# How to extract a single file with its history from a git repository
These steps show two less common interactions with git to extract a single file which is inside a subfolder from a git repository. These steps essentially reduce the repository to just the desired files and should performed on a copy of the original repository (1.).

First the repository is reduced to just the subfolder containing the files in question using `git filter-branch --subdirectory-filter` (2.) which is a useful step by itself if just a subfolder needs to be extracted. This step moves the desired files to the top level of the repository.

Finally all remaining files are listed using `git ls`, the files to keep are removed from that using `grep -v` and the resulting list is passed to `git rm` which is invoked by `git filter-branch --index-filter` (3.). A bit convoluted but it does the trick.



### 1. copy the repository to extract the file from and go to the desired branch
```sh
➜  /tmp  git clone git@github.com:ssp/pazpar2.git pazpar2g
Cloning into pazpar2g...
remote: Counting objects: 14950, done.
remote: Compressing objects: 100% (4092/4092), done.
remote: Total 14950 (delta 10938), reused 14719 (delta 10707)
Receiving objects: 100% (14950/14950), 3.30 MiB | 1.60 MiB/s, done.
Resolving deltas: 100% (10938/10938), done.
➜  /tmp  cd pazpar2g
➜  pazpar2g git:(master) git checkout ssp
Branch ssp set up to track remote branch ssp from origin.
Switched to a new branch 'ssp'
```

### 2. reduce the repository to just the subfolder »etc« which contains the interesting file(s)
```sh
➜  pazpar2g git:(ssp) git filter-branch --prune-empty --subdirectory-filter etc -- --all
Rewrite b3d4f2a89fdee662fb43122990fc28aa2c08bee5 (558/558)
Ref 'refs/heads/master' was rewritten
Ref 'refs/heads/ssp' was rewritten
Ref 'refs/remotes/origin/master' was rewritten
WARNING: Ref 'refs/remotes/origin/master' is unchanged
Ref 'refs/remotes/origin/ssp' was rewritten
Ref 'refs/tags/wildcard-matching' was rewritten
```

### 3. remove all files other than the ones you want to keep (tmarc.xsl, check-pazpar2.xsl)
```sh
➜  pazpar2g git:(ssp)  git filter-branch -f --prune-empty --index-filter 'git rm --cached --ignore-unmatch $(git ls-files | grep -v "tmarc.xsl\|check-pazpar2.sh")'
Rewrite f06a533323ad8257efa9e52c45ad2e22e2b09b1c (1/558)rm 'bibs.pz'
… [lenghty output omitted]
Ref 'refs/heads/ssp' was rewritten
➜  pazpar2g git:(ssp) ls -l
total 48
-rwxrwxr-x 1 ssp ssp  1359 2012-01-23 14:03 check-pazpar2.sh
-rw-rw-r-- 1 ssp ssp 41078 2012-01-23 14:03 tmarc.xsl
```