 1))
    done
    outer=$((outer + 1))
done

Reading Multiple Values

Parse structured input:

#!/bin/bash
# Data format: name:score
while IFS=':' read -r name score; do
    echo "$name scored $score"
done << EOF
Alice:95
Bob:87
Carol:92
EOF

Exercise: Until Condition Met

Loop until a condition changes:

Common Pitfalls

Variable Not Updating

# WRONG - runs forever!
COUNT=1
while [ $COUNT -le 5 ]; do
    echo $COUNT
    # Forgot to increment!
done

# CORRECT
COUNT=1
while [ $COUNT -le 5 ]; do
    echo $COUNT
    COUNT=$((COUNT + 1))
done

Pipeline Subshell Issue

# WRONG - variable doesn't persist
count=0
cat file.txt | while read -r line; do
    count=$((count + 1))
done
echo "Lines: $count"    # Always 0!

# CORRECT - use process substitution
count=0
while read -r line; do
    count=$((count + 1))
done < <(cat file.txt)
echo "Lines: $count"    # Correct count

Best Practices

1. Always have an exit condition: Prevent infinite loops
2. Use read -r: Prevents backslash interpretation
3. Quote variables: Handle spaces in data
4. Avoid pipe + while: Variables won't persist
5. Consider alternatives: for might be cleaner for fixed iterations

Key Takeaways

• while [ condition ]; do ... done loops while condition is true
• Use while read -r line to process files line by line
• while true creates infinite loops (use break to exit)
• Variables in piped while loops don't persist - use process substitution
• Use (( )) for cleaner arithmetic conditions
• Always ensure your loop has a working exit condition
• read -r prevents backslash interpretation