Kernel hacker's guide to git, updated

Just updated my KHGtG to include the latest goodies available in git-core, the Linux kernel standard SCM tool:

http://linux.yyz.us/git-howto.html

Several changes in git-core have made working with git a lot easier, so be sure to re-familiarize yourself with the development process.

Comments, corrections, and notes of omission welcome. This document mainly reflects my typical day-to-day git activities, and may not be very applicable outside of kernel work.

Jeff

Can you update the date on that page to reflect your latest updates? I was digging around with git yesterday and had a few surprises... like checking out the kernel and being told I modified a bunch of files I never touched.

- Dave

The KHGtG page is updated to September 29, 2005.

--
Don't be evil!!!

Awesome! Thanks. :-) I'm looking at cogito and darcs-git for working with v9fs and other goodies.

Dave

Am Donnerstag, 29. September 2005 13:03 schrieb Jeff Garzik:

Unfortunately, following the instructions to the letter produces this:
oliver@oenone:~/linux-2.6> git checkout
usage: read-tree (<sha> | -m <sha1> [<sha2> <sha3>])

Regards
Oliver

Yeah. See if you still have a .git/HEADS that symlinks to a valid place or not...?

jdl

Am Donnerstag, 29. September 2005 21:34 schrieb Jon Loeliger:

oliver@oenone:~/linux-2.6> ls -la .git/
insgesamt 14

drwxrwxr-x    6 oliver users  224 2005-09-29 21:06 .
drwxr-xr-x    3 oliver users   72 2005-09-29 20:45 ..
-rw-rw-r--    1 oliver users   19 2005-05-02 01:02 description
lrwxrwxrwx    1 oliver users   17 2005-09-29 21:06 HEAD -> refs/heads/master
-rw-------    1 oliver users   32 2005-09-29 21:06 index
drwxrwxr-x    2 oliver users  104 2005-09-11 21:41 info
drwxr-xr-x  260 oliver users 6240 2005-09-29 19:05 objects
drwxrwxr-x    4 oliver users   96 2005-05-02 02:15 refs
drwxr-xr-x    2 oliver users   72 2005-09-29 21:05 remotes
oliver@oenone:~/linux-2.6> ls -la .git/refs/
insgesamt 1
drwxrwxr-x  4 oliver users  96 2005-05-02 02:15 .
drwxrwxr-x  6 oliver users 224 2005-09-29 21:06 ..
drwxrwxr-x  2 oliver users  72 2005-09-29 19:05 heads
drwxrwxr-x  2 oliver users 600 2005-09-20 05:02 tags
oliver@oenone:~/linux-2.6> ls -la .git/refs/heads/
insgesamt 4
drwxrwxr-x  2 oliver users 72 2005-09-29 19:05 .
drwxrwxr-x  4 oliver users 96 2005-05-02 02:15 ..
-rw-rw-r--  1 oliver users 41 2005-09-29 19:05 master

oliver@oenone:~/linux-2.6> cat .git/refs/heads/master aa55a08687059aa169d10a313c41f238c2070488

Regards
Oliver

I wonder what that 32-byte small file called 'index' is doing there. If this is the kernel tree, its index file should weigh about 1.7MB if I recall correctly.

Am Freitag, 30. September 2005 09:37 schrieb Junio C Hamano:

I have no idea, but the instructions as stated didn't work for me. Can you replicate that?

Regards
Oliver

32 bytes is what you get for an empty index (it's a 12-byte header, followed by a 20-byte SHA1 of the contents).

In general, such an index file it should be 100% equivalent to not having an index file at all.

Linus

You wrote..

$ git clone rsync://rsync.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git linux-2.6
$ cd linux-2.6
$ rsync -a --verbose --stats --progress \
  rsync://rsync.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git/ \
  .git/

Could be just..

$ git clone rsync://rsync.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git linux-2.6
$ cd linux-2.6
$ git pull

Likewise, in the next section, git pull doesn't need an argument if pulling from the repo it cloned.

Dave

That is not actually the same. "git pull" for example will not download Linus' tags whilst the rsync would get everything.

Now that is definitely correct. (-:

Best regards,

Anton

Anton Altaparmakov <aia21 at cam.ac.uk> (replace at with @)
Unix Support, Computing Service, University of Cambridge, CB2 3QH, UK
Linux NTFS maintainer / IRC: #ntfs on irc.freenode.net
WWW: http://linux-ntfs.sf.net/ & http://www-stu.christs.cam.ac.uk/~aia21/

Ah. I didn't know this. Thanks.
Hmm, it'd be nice to have a shorthand 'not have to type the url, pull everything'. Something like 'git pull all'.

Dave

Something like this?

Except it's called "git fetch --all", and it's obviously totally untested.

Linus
--

@@ -5,6 +5,8 @@
 _x40='[0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f]'
 _x40="$_x40$_x40$_x40$_x40$_x40$_x40$_x40$_x40"
 
+all=
+tags=
 append=
 force=
 update_head_ok=

@@ -17,6 +19,12 @@ do
 	-f|--f|--fo|--for|--forc|--force)
 		force=t
 		;;
+	--tags)
+		tags=t
+		;;
+	--all)
+		all=t
+		;;
 	-u|--u|--up|--upd|--upda|--updat|--update|--update-|--update-h|\
 	--update-he|--update-hea|--update-head|--update-head-|\
 	--update-head-o|--update-head-ok)

@@ -158,7 +166,16 @@ case "$update_head_ok" in
 	;;
 esac
 
-for ref in $(get_remote_refs_for_fetch "$@")
+taglist=
+if [ "$tags$all" ]; then
+	pattern='/refs\/tags/'
+	if [ "$all" ]; then
+		pattern='/refs/'
+	fi
+	taglist=$(git-ls-remote "$remote" | awk "$pattern"' { print $2":"$2 }')
+fi
+
+for ref in $(get_remote_refs_for_fetch "$@" $taglist)
 do
     refs="$refs $ref"
 

Gaah. Using a new pine version, and it is back to corrupting whitespace.

Damn all mailer writers. Do they think email is just for grandma?

Linus

Of course - grandma uses pine, hackers use mutt ;)

*hides*

Best,
Elfyn

Elfyn McBratney
Gentoo Developer/Perl Team Lead
beu/irc.freenode.net                            http://dev.gentoo.org/~beu/
+------------O.o--------------------- http://dev.gentoo.org/~beu/pubkey.asc

PGP Key ID: 0x69DF17AD
PGP Key Fingerprint:
  DBD3 B756 ED58 B1B4 47B9  B3BD 8D41 E597 69DF 17AD

What I find amusing is that it was a patch rejection mail from you *years* back (circa 2000 iirc), telling me my pine corrupted whitespace, that made me switch MUA ;-)

All these years later, and it's still buggered ?

Dave

Actually, it seems better. It seems to be buggered by default, but it used to be that you had to actually recompile pine to make it behave. Now you can just disable "strip-whitespace-before-send" and _enable_ "quell-flowed-text" and those together seem to do the trick. No extra patches or recompiles necessary.

So there's progress.

Of course, pico is still pico. Which I find a bit sad: my editor of choise is still an improved version of uemacs, and pico actually comes from the same uemacs history, but has different key-bindings for just enough keys to be slightly confusing.

Still, that shared history means that I find pico a lot more to my taste than just about any other emailer editor out there. It may have a few differences, but it has more things in common..

Linus

Indeed. I use those two options like that, too. (-:

Why don't you enable "enable-alternate-editor-implicitly" and set editor = "your-editor-of-choice" in the pine config? It is integrated in a quite seamless way.

Best regards,

Anton

Anton Altaparmakov <aia21 at cam.ac.uk> (replace at with @)
Unix Support, Computing Service, University of Cambridge, CB2 3QH, UK
Linux NTFS maintainer / IRC: #ntfs on irc.freenode.net
WWW: http://linux-ntfs.sf.net/ & http://www-stu.christs.cam.ac.uk/~aia21/

You think so? I don't find it that way.

With an alternate editor you have to edit the headers separately, and things like postponing a message suddenly turns into a big deal, not just a trivial ^O. In fact, almost everything gets more involved.

And pico _is_ pretty close to uemacs.

Linus

Well using vim as alternate editor a postpone turns into a "ZZ" followed by ^O which I don't think is such a big deal but in general I agree that it adds hassle. It is a shame that the text based headers do not appear as part of the message in the editor in particular...

I wouldn't know about that. Never used uemacs. I am a vim addict myself. (-;

Best regards,

Anton

Anton Altaparmakov <aia21 at cam.ac.uk> (replace at with @)
Unix Support, Computing Service, University of Cambridge, CB2 3QH, UK
Linux NTFS maintainer / IRC: #ntfs on irc.freenode.net
WWW: http://linux-ntfs.sf.net/ & http://www-stu.christs.cam.ac.uk/~aia21/

Anybody interested in adding entry to SubmittingPatches MUA Specific Help section?

Hi,

Did I miss something? Straight from my local git directory:

-- snip --

Pine
----

(Johannes Schindelin)

I don't know how many people still use pine, but for those poor souls it may be good to mention that the quell-flowed-text is needed for recent versions.

... the "no-strip-whitespace-before-send" option, too. AFAIK it was introduced in 4.60.

-- snap --

Hth,
Dscho

Linus Torvalds <[email protected]> wrote:

[...]

Can I get a URL for the source for your editor? The one on kernel.org doesn't compile, and probably hasn't for a long time.

[Yes, there is some (perverse) fun in telling people you use the very same
editor than Linus Torvalds :-]>

Dr. Horst H. von Brand                   User #22616 counter.li.org
Departamento de Informatica                     Fono: +56 32 654431
Universidad Tecnica Federico Santa Maria              +56 32 654239
Casilla 110-V, Valparaiso, Chile                Fax:  +56 32 797513

Oh, it compiles with trivial modifications. Not cleanly, but it works.

I put my git repo on

kernel.org:/pub/software/editors/uemacs/uemacs.git

but it will take a moment to mirror out.

The "readme" says non-commercial only, but I asked Daniel Lawrence if it was ok to include it in commercial distributions a long time ago, and he said yes. Sadly, I've lost that email, so I don't have any paper trail for that. So you should consider the readme binding.

The thing is, it's not a wonderful editor. It's small, and good enough, but I really wished somebody wrote something that handled UTF-8, for example. But I've got the keybindings hardcoded in my spine, so I can't ever change.

Linus

Wouldn't underneath pub/scm/ be more appropriate and gitweb friendly?

Hmph. I vaguely recall in my distant past when I was still in school, I did my own Japanization of microEmacs. I wonder if yours share the same root -- I do not have the source of my version anymore so I have no way checking which ancient version I mucked with.

Well, I don't actually have write permissions to anything there except for udner the kernel tree. Also, I suspect there aren't all that many uemacs users out there, and anybody who isn't already a uemacs user probably shouldn't pick it up.

I don't think it has gotten any real development for the last ten years, and while it was arguably the best editor twenty years ago (not a lot of competition - what else worked on DOS, VMS and UNIX without being crazy and modal?) it's definitely showing its age.

So since I already had a uemacs subdirectory, I just put the git archive there too.

It's based on 3.9e, with some fixes by Petri Kutvonen at Helsinki University. With various (slight) modernizations by yours truly (eg posix termios support rather than the old "termio" stuff).

It _shouldn't_ be that hard to teach it about UTF-8, but every time I look at it I realize that there is a lot of stuff that should be improved. Converting it to K&R, getting rid of the things that made sense in the 80's but haven't made sense in ten years, re-indenting it (right now it's a strange mix of indentation styles)..

And if you open a binary file with it, it will literally spew binary characters to the tty, and can seriously confuse it due to the control chars. Or if you have a really long line, it can SIGSEGV on you. Cute.

But dammit, I'm not giving it up.

Linus

Gaah. You guys shamed me into actually trying to clean up the sources a bit.

It's by no means done, but some of it is even sparse-clean now.

I suspect there's a lot of it that still works mainly by luck than because it does anything right.

Linus

I'd been trying my hand at the original version. Need to merge my changes with yours...

Dr. Horst H. von Brand                   User #22616 counter.li.org
Departamento de Informatica                     Fono: +56 32 654431
Universidad Tecnica Federico Santa Maria              +56 32 654239
Casilla 110-V, Valparaiso, Chile                Fax:  +56 32 797513

[ Sorry about the spam, most of you probably have never cared ]

Heh, now that I got into cleaning things up, I've also been actually fixing things that have long irritated me, but the code was just too messy for me, and my level of care wasn't high enough to overcome.

I fixed the "outputs binary data in the 128-159 range to the terminal" yesterday (which would quite possibly corrupt terminal state if trying to edit binary files), and today I added a "utf-8" mode.

The utf-8 mode is purely about the _terminal_ being in UTF-8. The actual editor is still very much byte-char based, but now it understands 8-bit input that comes in UTF8-encoded, and can output things in UTF-8 format.

(Since UTF-8 is basically always used with Unicode, and the 8-bit subset of Unicode is iso-8859-1 aka Latin1, this is probably _less_ international than the non-UTF8, but hey, what can I say? If you want more, you'd need to teach uemacs about actual UTF8 contents).

The new "emacs.rc" will enable utf-8 mode automatically if $LANG contains the substring "UTF-8", since that _usually_ means that the terminal is in utf-8 mode. If it's wrong, you can add or delete the mode by hand (just like any global mode: Meta-X to add, Meta-Ctrl-X to delete).

Basically, it means that you don't have to switch the whole terminal into latin1 just to use uemacs.

Linus

Ok, disabling "text flowing" seems to have fixed it. It still leaves empty spaces at the end of lines when doing normal word-wrapping in the editor (and then seems to use those empty spaces as a "marker" for flowing), but that's at least just a small silly detail.

So how about this patch now? With it you can do

	git fetch --tags <linus-kernel-repo>

and it should fetch all my tags automatically.

Linus
---

@@ -5,6 +5,8 @@
 _x40='[0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f]'
 _x40="$_x40$_x40$_x40$_x40$_x40$_x40$_x40$_x40"
 
+all=
+tags=
 append=
 force=
 update_head_ok=

@@ -17,6 +19,12 @@ do
 	-f|--f|--fo|--for|--forc|--force)
 		force=t
 		;;
+	--tags)
+		tags=t
+		;;
+	--all)
+		all=t
+		;;
 	-u|--u|--up|--upd|--upda|--updat|--update|--update-|--update-h|\
 	--update-he|--update-hea|--update-head|--update-head-|\
 	--update-head-o|--update-head-ok)

@@ -158,7 +166,16 @@ case "$update_head_ok" in
 	;;
 esac
 
-for ref in $(get_remote_refs_for_fetch "$@")
+taglist=
+if [ "$tags$all" ]; then
+	pattern='/refs\/tags/'
+	if [ "$all" ]; then
+		pattern='/refs/'
+	fi
+	taglist=$(git-ls-remote "$remote" | awk "$pattern"' { print $2":"$2 }')
+fi
+
+for ref in $(get_remote_refs_for_fetch "$@" $taglist)
 do
     refs="$refs $ref"
 

Seems to work. I blew away git/refs/tags/v2.6.13* and git fetch --tags fetched them all back just fine.

Dave

Nope. It intentionally includes the manual rsync because clone/pull doesn't seem to grab tags. Or at least last time I checked...

Jeff

i just used the instructions at the bottom of your web page to grab new tags, and it completely wiped all my Stacked Git metadata. (fortunately i was trying this in a test repository, and it will be simple to recover the missing files).

so, this is probably something that should have a warning, or maybe you might consider providing an example that won't wipe out existing files and directories under .git/ ...

begin:vcard
fn:Chuck Lever
n:Lever;Charles
org:Network Appliance, Incorporated;Linux NFS Client Development
adr:535 West William Street, Suite 3100;;Center for Information Technology Integration;Ann Arbor;MI;48103-4943;USA email;internet:[email protected]
title:Member of Technical Staff
tel;work:+1 734 763 4415
tel;fax:+1 734 763 4434
tel;home:+1 734 668 1089
x-mozilla-html:FALSE
url:http://www.monkey.org/~cel/
version:2.1
end:vcard

clone does (or should), fetch (and hence pull) does not automatically. Running the following one-liner every once in a while would sync your set of tags with Linus:

git fetch origin git-ls-remote --tags origin | sed -ne 's|^.*refs/tags/|tag |p'

Thanks for all the comments. I just updated the KHGtG with the feedback I received. Go to

http://linux.yyz.us/git-howto.html

and click reload. Continued criticism^H^H^Hcomments welcome!

Two items of note:

1)

that's way too long and convoluted to deal with. Once the 'git fetch --tags' changes make it into the official repository (are they there already?), I'll remove all the remaining direct references to running rsync.

2) What is the easiest way to obtain a list of changes present in repository B, that are not present in repository A? I used to use git-changes-script [hacked cg-log script] for this:

	$ cd /repo/netdev-2.6
	$ git-changes-script -L ../linux-2.6

would display all changes in /repo/netdev-2.6 which are not present in /repo/linux-2.6. This is similar to 'git log master..HEAD', except for repositories rather than branches.

Jeff

Sounds like a thinly veiled threat and/or very effective prodding ;-).

It is not there yet only because I simply have not got around to it, but it will happen before 0.99.8.

I suspect the version Linus posted has a funny interaction with 'git-pull'; 'git pull --tags' by mistake, or intentionally to file a bug report to annoy me ;-), would create an Octopus out of those tags, if I am not mistaken.

I think I still have the copy you sent to the list. If you do not mind me placing in the master branch just holler -- better yet please send a patch with commit log and signoff to add the latest, and I will apply it.

-jc

It's archived here:
http://www.kernel.org/pub/linux/kernel/people/jgarzik/git-changes-script

but it needs a git expert (read: not me :)) to fix it up for the very latest git-core stuff.

Currently, using 'git-changes-script -L ../linux-2.6' spits out

Regards,

Jeff

I haven't really *read* that script, but I think it is trying to make a list of commits from both repositories and trying to find the set that are in one side and not in the other using diff (a real shell programer probably would have used "comm" for this kind of task, not "diff"), then doing a handcrafted git-log on each of them.

Attached is my quick hack, based on your original question, without really trying to understand what the script is doing, so I cannot claim it is a rewrite nor even attempting to be compatible. Please take a look at it and tell me if this is any close to what you need.

I have a suspition that this might be better done as a natural extension of git-log, though.

------------
#!/bin/sh
#
# Copyright (c) 2005 Junio C Hamano
#

. git-sh-setup || die "Not a git archive"

usage () {
	echo >&2 "$0 ( -L | -R ) <dir> [<ref>] [<ref>]

-L shows changes in local not in remote.
-R shows changes in remote not in local.
<dir> names the remote repository.

If given no refs, local and remote HEADs are compared.
If given one ref, local HEAD and named remote ref are compared. If given two refs, the first names a local ref, and the second names remote ref to be compared.
"
exit 1
}

case "$1" in
-L | -R)

	;;
*)
	usage ;;

esac

other="$2"

(
unset GIT_DIR GIT_OBJECT_DIRECTORY cd "$other" && . git-sh-setup || die "$other is not a valid git repository."
)

local=${3:-HEAD}
remote=${4:-HEAD}

# Basic validation.
local=$(git-rev-parse --verify "$local^0" 2>/dev/null) ||
die "local ref $local is not valid."
remote=$(GIT_DIR="$other" git-rev-parse --verify "$remote^0" 2>/dev/null) || die "remote ref $remote is not valid."

case "$1" in
-L)

	list_args="$local ^$remote" ;;
-R)
	list_args="^$local $remote" ;;

esac

GAOD="$GIT_ALTERNATE_OBJECT_DIRECTORIES"

GIT_ALTERNATE_OBJECT_DIRECTORIES="$other/.git/objects:$GAOD" \
git-rev-list --pretty $list_args |
LESS=-S ${PAGER:-less}

Hey, even more impressive is "git pull --all", which will happily try to create an octopus of every single ref available at the other end.

Now, I think that octopus merges in _general_ are likely to be driver error, and that it might make sense to have a separate flag to enable them in the first place. That would solve the confusion..

So then you could do

	git pull --all --octopus xyzzy

if you _really_ meant to do that.

Linus

True.

However, I think --all is a mistake even if you use it without merging in 'git fetch', so I am not planning to do refs/heads/ side, at least not yet. Even if you prevent an Octopus, what would you do then? If you choose to merge one of them, which one? Not merging any that is not explicitly specified on the command line, seems to me the most sensible and safe option.

The rule for 'pull' to decide which refs to merge is:

(1) if command line has explicit refspecs (--tags and --heads do not count), they are all merged.

(2) if command line has no explicit refspecs (--tags and --heads do not count), the default one found from either remotes or branches file is merged.

Notice that I am forbidding remotes file to say "by default I always merge these three heads from there to make an Octopus" by the above rule (branches file cannot even name more than one head so this is not an issue). Since everybody seems to agree that Octopus is not something that is done mechanically and routinely anyway [*1*], I think this is a sensible way to guard against accidental Octopus.

We could consider fetching all heads, by minimally renaming remote master to origin and getting everything else under the same name, but I'd really want to keep the local namespace for branches isolated from each other. Many kernel.org public repositories seem to have 'test' and 'release' branches and if you are a maintainer of such a tree, and if you are interested in another maintainer's tree, and if that other maintainer has the 'test' and 'release' branches, --heads (or --tags) overwriting your 'test' with his 'test' is obviously not what you want.

Possibly, something like this could be arranged later:

* git fetch --heads=$ns $remote "$@"

In addition to the usual refspecs (the rest of the command line arguments), fetch all remote heads and store remote refs/heads/$a under local refs/heads/$ns/$a for all $a. If $ns is empty, remote "master" is renamed "origin".

* git fetch --heads $remote "$@"

shorthand for empty $ns

[Footnote]

*1* I do make many Octopus merges, but they happen across my local topic branches. Topics merged change day-by-day, and even the set of topics alive at the time changes everyday. IOW, it is not something I would want to do with the same sets of heads every time by describing them in the remotes file.

Am Freitag, 30. September 2005 13:15 schrieb Jeff Garzik:

The error is still there.

oliver@oenone:~/linux-2.6> git checkout -f master
usage: read-tree (<sha> | -m <sha1> [<sha2> <sha3>])

Regards
Oliver

Something's wrong with your installation, not the instructions. Works fine here.

Jeff

Right , you must have a corrupted copy of the git repo. Do the initial cloning of the linux repository again, and then retry the git checkout.

FYI, the rsync command to get the tags blows away .git/branches/origin, so on the next "git pull", git will tell you "Where do you want to fetch from today?".

In the mean time, better make it two separate rsync commands (sorry for the long lines):

rsync -a --delete --verbose --stats --progress \
rsync://rsync.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git/tags/ \
.git/tags/

rsync -a --delete --verbose --stats --progress \
rsync://rsync.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git/heads/ \
.git/heads/

Erik

+-- Erik Mouw -- www.harddisk-recovery.com -- +31 70 370 12 90 --
| Lab address: Delftechpark 26, 2628 XH, Delft, The Netherlands

Agreed, though I always run 'git pull $url' each time, so I never noticed this.

Since git clone pulls the tags, I have eliminated the first rsync completely.

Jeff

- To know the current branch, "git branch" is enough (the one '*'-ed)
- rsync(1) a repository is dangerous, it might catch it in the middle of a update and give you an incomplete/messed up copy. Repeat rsync until no change, perhaps?
- I understand "git checkout -f" blows away any local changes, no questions asked. Not very nice to suggest that to a newbie...

Thanks for the docu!

Dr. Horst H. von Brand                   User #22616 counter.li.org
Departamento de Informatica                     Fono: +56 32 654431
Universidad Tecnica Federico Santa Maria              +56 32 654239
Casilla 110-V, Valparaiso, Chile                Fax:  +56 32 797513

Click reload, this is already mentioned.

Usually that's just unlucky. I have caught kernel.org in the middle of a sync once, maybe twice.

I constantly run into problems if I -do not- use the "-f" flag. I habitually use it at all times, now.

Thanks,

Jeff

Jeff Garzik <[email protected]> :
[...]

The basic tasks make no use of git-whatchanged.

Even if it is a CVSism, it is quite handful to retrieve the history of the patchsets by aiming at a specific file.

--
Ueimor

Hi!
I tried to follow the instructions step by step, but I do not have a "git-update-cache" command. The closest thing I found (comparing to the "Short Git Tutorial" [1] ) is "git-update-index"...

-Rolf

[1] http://www.kernel.org/pub/software/scm/git/docs/tutorial.html