AWS S3 sync: Sync a Directory to S3

Syncing Directories with aws s3 sync

The aws s3 sync command intelligently synchronizes files between a local directory and an S3 prefix (or between two S3 locations). It only transfers files that are new or have changed, making it much faster than cp --recursive for incremental updates.

Basic Sync Command

aws s3 sync ./dist/ s3://my-website-bucket/

This uploads only files that have been added or modified since the last sync.

Mirror Sync with --delete

aws s3 sync ./dist/ s3://my-website-bucket/ --delete

The --delete flag removes files from the destination that no longer exist in the source, creating an exact mirror. Use with caution — this will permanently delete S3 objects.

Selective Sync with Filters

aws s3 sync ./project/ s3://my-bucket/project/ \
  --exclude "*.tmp" \
  --exclude ".git/*" \
  --exclude "node_modules/*" \
  --include "*.js" \
  --include "*.css"

Filters are applied in order. First all files are included, then --exclude patterns remove files, then --include patterns add files back.

Dry Run Preview

aws s3 sync ./data/ s3://my-bucket/data/ --dryrun

The --dryrun flag shows what would be transferred without actually copying any files. Always use this before running with --delete on production data.

How Sync Determines Changes

S3 sync compares:

1. File size — different sizes trigger a transfer
2. Last modified time — newer local files are uploaded
3. It does NOT compare file content (checksums) by default