Copy a directory tree (preserving permissions) in Go.

copy.go

/* MIT License
 *
 * Copyright (c) 2017 Roland Singer [roland.singer@desertbit.com]
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */

// CopyFile copies the contents of the file named src to the file named
// by dst. The file will be created if it does not already exist. If the
// destination file exists, all it's contents will be replaced by the contents
// of the source file. The file mode will be copied from the source and
// the copied data is synced/flushed to stable storage.
func CopyFile(src, dst string) (err error) {
	in, err := os.Open(src)
	if err != nil {
		return
	}
	defer in.Close()

	out, err := os.Create(dst)
	if err != nil {
		return
	}
	defer func() {
		if e := out.Close(); e != nil {
			err = e
		}
	}()

	_, err = io.Copy(out, in)
	if err != nil {
		return
	}

	err = out.Sync()
	if err != nil {
		return
	}

	si, err := os.Stat(src)
	if err != nil {
		return
	}
	err = os.Chmod(dst, si.Mode())
	if err != nil {
		return
	}

	return
}

// CopyDir recursively copies a directory tree, attempting to preserve permissions.
// Source directory must exist, destination directory must *not* exist.
// Symlinks are ignored and skipped.
func CopyDir(src string, dst string) (err error) {
	src = filepath.Clean(src)
	dst = filepath.Clean(dst)

	si, err := os.Stat(src)
	if err != nil {
		return err
	}
	if !si.IsDir() {
		return fmt.Errorf("source is not a directory")
	}

	_, err = os.Stat(dst)
	if err != nil && !os.IsNotExist(err) {
		return
	}
	if err == nil {
		return fmt.Errorf("destination already exists")
	}

	err = os.MkdirAll(dst, si.Mode())
	if err != nil {
		return
	}

	entries, err := ioutil.ReadDir(src)
	if err != nil {
		return
	}

	for _, entry := range entries {
		srcPath := filepath.Join(src, entry.Name())
		dstPath := filepath.Join(dst, entry.Name())

		if entry.IsDir() {
			err = CopyDir(srcPath, dstPath)
			if err != nil {
				return
			}
		} else {
			// Skip symlinks.
			if entry.Mode()&os.ModeSymlink != 0 {
				continue
			}

			err = CopyFile(srcPath, dstPath)
			if err != nil {
				return
			}
		}
	}

	return
}

@m4ng0squ4sh thank you :)

@m4ng0squ4sh im very new with golang and have a question to you => sometimes you return void in CopyDir and CopyFile ! Can you explain exactly what that does?

@SerkanSipahi: Since I'm using this snippet myself (thanks!): If you use "return" without a value, go will return the variable specified in the function definition (err in this case). This also declares err in the function. This is why you see no "var err error" inside of the function, although it is the target of multiple assignments.

Edit: So, in line 55 you could emit the err and just write "return" to make it more consistent.

Awesome! Works like a charm, thank you!

@r0l1 mind adding a license statement/clause?

Very nice! Thanks for your work!

Thanks for the feedback!

@mikeatlas Added a license.

It works perfectly. Thank you for your work and sharing it.

3Q~

Do you know if it is possible to make it skip over certain files? For example I am trying to copy a whole user folder, however, it starts copying App data, which is not necessary in the transfer

thank you!

Hi and thanks for this nice utility! I have incorporated it into my little goasciidoc project to be used in testing. I hoper that is ok.

Cheers,
Mario

Yes, thanks for this -- handy.

This is great! Thank you!

How does this differ from https://github.com/golang/dep/blob/v0.5.4/internal/fs/fs.go#L356, @r0l1?

@quackduck seams to be quite similar. However there are some minor differences (LStat, Sync, wrapped error handling).

Are the changes better? Do they improve anything?

The error handling is a nice addition with the extra prefix. Mind sharing a revision?
For sync and Lstat please check the docs. It depends on what you need. Performance is better, if you remove the sync call.

What do you mean by sharing a revision? I don't know gists so well. Do you really need sync? Also, this is the reasoning for using Lstat from their documentation:

// We use os.Lstat() here to ensure we don't fall in a loop where a symlink
// actually links to a one of its parent directories.