What are the finest-grained operations on the .git/index file? -

certain git commands, such git add ... modify state of .git/index file, imagine such operations may thought sequence of smaller operations. example,

git add foo bar 

may decomposable into

git add foo git add bar 

in fact, there may (for know) commands above may broken down "finer-grained" (but still ".git/index-modifying") git commands.

my question is, finest-grained modifications can performed on .git/index file using git commands? iow, "atomic" command-line operations on .git/index?

i imagine these commands plumbing commands. also, if had guess, i'd imagine gid add <file> , git rm --cached <file> approximate 2 of operations. on other hand, git mv <old> <new> may composite of "atomic" deletion followed "atomic" addition...

edit: motivation question following.

the aspects of git's behavior find confusing result in modification of .git/index file. else git bit more open inspection. .git/index file, however, pretty opaque.

for reason, i'd better understanding of how .git/index file gets modified.

the main index composed of several index entries, , collection of entries represents contents of next commit. when git add file, adds (or updates) index entry given filename.

the index entry contains several fields both related staged change (the object id , mode) related contents in working directory (the timestamp, file size, etc). can see both of these git ls-files command:

c:\temp\testrepo>git add file.txt  c:\temp\testrepo>git ls-files --stage 100644 9b72baa6b025e0eb1dd8f1fd23bf5d5515012cd6 0       file.txt  c:\temp\testrepo>git ls-files --debug file.txt   ctime: 1391645170:0   mtime: 1391645172:0   dev: 0        ino: 0   uid: 0        gid: 0   size: 7       flags: 0 

generally speaking, 1 uses git add , git rm commands add, update , remove entries main index. however, commands update parts of index entries. example, git checkout , git status write working directory cache contents of index entry. example:

c:\temp\testrepo>touch file.txt  c:\temp\testrepo>git status # on branch master nothing commit, working directory clean  c:\temp\testrepo>git ls-files --debug file.txt   ctime: 1391645170:0   mtime: 1391645458:0   dev: 0        ino: 0   uid: 0        gid: 0   size: 7       flags: 0 

(note updated mtime (modified time) field in index entry).

the smallest single change can make index in single command flip assume-unchanged bit on file:

c:\temp\testrepo>git update-index --assume-unchanged file.txt  c:\temp\testrepo>git ls-files --debug file.txt   ctime: 1391645170:0   mtime: 1391645458:0   dev: 0        ino: 0   uid: 0        gid: 0   size: 7       flags: 8000 

(note change flags field 0x0000 0x8000, flipping single bit.)