Patch option in Git

29 June 2018

git , development

Patch

I have (almost) always been using git add -p or git add --patch.

This option allows you to interactively select which pieces of your changes to be added to the index. (Before writing this article, I was even convinced that -p stood for “partial”…)

This is very convenient to 1) make sure that you will not commit unwanted code, 2) partially save your changes without losing your work in progress.

I recently learned that this -p/--patch option is available for the checkout and the reset commands as well!

With these, we can respectively get rid of only part of our changes and remove pieces of code from the index.

Examples

Here is the example setup: I created a git repository in which I have committed a single file.

$ ls
example.txt

$ git status
On branch master
nothing to commit, working tree clean

$ cat example.txt
This is an exmple fil.
Containing multiple lines.

Very interest.

git add -p

Let’s say we edited our example.txt to add some content that we would like to commit.

$ cat example.txt
File

This is an example file.
Containing multiple lines.

Very interesting.

We did multiple things here: added a “title” an corrected several words. To keep things clean, we would like to make one commit for each one of these changes.

Here is how I would use -p:

• use “split” and “edit” to keep only the changes related to correct the words
• commit those changes
• verify that only the title is added
• commit this change

git checkout -p

Similarly, we can use git checkout -p to discard part of the changes that we have performed on a file. Let’s say we have edited our example.txt to add a line in the middle and modify the last one.

$ cat example.txt
File

This is an example file.
Bwaaaaaaaaaah!
Containing multiple lines.

Some very interesting changes.

Then, we can use git checkout -p to get rid of the rubbish line that has been introduced:

• use split
• get rid of the first part
• but not of the second

git reset -p

At last, we added our previous change to the index, as well as our edit of the third line, containing a grammar error…

$ cat example.txt
File

This is a example marvelous file.
Containing multiple lines.

Some very interesting changes.

$ git add example.txt

We changed our mind: this is not OK to commit broken English.

Let’s use git reset -p to remove the unwanted content from the index so we can commit peacefully:

• split the content
• reset the first part
• keep the second one
• commit

Et voilà!