How to Iterate with Counter in Bash

Quick Answer: How to Create a Counter Loop

Use a C-style for loop: for ((i=0; i<10; i++)); do echo $i; done. This counts from 0 to 9, incrementing the counter each iteration.

Quick Comparison: Loop Methods

Method	Best For	Speed	Readability
C-style for	Fixed range	Fast	Excellent
while loop	Conditional iteration	Fast	Good
for…in	Array iteration	Fast	Excellent
seq (legacy)	Generating sequences	Medium	Fair

Bottom line: Use C-style for loops for counting. Use for...in for arrays. Use while when you need conditional checks.

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Why Use Counter Loops?

Counter loops are fundamental to programming. They let you run code a specific number of times, process ranges of numbers, or iterate through arrays by index. Whether processing files, retrying operations, or generating sequences, counter loops are essential.

C-Style For Loop (Recommended)

The C-style for loop is the most common and readable approach for counter-based iteration.

C-Style For Loop (Recommended)

The C-style for loop is the most common and readable approach. It works just like in C, Java, or Python.

for ((i=0; i<10; i++)); do
  echo "Count: $i"
done

Output:

Count: 0
Count: 1
Count: 2
...
Count: 9

The syntax breaks down as:

• ((i=0)) - Initialize counter to 0
• i<10 - Continue while condition is true (counter less than 10)
• i++ - Increment counter after each iteration

This is the most readable and efficient counter loop syntax in Bash.

Common variations:

# Count from 1 to 10 (inclusive)
for ((i=1; i<=10; i++)); do
  echo $i
done

# Count by steps of 2
for ((i=0; i<10; i+=2)); do
  echo $i  # Prints: 0, 2, 4, 6, 8
done

# Countdown
for ((i=5; i>0; i--)); do
  echo "$i..."
done
echo "Blastoff!"

# Count a specific number of times
for ((i=1; i<=3; i++)); do
  echo "Run #$i"
done

When to Use Counter Loops

Use C-style for loops when:

• You need to run code a specific number of times
• Counting from A to B is the core logic
• You need the index/counter value inside the loop
• Performance matters (it’s very fast)

Use while loops when:

• The exit condition is complex or data-dependent
• You need to update multiple variables per iteration
• Clarity is more important than brevity

---

Quick Reference

# Count from 0 to 9
for ((i=0; i<10; i++)); do echo $i; done

# Count from 1 to 10 (inclusive)
for ((i=1; i<=10; i++)); do echo $i; done

# Count by 2 (0, 2, 4, 6, 8)
for ((i=0; i<10; i+=2)); do echo $i; done

# Countdown from 5
for ((i=5; i>0; i--)); do echo "$i..."; done

# Using counter with arrays
array=(a b c d e)
for ((i=0; i<${#array[@]}; i++)); do
  echo "${array[$i]}"
done

---

Practical Example: Retrying Failed Commands

Here’s a real example where counter loops are crucial:

#!/bin/bash

# Retry a command up to 5 times
MAX_ATTEMPTS=5
ATTEMPT=0
SUCCESS=0

while (( ATTEMPT < MAX_ATTEMPTS )); do
  ((ATTEMPT++))
  echo "Attempt $ATTEMPT of $MAX_ATTEMPTS..."

  # Try to download a file
  if wget -q https://example.com/data.zip; then
    echo "Download successful!"
    SUCCESS=1
    break  # Exit loop on success
  else
    echo "Download failed. Waiting before retry..."
    sleep 2
  fi
done

if (( SUCCESS == 0 )); then
  echo "ERROR: Failed after $MAX_ATTEMPTS attempts"
  exit 1
fi

While Loop with Counter

The while loop approach is more verbose but sometimes clearer:

i=0
while (( i < 5 )); do
  echo "Iteration $i"
  ((i++))
done

# Output:
# Iteration 0
# Iteration 1
# Iteration 2
# Iteration 3
# Iteration 4

Processing Arrays with Counter Loops

Counter loops are perfect for accessing array elements by index:

#!/bin/bash

files=("file1.txt" "file2.txt" "file3.txt")

# Process each file
for ((i=0; i<${#files[@]}; i++)); do
  echo "Processing: ${files[$i]}"
done

# Output:
# Processing: file1.txt
# Processing: file2.txt
# Processing: file3.txt

Nested Counters

Sometimes you need loops within loops:

#!/bin/bash

# Create a multiplication table
for ((i=1; i<=5; i++)); do
  for ((j=1; j<=5; j++)); do
    result=$((i * j))
    printf "%2d " $result
  done
  echo  # Newline after each row
done

Output:

 1  2  3  4  5
 2  4  6  8 10
 3  6  9 12 15
 4  8 12 16 20
 5 10 15 20 25

Another nested example - processing multiple directories:

#!/bin/bash

# Process files in multiple directories
dirs=("/tmp" "/home" "/var")

for ((d=0; d<${#dirs[@]}; d++)); do
  dir="${dirs[$d]}"
  echo "Directory: $dir"

  for ((f=1; f<=3; f++)); do
    echo "  File $f"
  done
done

Break and Continue in Counter Loops

You can exit or skip iterations:

#!/bin/bash

# Break: exit loop early
for ((i=0; i<10; i++)); do
  if (( i == 5 )); then
    echo "Stopping at $i"
    break
  fi
  echo "Count: $i"
done

# Continue: skip to next iteration
for ((i=0; i<5; i++)); do
  if (( i == 2 )); then
    echo "Skipping $i"
    continue
  fi
  echo "Count: $i"
done

Real-World Example: Batch Processing

#!/bin/bash

# Process log files in batches
LOG_DIR="/var/log"
BATCH_SIZE=10
COUNT=0

for ((i=1; i<=100; i++)); do
  filename="$LOG_DIR/app_$(printf "%04d" $i).log"

  # Process file
  echo "Processing: $filename"

  ((COUNT++))

  # Every BATCH_SIZE files, do cleanup
  if (( COUNT % BATCH_SIZE == 0 )); then
    echo "Processed $COUNT files, running maintenance..."
    # Do cleanup or progress reporting
  fi
done

echo "Total processed: $COUNT files"

Performance Comparison: For Loop vs C-Style

#!/bin/bash

# C-style for loop (fast, recommended)
time for ((i=0; i<1000; i++)); do
  : # Do nothing
done

# While loop (also fast)
time {
  i=0
  while (( i < 1000 )); do
    : # Do nothing
    ((i++))
  done
}

# For-in loop (slower for large ranges - spawns subshell for each iteration)
time for i in {1..1000}; do
  : # Do nothing
done

Important Notes

• Always use (( )) for arithmetic - it’s cleaner and more efficient
• Initialize your counter before the loop
• Don’t forget to increment/decrement the counter
• Use meaningful variable names: for ((attempt=0; attempt<5; attempt++)) is clearer than for ((i=0; i<5; i++))
• C-style for loops are the fastest; avoid for i in {1..1000} for large ranges

Quick Reference

# Count from 0 to 9
for ((i=0; i<10; i++)); do echo $i; done

# Count from 1 to 10
for ((i=1; i<=10; i++)); do echo $i; done

# Count by 5s
for ((i=0; i<50; i+=5)); do echo $i; done

# Countdown
for ((i=10; i>0; i--)); do echo $i; done

# While with counter
i=0
while (( i < 10 )); do
  echo $i
  ((i++))
done

Summary

Counter loops are one of the most fundamental patterns in shell scripting. The C-style for loop for ((i=0; i<N; i++)) is typically the clearest and most efficient choice. Master this pattern and you’ll be able to automate repetitive tasks effectively.