file-syncer

Utility for syncing files -- mainly live-sync of folder symlinks to hardlinks. (for Docker and other stubborn tools)

Install

Note: Installation is not necessary if using npx file-syncer to run.

yarn add file-syncer # or: npm i file-syncer

General usage

// basic
npx file-syncer --from XXX YYY ZZZ --to HardLinked [--watch] [--async] [--autoKill] [etc...]

// node-modules
npx file-syncer --from node_modules/XXX "node_modules/spa ces" --to HardLinked

Run npx file-syncer --help for more details. (or check the option list in the source code)

Docker example

From: https://stackoverflow.com/a/68765508

Given the existing directory structure:

parent_dir
	- common_files
		- file.txt
	- my-app
		- Dockerfile
		- common_files -> symlink to ../common_files

Basic usage:

cd parent_dir

// starts live-sync of files under "common_files" to "my-app/HardLinked/common_files"
npx file-syncer --from common_files --to my-app/HardLinked

Then in your Dockerfile:

[regular commands here...]

# have docker copy/overlay the HardLinked folder's contents (common_files) into my-app itself
COPY HardLinked /

Q/A

• How is this better than just copying parent_dir/common-files to parent_dir/my-app/common_files before Docker runs?

That would mean giving up the regular symlink, which would be a loss, since symlinks are helpful and work fine with most tools. For example, it would mean you can't see/edit the source files of common_files from the in-my-app copy, which has some drawbacks. (see below)

• How is this better than copying parent_dir/common-files to parent_dir/my-app/common_files_Copy before Docker runs, then having Docker copy that over to parent_dir/my-app/common_files at build time?

There are two advantages:

1. file-syncer does not "copy" the files in the regular sense. Rather, it creates hard links from the source folder's files. This means that if you edit the files under parent_dir/my-app/HardLinked/common_files, the files under parent_dir/common_files are instantly updated, and vice-versa, because they reference the same file/inode. (this can be helpful for debugging purposes and cross-project editing [especially if the folders you are syncing are symlinked node-modules that you're actively editing], and ensures that your version of the files is always in-sync/identical-to the source files)
2. Because file-syncer only updates the hard-link files for the exact files that get changed, file-watcher tools like Tilt or Skaffold detect changes for the minimal set of files, which can mean faster live-update-push times than you'd get with a basic "copy whole folder on file change" tool would.

• How is this better than a regular file-sync tool like Syncthing?

Some of those tools may be usable, but most have issues of one kind or another. The most common one is that the tool either cannot produce hard-links of existing files, or it's unable to "push an update" for a file that is already hard-linked (since hard-linked files do not notify file-watchers of their changes automatically, if the edited-at and watched-at paths differ). Another is that many of these sync tools are not designed for instant responding, and/or do not have run flags that make them easy to use in restricted build tools. (eg. for Tilt, the --async flag of file-syncer enables it to be used in a local(...) invokation in the project's Tiltfile)

Tasks

1. Add better documentation.