"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"
