How git log -S and git log -G Work

Overview

This repository shows how to use git log -S and git log -G.

I referred to the following articles:

• [1] git log -S
• [2] git log -G
• [3] diffcore-pickaxe: For Detecting Addition/Deletion of Specified String
• [4] Adding git log -S to your code detective toolbox

The Example File

The file some-state-parks-in-pa.md was created and edited in the following commits (git log --no-show-signature -- ":(exclude)README.md"(*)):

commit cc7fe6f34fbda843fea68d2efb4d6b023a99fc3b (HEAD -> master)
Author: Yaobin Wen <robin.wyb@gmail.com>
Date:   Thu Oct 7 19:30:21 2021 -0400

    Remove `Lake Erie State Park` because it's not in PA

commit 950b473e93413f5114a92cabaed8618065a174f6
Author: Yaobin Wen <robin.wyb@gmail.com>
Date:   Thu Oct 7 19:29:12 2021 -0400

    Reorder the state parks alphabetically

commit 5d018d6020fa3b6800b7d04c649c96b42f706610
Author: Yaobin Wen <robin.wyb@gmail.com>
Date:   Thu Oct 7 19:27:59 2021 -0400

    Add more state parks in Pennsylvania

commit 0b3c32d34bf3fa188650cadbeea249cb924bdd31
Author: Yaobin Wen <robin.wyb@gmail.com>
Date:   Thu Oct 7 19:23:45 2021 -0400

    Add some state parks in Pennsylvania

    (Just a few of them. Not all of them.)

Note (*): I need to use the path spec :(exclude)README.md to exclude the commits that include this README.md. Otherwise these commits would also show up in the results.

Among all the state parks that were added to this list, the "Lake Erie State Park" was used to show how these two git log options work. Note that "Lake Erie State Park" no longer exists in the current file because the commit cc7fe6f removed it (as it's not a park in PA but in NY).

How to Use git log -S

As [1] says:

Look for differences that change the number of occurrences of the specified string (i.e. addition/deletion) in a file. Intended for the scripter's use.

What it means is: git log -S<word> only lists the commits in which the keyword <word> is added or deleted in a file. If <word> is moved inside the same file in a commit, this commit won't be listed. This is useful when you want to figure out which commit introduces or gets rid of something.

[3] explains it as follows:

"-S<block of text>" detects filepairs whose preimage and postimage have different number of occurrences of the specified block of text. By definition, it will not detect in-file moves. Also, when a changeset moves a file wholesale without affecting the interesting string, diffcore-rename kicks in as usual, and -S omits the filepair (since the number of occurrences of that string didn't change in that rename-detected filepair).

For example, running git log --no-show-signature -SErie --stat -- ':(exclude)README.md' in this repository lists the following commits:

commit cc7fe6f34fbda843fea68d2efb4d6b023a99fc3b
Author: Yaobin Wen <robin.wyb@gmail.com>
Date:   Thu Oct 7 19:30:21 2021 -0400

    Remove `Lake Erie State Park` because it's not in PA

 some-state-parks-in-pa.md | 1 -
 1 file changed, 1 deletion(-)

commit 5d018d6020fa3b6800b7d04c649c96b42f706610
Author: Yaobin Wen <robin.wyb@gmail.com>
Date:   Thu Oct 7 19:27:59 2021 -0400

    Add more state parks in Pennsylvania

 some-state-parks-in-pa.md | 6 ++++++
 1 file changed, 6 insertions(+)

The commit 5d018d6 is listed because the keyword Erie is added in this commit. In other words, the number of occurrences of "Erie" in the file "some-state-parks-in-pa.md" changes from 0 to 1 in this commit.

The commit cc7fe6f is listed because the keyword Erie is deleted in this commit. In other words, the number of occurrences of "Erie" in the file "some-state-parks-in-pa.md" changes from 1 to 0 in this commit.

Although the keyword Erie also appears in the commit 950b473 ("Reorder the state parks alphabetically"), this commit is not listed because the keyword "Erie" was simply moved from one line to another line thus the number of occurrences of "Erie" in the file "some-state-parks-in-pa.md" remained unchanged.

How to Use git log -G

As [2] says:

Look for differences whose patch text contains added/removed lines that match <regex>.

What it means is: git log -G<regex> lists the commits as long as the content changes include the keyword that matches <regex>. Personally, when I want to look for something but can only vaguely recall some keywords, I use this command to do a blanket search to give me an initial scope to look further. I then examine this initial search result (which could be long) and gradually narrow down the scope until I find what I'm looking for.

For example, running git log --no-show-signature -GErie --stat -- ':(exclude)README.md' in this repository lists the following commits:

commit cc7fe6f34fbda843fea68d2efb4d6b023a99fc3b (HEAD -> master)
Author: Yaobin Wen <robin.wyb@gmail.com>
Date:   Thu Oct 7 19:30:21 2021 -0400

    Remove `Lake Erie State Park` because it's not in PA

 some-state-parks-in-pa.md | 1 -
 1 file changed, 1 deletion(-)

commit 950b473e93413f5114a92cabaed8618065a174f6
Author: Yaobin Wen <robin.wyb@gmail.com>
Date:   Thu Oct 7 19:29:12 2021 -0400

    Reorder the state parks alphabetically

 some-state-parks-in-pa.md | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

commit 5d018d6020fa3b6800b7d04c649c96b42f706610
Author: Yaobin Wen <robin.wyb@gmail.com>
Date:   Thu Oct 7 19:27:59 2021 -0400

    Add more state parks in Pennsylvania

 some-state-parks-in-pa.md | 6 ++++++
 1 file changed, 6 insertions(+)

Note that it lists cc7fe6f and 5d018d6 which are also listed by git log -S but for a different reason: the keyword "Erie" occurs in the diff output of the two commits:

commit cc7fe6f34fbda843fea68d2efb4d6b023a99fc3b (HEAD -> master)
Author: Yaobin Wen <robin.wyb@gmail.com>
Date:   Thu Oct 7 19:30:21 2021 -0400

    Remove `Lake Erie State Park` because it's not in PA

diff --git a/some-state-parks-in-pa.md b/some-state-parks-in-pa.md
index 87139fe..1bb06fc 100644
--- a/some-state-parks-in-pa.md
+++ b/some-state-parks-in-pa.md
@@ -3,7 +3,6 @@
 Cherry Springs State Park
 Keystone State Park
 Kinzua Bridge State Park
-Lake Erie State Park
 Leonard Harrison State Park
 Linn Run State Park
 McConnells Mill State Park

commit 5d018d6020fa3b6800b7d04c649c96b42f706610
Author: Yaobin Wen <robin.wyb@gmail.com>
Date:   Thu Oct 7 19:27:59 2021 -0400

    Add more state parks in Pennsylvania

diff --git a/some-state-parks-in-pa.md b/some-state-parks-in-pa.md
index 4001bf4..41eb83f 100644
--- a/some-state-parks-in-pa.md
+++ b/some-state-parks-in-pa.md
@@ -6,3 +6,9 @@ Keystone State Park
 McConnells Mill State Park
 Moraine State Park
 Ohiopyle State Park
+Linn Run State Park
+Kinzua Bridge State Park
+Cherry Springs State Park
+Leonard Harrison State Park
+Presque Isle State Park
+Lake Erie State Park

The commit 950b473 is also listed because its diff output contains the keyword "Erie":

commit 950b473e93413f5114a92cabaed8618065a174f6
Author: Yaobin Wen <robin.wyb@gmail.com>
Date:   Thu Oct 7 19:29:12 2021 -0400

    Reorder the state parks alphabetically

diff --git a/some-state-parks-in-pa.md b/some-state-parks-in-pa.md
index 41eb83f..87139fe 100644
--- a/some-state-parks-in-pa.md
+++ b/some-state-parks-in-pa.md
@@ -1,14 +1,14 @@
 # Some State Parks in Pennsylvania

-Point State Park
-Raccoon Creek State Park
+Cherry Springs State Park
 Keystone State Park
+Kinzua Bridge State Park
+Lake Erie State Park
+Leonard Harrison State Park
+Linn Run State Park
 McConnells Mill State Park
 Moraine State Park
 Ohiopyle State Park
-Linn Run State Park
-Kinzua Bridge State Park
-Cherry Springs State Park
-Leonard Harrison State Park
 Presque Isle State Park
-Lake Erie State Park
+Point State Park
+Raccoon Creek State Park