Applying an edited Git patch in Vim
by Alex Harvey
From time to time I have needed to take a patch in unified format and then edit it before git applying it to a file. I discovered some tricks today when doing this and intend to document these in this post.
Patch problem
By way of setting up an example, imagine we start with the following YAML file:
---
series: "Star Trek: The Next Generation"
episodes:
-
Name: "The Naked Now"
StarDate: 41209.2
AirDate: "October 5, 1987"
-
Name: "Code of Honor"
StarDate: 41235.25
AirDate: "October 12, 1987"
-
Name: "The Last Outpost"
StarDate: 41386.4
AirDate: "October 19, 1987"
-
Name: "Where No One Has Gone Before"
StarDate: 41263.1
AirDate: "October 26, 1987"
-
Name: "Lonely Among Us"
StarDate: 41249.3
AirDate: "November 2, 1987"
creator: "Gene Roddenberry"
Someone then emails a patch that is intended to update this file:
commit cee8d7f5067c10d7c63cdc0856ffa27e949bbd7f
Author: Jean-luc Picard
Date: Fri Aug 17 00:57:20 2018 +1000
Please update this file!
diff --git a/star_trek.yml b/star_trek.yml
index 51a9f7b..33d8b12 100644
--- a/star_trek.yml
+++ b/star_trek.yml
@@ -3,6 +3,10 @@
series: "Star Trek: The Next Generation"
episodes:
+ -
+ Name: "Encounter at Farpoint"
+ StarDate: 41153.7
+ AirDate: "September 28, 1987"
-
Name: "The Naked Now"
StarDate: 41209.2
@@ -23,5 +27,13 @@ episodes:
Name: "Lonely Among Us"
StarDate: 41249.3
AirDate: "November 2, 1987"
+ -
+ Name: "Justice"
+ StarDate: 41255.6
+ AirDate: "November 9, 1987"
+ -
+ Name: "The Battle"
+ StarDate: 41723.9
+ AirDate: "November 16, 1987"
creator: "Gene Roddenberry"
In the mean time, someone else has changed the file to:
---
studio: "Paramount"
series: "Star Trek: The Next Generation"
episodes:
SEASON1:
-
Name: "The Naked Now"
StarDate: 41209.2
AirDate: "October 5, 1987"
-
Name: "Code of Honor"
StarDate: 41235.25
AirDate: "October 12, 1987"
-
Name: "The Last Outpost"
StarDate: 41386.4
AirDate: "October 19, 1987"
-
Name: "Where No One Has Gone Before"
StarDate: 41263.1
AirDate: "October 26, 1987"
-
Name: "Lonely Among Us"
StarDate: 41249.3
AirDate: "November 2, 1987"
creator: "Gene Roddenberry"
I now want to apply the patch that was emailed to the current version of the file, but of course it won’t apply:
$ git apply patch
error: patch failed: star_trek.yml:3
error: star_trek.yml: patch does not apply
Editing the patch in vim
Problem 1: Lines shifted
The first problem is that lines have shifted.
This means that 3, 23 and 27 are incorrect now in @@ -3,6 +3,10 @@ and @@ -23,5 +27,13 @@.
To find out what to change the first one to, I used grep -n on the first line of context in the first hunk:
$ grep -n 'series: "Star Trek: The Next Generation"' star_trek.yml
4:series: "Star Trek: The Next Generation"
So I change the first hunk’s header to @@ -4,6 +4,10 @@. I’ll need to do the second one later as step 2 will introduce a complicating factor.
The first hunk is now:
@@ -4,6 +4,10 @@
series: "Star Trek: The Next Generation"
episodes:
+ -
+ Name: "Encounter at Farpoint"
+ StarDate: 41153.7
+ AirDate: "September 28, 1987"
-
Name: "The Naked Now"
StarDate: 41209.2
Problem 2: Lines context changed
The second problem is the context around the first hunk changed. A line SEASON1: was added after episodes, so I need to manually insert that line into the patch. The first hunk becomes:
@@ -4,6 +4,10 @@
series: "Star Trek: The Next Generation"
episodes:
SEASON1:
+ -
+ Name: "Encounter at Farpoint"
+ StarDate: 41153.7
+ AirDate: "September 28, 1987"
-
Name: "The Naked Now"
StarDate: 41209.2
Problem 3: Indentation changed
Having inserted a key SEASON1 inside the episodes hash, I need to then correct the indentation. I can fix this by selecting the lines in visual mode and then running the following vim editor command:
:'<,'>s/^\([^@]\)/\1 /
I do this on both hunks. They become:
@@ -4,6 +4,10 @@
series: "Star Trek: The Next Generation"
episodes:
SEASON1:
+ -
+ Name: "Encounter at Farpoint"
+ StarDate: 41153.7
+ AirDate: "September 28, 1987"
-
Name: "The Naked Now"
StarDate: 41209.2
@@ -23,5 +27,13 @@ episodes:
Name: "Lonely Among Us"
StarDate: 41249.3
AirDate: "November 2, 1987"
+ -
+ Name: "Justice"
+ StarDate: 41255.6
+ AirDate: "November 9, 1987"
+ -
+ Name: "The Battle"
+ StarDate: 41723.9
+ AirDate: "November 16, 1987"
creator: "Gene Roddenberry"
Problem 4: Before and after line counts changed
The next problem is the before and after line counts changed. These are 4 and 10 in the first hunk and 5 and 13 in the second hunk.
To find out the new counts, it’s necessary to know:
before lines = unchanged lines + deleted lines
And:
after lines = unchanged lines + added lines
I can get the first by selecting all the lines in visual mode and then:
:'<,'>w ! grep -v "^-" | wc -
or:
:'<,'>w ! grep -v "^+" | wc -
The first hunk is the only one that changes because it’s the one where I edited the context by adding the SEASON1 line. I find that @@ -4,6 +4,10 @@ becomes @@ -4,7 +4,11 @@.
Problem 5: Lines shifted of second hunk
Now the second hunk has shifted both by the 1 line that the first hunk was shifted by, but also by the additional SEASON1 line I inserted. So I add 2 to each of the markers: @@ -23,5 +27,13 @@ becomes @@ -25,5 +29,13 @@.
Final patch
The final patch now applies and it looks like:
--- a/star_trek.yml
+++ b/star_trek.yml
@@ -4,7 +4,11 @@
series: "Star Trek: The Next Generation"
episodes:
SEASON1:
+ -
+ Name: "Encounter at Farpoint"
+ StarDate: 41153.7
+ AirDate: "September 28, 1987"
-
Name: "The Naked Now"
StarDate: 41209.2
@@ -25,5 +29,13 @@ episodes:
Name: "Lonely Among Us"
StarDate: 41249.3
AirDate: "November 2, 1987"
+ -
+ Name: "Justice"
+ StarDate: 41255.6
+ AirDate: "November 9, 1987"
+ -
+ Name: "The Battle"
+ StarDate: 41723.9
+ AirDate: "November 16, 1987"
creator: "Gene Roddenberry"
And after git applying it, the final version of the file is:
---
studio: "Paramount"
series: "Star Trek: The Next Generation"
episodes:
SEASON1:
-
Name: "Encounter at Farpoint"
StarDate: 41153.7
AirDate: "September 28, 1987"
-
Name: "The Naked Now"
StarDate: 41209.2
AirDate: "October 5, 1987"
-
Name: "Code of Honor"
StarDate: 41235.25
AirDate: "October 12, 1987"
-
Name: "The Last Outpost"
StarDate: 41386.4
AirDate: "October 19, 1987"
-
Name: "Where No One Has Gone Before"
StarDate: 41263.1
AirDate: "October 26, 1987"
-
Name: "Lonely Among Us"
StarDate: 41249.3
AirDate: "November 2, 1987"
-
Name: "Justice"
StarDate: 41255.6
AirDate: "November 9, 1987"
-
Name: "The Battle"
StarDate: 41723.9
AirDate: "November 16, 1987"
creator: "Gene Roddenberry"