Ever wonder what the six-digit file modes are in a Git commit? The mysterious 100644 and 100755 modes?

diff --git a/foo/bar.py b/foo/bar.py
old mode 100644
new mode 100755
index b829edea4..ee6bda024
--- a/foo/bar.py
+++ b/foo/bar.py
@@ -1,3 +1,4 @@
...

I had made foo/bar.py executable by using chmod +x and adding a #!/usr/bin/env python shebang. The last three digits are obviously the same octal digits that you can use with chmod. But what’s that 100 prefix?

The ex­pla­na­tion can be found in a Stack­Over­flow answer:

100644₈  regular file (non-executable)  S_IFREG | S_IRUSR | S_IWUSR
                                                | S_IRGRP | S_IWGRP
                                                | S_IROTH | S_IWOTH
100755₈  regular file (executable)      S_IFREG | S_IRUSR | S_IWUSR | S_IXUSR
                                                | S_IRGRP | S_IWGRP | S_IXGRP
                                                | S_IROTH | S_IWOTH | S_IXOTH
120000₈  symbolic link                  S_IFLNK
160000₈  gitlink                        No stat(2) equivalent
040000₈  directory                      S_IFDIR

A gitlink is used to refer to a commit in another repos­i­to­ry; it’s how submodules are im­ple­ment­ed.