how to see the file which would be committed

Question

I have modified a file (or some files) in my directory, and I've used git add to stage some of the changes lines from the file, but not all the changed line.

I can use git diff --staged my-file to see the diff of what's changed. git diff --staged my-file ignores lines which were changed but not staged. Here is an example of the output of git diff --staged my-file

diff --git a/ens/cours/ens_cours_JN.csv b/ens/cours/ens_cours_JN.csv
index dcea574..ff33469 100644
--- a/ens/cours/ens_cours_JN.csv
+++ b/ens/cours/ens_cours_JN.csv
@@ -24,6 +24,7 @@ SCALIN_E;EPITA;préparation pédagogique;JN;ING1;2020-05-13;;False;True;PT4H;;
 SCALIN_E;EPITA;préparation pédagogique;JN;ING1;2020-05-20;;False;True;PT4H;;
 SCALIN_E;EPITA;préparation pédagogique;JN;ING1;2020-05-27;;False;True;PT4H;;
 SCALIN_E;EPITA;préparation pédagogique;JN;ING1;2020-06-03;;False;True;PT4H;;
+SCALIN_E;EPITA;préparation pédagogique;JN;ING1;2020-06-03;;False;True;PT4H;;commit this line
 THLR;EPITA;préparation pédagogique;JN;ING1;2020-07-20;;False;True;PT8H;;Recording TDs
 THLR;EPITA;préparation pédagogique;JN;ING1;2020-07-21;;False;True;PT8H;;Recording TDs
 THLR;EPITA;préparation pédagogique;JN;ING1;2020-07-22;;False;True;PT8H;;Recording TDs

Question: How can I generate the text of the file which would be committed? I'd like a check-in hook to eventually process that file before allowing the commit.

I suspect there is some simple incantation using git apply. However, a simple use of git apply produces the following diagnostic messages.

jnewton@Marcello cours % git diff --staged > ens_cours_JN.csv.patch
git diff --staged > ens_cours_JN.csv.patch
jnewton@Marcello cours % git apply ens_cours_JN.csv.patch
git apply ens_cours_JN.csv.patch
error: patch failed: ens/cours/ens_cours_JN.csv:24
error: ens/cours/ens_cours_JN.csv: patch does not apply

I have a solution that seems far too complicated.

1. generate the .patch file with git diff --staged > my-file.patch
2. save the original with cp my-file my-file.save
3. stash the changes with git stash save my-file
4. apply the patch with git apply my-file.patch
5. saved the desired result with cp my-file my-file.to-commit
6. restore the file to the pre-add state with mv my-file.save my-file
7. restore the fiel to the post-add state with git stash apply

Now, my file.to-commit is a copy of the fill which would be committed. Is this really the correct way to do this? It seems like I'm doing too much work.

Answer 1

You can take advantage of the :[<n>:]<path> construct to access to corresponding staged blob, and just do

git show :my-file

As described here :

:[<n>:]<path>, e.g. :0:README, :README
A colon, optionally followed by a stage number (0 to 3) and a colon, followed by a path, names a blob object in the index at the given path. A missing stage number (and the colon that follows it) names a stage 0 entry. During a merge, stage 1 is the common ancestor, stage 2 is the target branch’s version (typically the current branch), and stage 3 is the version from the branch which is being merged.

So git show :0:path/to/file or the shorter git show :path/to/file outputs the full staged version of the file.

Answer 2

For individual files, you can use git checkout-index :

# will overwrite 'that/file.txt' in place with the indexed version :
git checkout-index -- that/file.txt

# will create '/tmp/that/file.txt' :
git checkout-index --prefix=/tmp/ -- that/file.txt

Or you can mention explicitly the target --work-tree and --git-dir (options to git itself, not to its subcommand), and use git checkout :

git --git-dir=.git/ --work-tree=/tmp/myindex/ checkout -- .

---

Also worth mentioning :

• git stash -k will store your unstaged changes in a stash, preserve the index as it is and restore the content on disk to the indexed content
  you can then use git stash apply or git stash pop to get your unstaged changes back
• git checkout -- some/path will get the version from the index on disk (it will overwrite your local changes, use git stash -k or some other way to checkout your files if that's a problem)