What are the common loop structures in Shell scripts? How to use for, while, and until loops?

Common loop structures in Shell scripts include for loops, while loops, and until loops.

for Loop

Basic Syntax

bash
for variable in list
do
    commands
done

Iterating Over Lists

bash
# Iterate over string list
for fruit in apple banana orange
do
    echo "Fruit: $fruit"
done

# Iterate over number sequence
for i in 1 2 3 4 5
do
    echo "Number: $i"
done

# Use seq command to generate sequence
for i in $(seq 1 10)
do
    echo "Number: $i"
done

# Use {start..end} syntax (Bash 3.0+)
for i in {1..10}
do
    echo "Number: $i"
done

# Specify step
for i in {0..10..2}
do
    echo "Number: $i"
done

C-style for Loop

bash
# C-style syntax
for (( i=0; i<10; i++ ))
do
    echo "Number: $i"
done

# Multiple variables
for (( i=0, j=10; i<10; i++, j-- ))
do
    echo "i=$i, j=$j"
done

Iterating Over Files and Directories

bash
# Iterate over files in current directory
for file in *.txt
do
    echo "Processing: $file"
done

# Iterate over command output
for user in $(cut -d: -f1 /etc/passwd)
do
    echo "User: $user"
done

# Recursively iterate over directories
for file in $(find . -name "*.sh")
do
    echo "Shell script: $file"
done

while Loop

Basic Syntax

bash
while condition
do
    commands
done

while Loop Examples

bash
# Counter
count=0
while [ $count -lt 5 ]
do
    echo "Count: $count"
    count=$((count + 1))
done

# Read file lines
while IFS= read -r line
do
    echo "Line: $line"
done < input.txt

# Read user input
while true
do
    read -p "Enter 'quit' to exit: " input
    if [ "$input" = "quit" ]; then
        break
    fi
    echo "You entered: $input"
done

# Wait for process to finish
while ps -p $PID > /dev/null
do
    echo "Process is running..."
    sleep 1
done
echo "Process finished"

until Loop

Basic Syntax

bash
until condition
do
    commands
done

until Loop Examples

bash
# Wait for condition to be true
count=0
until [ $count -ge 5 ]
do
    echo "Count: $count"
    count=$((count + 1))
done

# Wait for service to start
until curl -s http://localhost:8080 > /dev/null
do
    echo "Waiting for service..."
    sleep 2
done
echo "Service is ready"

Loop Control Statements

break Statement

bash
# Break out of loop
for i in {1..10}
do
    if [ $i -eq 5 ]; then
        break
    fi
    echo "Number: $i"
done
# Output: 1 2 3 4

continue Statement

bash
# Skip current iteration
for i in {1..10}
do
    if [ $((i % 2)) -eq 0 ]; then
        continue
    fi
    echo "Odd number: $i"
done
# Output: 1 3 5 7 9

break and continue with Levels

bash
# Break out of nested loops
for i in {1..3}
do
    for j in {1..3}
    do
        if [ $i -eq 2 ] && [ $j -eq 2 ]; then
            break 2
        fi
        echo "i=$i, j=$j"
    done
done

Practical Application Examples

bash
#!/bin/bash

# Example 1: Batch process files
echo "Processing text files..."
for file in *.txt
do
    if [ -f "$file" ]; then
        echo "Processing: $file"
        # Add processing logic here
        cp "$file" "backup_$file"
    fi
done

# Example 2: Monitor process
echo "Monitoring process..."
while true
do
    if ps aux | grep -q "[m]yprocess"; then
        echo "Process is running"
    else
        echo "Process stopped"
        break
    fi
    sleep 5
done

# Example 3: Wait for multiple services
services=("service1" "service2" "service3")
for service in "${services[@]}"
do
    echo "Waiting for $service..."
    until systemctl is-active --quiet "$service"
    do
        sleep 2
    done
    echo "$service is ready"
done

# Example 4: Read configuration file
echo "Reading configuration..."
while IFS='=' read -r key value
do
    # Skip comments and empty lines
    [[ $key =~ ^#.*$ ]] && continue
    [[ -z $key ]] && continue
    
    echo "Config: $key = $value"
done < config.txt

# Example 5: Parallel processing
max_parallel=3
count=0
for task in task1 task2 task3 task4 task5
do
    # Start background task
    echo "Starting $task..."
    (
        sleep 2
        echo "$task completed"
    ) &
    
    count=$((count + 1))
    
    # Control parallelism
    if [ $((count % max_parallel)) -eq 0 ]; then
        wait
    fi
done
wait
echo "All tasks completed"

Loop Best Practices

1. Quote variables: Prevent issues with spaces and special characters
2. Set IFS for special delimiters: IFS= read -r line
3. Use break and continue: Improve code readability
4. Avoid infinite loops: Ensure while and until loops have exit conditions
5. Use wait for background processes: Ensure all tasks complete
6. Prefer C-style for loops: Clearer for number sequences
7. Use array iteration: for item in "${array[@]}"