Mastering For Loops in Bash

For loops are one of the most powerful constructs in Bash scripting, allowing you to automate repetitive tasks and process multiple files with ease. In this comprehensive guide, we’ll explore various ways to use for loops effectively.

Basic For Loop Syntax

The most basic for loop in Bash follows this pattern:

for variable in list
do
    command1
    command2
    ...
done

Example: Simple Iteration

#!/bin/bash
for fruit in apple banana cherry
do
    echo "I like $fruit"
done

Output:

I like apple
I like banana
I like cherry

Looping Through Files

One of the most common use cases is processing all files in a directory. Here’s how to iterate through all .txt files:

#!/bin/bash
for file in *.txt
do
    echo "Processing: $file"
    # Add your file processing commands here
    wc -l "$file"
done

Important: Handling Spaces in Filenames

Always quote your variables when dealing with filenames that might contain spaces:

for file in *.pdf
do
    # GOOD - handles spaces correctly
    mv "$file" "backup/$file"
    
    # BAD - will break on spaces
    # mv $file backup/$file
done

C-Style For Loops

Bash also supports C-style for loops for numeric iterations:

#!/bin/bash
for ((i=1; i<=5; i++))
do
    echo "Iteration $i"
done

Output:

Iteration 1
Iteration 2
Iteration 3
Iteration 4
Iteration 5

Using Ranges with Brace Expansion

Generate sequences easily with brace expansion:

# Numbers
for i in {1..10}
do
    echo "Number: $i"
done

# Letters
for letter in {a..z}
do
    echo "Letter: $letter"
done

# With step
for num in {0..100..10}
do
    echo "Count: $num"
done

Practical Examples

1. Batch Rename Files

#!/bin/bash
# Add prefix to all .jpg files
for file in *.jpg
do
    mv "$file" "photo_$file"
done

2. Create Multiple Directories

#!/bin/bash
# Create project directories
for dir in src tests docs config
do
    mkdir -p "project/$dir"
    echo "Created: project/$dir"
done

3. Process Log Files

#!/bin/bash
# Extract errors from multiple log files
for logfile in /var/log/*.log
do
    echo "Checking $logfile..."
    grep -i "error" "$logfile" > "errors_$(basename $logfile)"
done

4. Backup Multiple Databases

#!/bin/bash
# Backup multiple databases
for db in users products orders analytics
do
    echo "Backing up database: $db"
    mysqldump -u root -p "$db" > "backup_${db}_$(date +%Y%m%d).sql"
done

Nested For Loops

You can nest for loops for more complex operations:

#!/bin/bash
# Create a multiplication table
for i in {1..5}
do
    for j in {1..5}
    do
        result=$((i * j))
        echo -n "$result "
    done
    echo ""  # New line after each row
done

Reading from Command Output

Use command substitution to loop through command results:

#!/bin/bash
# Loop through all users
for user in $(cat /etc/passwd | cut -d: -f1)
do
    echo "User: $user"
done

# Better approach using while read
while IFS=: read -r username rest
do
    echo "User: $username"
done < /etc/passwd

Loop Control: Break and Continue

Break - Exit the loop

for i in {1..10}
do
    if [ $i -eq 5 ]; then
        echo "Breaking at $i"
        break
    fi
    echo "Number: $i"
done

for i in {1..10}
do
    if [ $i -eq 5 ]; then
        echo "Skipping $i"
        continue
    fi
    echo "Number: $i"
done

Common Pitfalls and Best Practices

❌ Don’t: Parse `ls` output

# BAD - breaks on spaces and special characters
for file in $(ls *.txt)
do
    echo "$file"
done

✅ Do: Use glob patterns

# GOOD - handles all filenames correctly
for file in *.txt
do
    echo "$file"
done

✅ Always quote variables

for file in *.mp3
do
    # GOOD
    cp "$file" "/backup/$file"
    
    # BAD
    # cp $file /backup/$file
done

Performance Tips

Avoid unnecessary subshells: Use built-in commands when possible
Use arrays for complex data: Store results in arrays for reuse
Limit loop iterations: Use break to exit early when condition is met

# Efficient: stops when found
for file in *.log
do
    if grep -q "ERROR" "$file"; then
        echo "Found errors in $file"
        break
    fi
done

Download Sample Scripts

I’ve prepared a collection of ready-to-use for loop scripts for you to practice with:

📦 Download Sample Scripts (ZIP)

The ZIP file includes:

basic_loop.sh - Simple iteration examples
file_processor.sh - File processing templates
batch_rename.sh - Batch file renaming script
backup_script.sh - Automated backup examples
README.md - Usage instructions

Video Tutorial

Summary

For loops are essential for:

✅ Processing multiple files in batch
✅ Automating repetitive tasks
✅ Generating sequences and ranges
✅ Building complex automation scripts

Master these patterns and you’ll dramatically improve your Bash scripting efficiency!

Mastering For Loops in Bash

Mastering For Loops in Bash

Basic For Loop Syntax

Example: Simple Iteration

Looping Through Files

Important: Handling Spaces in Filenames

C-Style For Loops

Using Ranges with Brace Expansion

Practical Examples

1. Batch Rename Files

2. Create Multiple Directories

3. Process Log Files

4. Backup Multiple Databases

Nested For Loops

Reading from Command Output

Loop Control: Break and Continue

Break - Exit the loop

Continue - Skip to next iteration

Common Pitfalls and Best Practices

❌ Don’t: Parse ls output

✅ Do: Use glob patterns

✅ Always quote variables

Performance Tips

Download Sample Scripts

Video Tutorial

Summary

Related Articles

Download Exercise Files

❌ Don’t: Parse `ls` output