macos - Why do I get files like ._foo in my tarball on OS X?

When I tar certain files in OS X:

tar cvf foo.tar foo

It produces an extra file ._foo in the tarball:

./._foo
foo

which only shows up if I extract it on a non-Mac operating system. But ._foo doesn't exist on my file system! What's going on? How can I get rid of it?

Answer

OS X's tar uses the AppleDouble format to store extended attributes and ACLs.

$ touch file1 file2 file3
$ xattr -w key value file1
$ chmod +a 'admin allow delete' file2
$ ls -le@ *
-rw-r--r--@ 1 lauri  staff  0 May 25 07:09 file1
    key 5
-rw-r--r--+ 1 lauri  staff  0 May 25 07:09 file2
 0: group:admin allow delete
-rw-r--r--  1 lauri  staff  0 May 25 07:09 file3
$ tar -cf 1.tar *
$ tar -tf 1.tar
./._file1
file1
./._file2
file2
file3

OS X's tar also knows how to convert the ._ members back to native formats, but the ._ files are usually kept when archives are extracted on other platforms. You can tell tar to not include the metadata by setting COPYFILE_DISABLE to some value:

$ COPYFILE_DISABLE=1 tar -cf 2.tar file*    
$ tar -tf 2.tar
file1
file2
file3

• The copyfile functions are described in man copyfile


• ls -l@ shows the keys and sizes of extended attributes, ls -le prints ACLs


• xattr -l lists the keys and values of extended attributes


• xattr -c clears all extended attributes (-d can't be used alone)


• chmod -N deletes ACLs


• Zip files created on OS X use a __MACOSX folder to store similar metadata

Information stored as extended attributes:

• Resource forks (resource forks have been extended attributes since 10.4)
  
  
  
  • Custom icons set in Finder and the images of Icon\r files
  
  
  • Metadata in PSD files
  
  
  • Objects stored in scpt files, AppleScript Editor window state, descriptions of scripts
  
  
  
  


• Information about aliases (aliases stop working if extended attributes are removed)


• Quarantine status or source URLs of files downloaded from the internet


• Spotlight comments


• Encoding of files saved with TextEdit


• Caret position of files opened with TextMate


• Skim notes