Extended Attributes
From OS X Scientific Computing
Contents |
HFS+ and metadata
One of the advantages of the HFS+ (and the zfs) is its support for arbitrarily extensible metadata attributes. Put simply, you can associate pretty much any information that suits you by using extended attributes.
A subset of possible attributes includes those used by SpotLight and its command-line counterpart, mdfind, to do quick searches.
The unix command xattr is our key to unlocking this potential.
A simple example
Let's start with a simple example. I downloaded a pdf from Science to my desktop to read. As is typical, it gets a less than descriptive filename:
% ls -lF *.pdf -rw-r--r--@ 1 wgscott staff 222K Feb 27 17:08 1229.pdf
The @ tells you that the file, despite just having been downloaded, has some form of extended attribute associated with it. Let's have a look:
% xattr -l 1229.pdf com.apple.metadata:kMDItemWhereFroms: 0000 62 70 6C 69 73 74 30 30 A1 01 5F 10 37 68 74 74 bplist00.._.7htt 0010 70 3A 2F 2F 77 77 77 2E 73 63 69 65 6E 63 65 6D p://www.sciencem 0020 61 67 2E 6F 72 67 2F 63 67 69 2F 72 65 70 72 69 ag.org/cgi/repri 0030 6E 74 2F 33 32 33 2F 35 39 31 38 2F 31 32 32 39 nt/323/5918/1229 0040 2E 70 64 66 08 0A 00 00 00 00 00 00 01 01 00 00 .pdf............ 0050 00 00 00 00 00 02 00 00 00 00 00 00 00 00 00 00 ................ 0060 00 00 00 00 00 44 .....D com.apple.quarantine: 0000;49a88e87;Safari.app;|com.apple.Safari
In addition to a fair amount of jibberish, we find the URL for the pdf from Science, and the fact is was downloaded by Safari.
We can make this more useful. For example, I can add in some attribute that tells me who the senior author is:
% xattr -w "Senior Author" "Gerald F. Joyce" 1229.pdf
zsh-% xattr -l 1229.pdf
Senior Author: Gerald F. Joyce
com.apple.metadata:kMDItemWhereFroms:
0000 62 70 6C 69 73 74 30 30 A1 01 5F 10 37 68 74 74 bplist00.._.7htt...
So you can see how that might be useful.
Similarly, I can delete information:
% xattr -d "Senior Author" 1229.pdf
% xattr -l 1229.pdf
com.apple.metadata:kMDItemWhereFroms:
0000 62 70 6C 69 73 74 30 30 A1 01 5F 10 37 68 74 74 bplist00.._.7htt...
% xattr -d "com.apple.metadata:kMDItemWhereFroms" 1229.pdf
% xattr -l 1229.pdf
<prints nothing>
Use with Spotlight and mdfind
Unfortunately, Spotlight and mdfind won't make use of our "Senior Author" extended attribute that we just made up. We are restricted to the kMDItem types that Spotlight is designed to make use of. mdls comes to our rescue:
% mdls 1229.pdf | grep Author <prints nothing>
% xattr -w "com.apple.metadata:kMDItemAuthors" "Joyce" 1229.pdf % mdls 1229.pdf | grep Author kMDItemAuthors = "Joyce"
Now if you use spotlight or the Finder's "find" GUI, you can find this file by looking for an author named Joyce.
A convenient shell script for changing Spotlight metadata on a file
I wrote a shell script called mdattr
% mdattr
mdattr is a function that can create, delete or revise Apple-style metadata attributes
of the form kMDItemAttributeName and assign each to a value of AttributeValue
eg: kMDItemKind = "Document"
Usage: mdattr <AttributeName> <AttributeValue> filename(s)
mdattr -d <AttributeName> filename(s)
Here is a link to the zsh shell script mdattr and its associated _mdattr zsh completion function.
It works like this:
% mdattr -d Authors 1229.pdf The metadata of the file 1229.pdf have been modified as follows: % mdls 1229.pdf | grep Author % mdattr Authors Joyce 1229.pdf The metadata of the file 1229.pdf have been modified as follows: kMDItemAuthors = "Joyce"
