What is the difference between the ‘COPY’ and ‘ADD’ commands in a Dockerfile?

The COPY and ADD commands in a Dockerfile both copy files from the host into a Docker image, but they differ in functionality and use cases:

1. COPY

• Purpose: Copies files/directories from the host machine into the image.
• Features:
• Simple file/directory copying.
• Preserves file permissions and metadata.
• Requires explicit extraction of compressed files (e.g., .tar, .zip).
• Best For:
• Local files or directories.
• Transparent and predictable file transfers.
• Example:

  COPY ./src /app/src

2. ADD

• Purpose: Copies files/directories and:
• Automatically extracts compressed archives (e.g., .tar, .gz, .zip).
• Can fetch files from URLs (but avoids this for remote URLs—use RUN curl/wget instead).
• Features:
• Extra functionality beyond copying (extraction, URL support).
• Less predictable behavior (e.g., unintended extraction of archives).
• Best For:
• Local compressed archives needing extraction.
• Rare edge cases requiring URL downloads (not recommended).
• Example:

  ADD ./data.tar.gz /app/data  # Extracts the archive into /app/data

Key Differences

Aspect	COPY	ADD
Extraction	❌ No auto-extraction.	✅ Extracts .tar, .gz, .zip archives.
URL Support	❌ Only local files.	✅ Can fetch from URLs (use with caution).
Transparency	✅ Predictable.	❌ May unintentionally extract archives.
Best Practice	✅ Preferred for most use cases.	❗ Use only for extraction or edge cases.

When to Use Which

• Use COPY:
• For most file/directory copying (simpler and safer).

  COPY ./config /app/config

• Use ADD:
• To extract a local compressed archive into the image.

  ADD ./package.tar.gz /app

• Avoid using ADD for URLs—prefer RUN curl/wget for better control:

  RUN curl -O https://example.com/file.txt

Example Workflow

# Prefer COPY for local files
COPY requirements.txt /app/

# Use ADD to extract a local .tar.gz
ADD ./dependencies.tar.gz /app/

# Avoid ADD for URLs; use RUN + curl instead
RUN curl -SL https://example.com/data.zip -o /app/data.zip \
    && unzip /app/data.zip \
    && rm /app/data.zip

Summary

• COPY: Simple, secure, and recommended for most cases.
• ADD: Use sparingly for archive extraction, but avoid for URLs.
  Always default to COPY unless you specifically need ADD‘s extra features.