「git-fetch」

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git-fetch,从另一个仓库下载object和ref。

描述

Fetch branches and/or tags (collectively, “refs”) from one or more other repositories, along with the objects
necessary to complete their histories. Remote-tracking branches are updated (see the description of <refspec>
below for ways to control this behavior).

By default, any tag that points into the histories being fetched is also fetched; the effect is to fetch tags
that point at branches that you are interested in. This default behavior can be changed by using the –tags
or –no-tags options or by configuring remote.<name>.tagOpt. By using a refspec that fetches tags explicitly,
you can fetch tags that do not point into branches you are interested in as well.

git fetch can fetch from either a single named repository or URL, or from several repositories at once if
<group> is given and there is a remotes.<group> entry in the configuration file. (See git-config(1)).

When no remote is specified, by default the origin remote will be used, unless there’s an upstream branch
configured for the current branch.

The names of refs that are fetched, together with the object names they point at, are written to
.git/FETCH_HEAD. This information may be used by scripts or other git commands, such as git-pull(1).

命令语法格式

git fetch [<options>] [<repository> [<refspec>…]]
git fetch [<options>] <group>
git fetch –multiple [<options>] [(<repository> | <group>)…]
git fetch –all [<options>]

命令选项及含义

–all
获取远程所有的。

-a, –append
Append ref names and object names of fetched refs to the existing contents of .git/FETCH_HEAD. Without
this option old data in .git/FETCH_HEAD will be overwritten.

–depth=<depth>
Limit fetching to the specified number of commits from the tip of each remote branch history. If fetching
to a shallow repository created by git clone with –depth=<depth> option (see git-clone(1)), deepen or
shorten the history to the specified number of commits. Tags for the deepened commits are not fetched.

–unshallow
If the source repository is complete, convert a shallow repository to a complete one, removing all the
limitations imposed by shallow repositories.

If the source repository is shallow, fetch as much as possible so that the current repository has the
same history as the source repository.

–update-shallow
By default when fetching from a shallow repository, git fetch refuses refs that require updating
.git/shallow. This option updates .git/shallow and accept such refs.

–dry-run
Show what would be done, without making any changes.

-f, –force
When git fetch is used with <rbranch>:<lbranch> refspec, it refuses to update the local branch <lbranch>
unless the remote branch <rbranch> it fetches is a descendant of <lbranch>. This option overrides that
check.

-k, –keep
Keep downloaded pack.

–multiple
Allow several <repository> and <group> arguments to be specified. No <refspec>s may be specified.

-p, –prune
Before fetching, remove any remote-tracking references that no longer exist on the remote. Tags are not
subject to pruning if they are fetched only because of the default tag auto-following or due to a –tags
option. However, if tags are fetched due to an explicit refspec (either on the command line or in the
remote configuration, for example if the remote was cloned with the –mirror option), then they are also
subject to pruning.

-n, –no-tags
By default, tags that point at objects that are downloaded from the remote repository are fetched and
stored locally. This option disables this automatic tag following. The default behavior for a remote may
be specified with the remote.<name>.tagOpt setting. See git-config(1).

–refmap=<refspec>
When fetching refs listed on the command line, use the specified refspec (can be given more than once) to
map the refs to remote-tracking branches, instead of the values of remote.*.fetch configuration variables
for the remote repository. See section on “Configured Remote-tracking Branches” for details.

-t, –tags
Fetch all tags from the remote (i.e., fetch remote tags refs/tags/* into local tags with the same name),
in addition to whatever else would otherwise be fetched. Using this option alone does not subject tags to
pruning, even if –prune is used (though tags may be pruned anyway if they are also the destination of an
explicit refspec; see –prune).

–recurse-submodules[=yes|on-demand|no]
This option controls if and under what conditions new commits of populated submodules should be fetched
too. It can be used as a boolean option to completely disable recursion when set to no or to
unconditionally recurse into all populated submodules when set to yes, which is the default when this
option is used without any value. Use on-demand to only recurse into a populated submodule when the
superproject retrieves a commit that updates the submodule’s reference to a commit that isn’t already in
the local submodule clone.

-j, –jobs=<n>
Number of parallel children to be used for fetching submodules. Each will fetch from different
submodules, such that fetching many submodules will be faster. By default submodules will be fetched one
at a time.

–no-recurse-submodules
Disable recursive fetching of submodules (this has the same effect as using the –recurse-submodules=no
option).

–submodule-prefix=<path>
Prepend <path> to paths printed in informative messages such as “Fetching submodule foo”. This option is
used internally when recursing over submodules.

–recurse-submodules-default=[yes|on-demand]
This option is used internally to temporarily provide a non-negative default value for the
–recurse-submodules option. All other methods of configuring fetch’s submodule recursion (such as
settings in gitmodules(5) and git-config(1)) override this option, as does specifying
–[no-]recurse-submodules directly.

-u, –update-head-ok
By default git fetch refuses to update the head which corresponds to the current branch. This flag
disables the check. This is purely for the internal use for git pull to communicate with git fetch, and
unless you are implementing your own Porcelain you are not supposed to use it.

–upload-pack <upload-pack>
When given, and the repository to fetch from is handled by git fetch-pack, –exec=<upload-pack> is passed
to the command to specify non-default path for the command run on the other end.

-q, –quiet
Pass –quiet to git-fetch-pack and silence any other internally used git commands. Progress is not
reported to the standard error stream.

-v, –verbose
Be verbose.

–progress
Progress status is reported on the standard error stream by default when it is attached to a terminal,
unless -q is specified. This flag forces progress status even if the standard error stream is not
directed to a terminal.

-4, –ipv4
Use IPv4 addresses only, ignoring IPv6 addresses.

-6, –ipv6
Use IPv6 addresses only, ignoring IPv4 addresses.

<repository>
The “remote” repository that is the source of a fetch or pull operation. This parameter can be either a
URL (see the section GIT URLS below) or the name of a remote (see the section REMOTES below).

<group>
A name referring to a list of repositories as the value of remotes.<group> in the configuration file.
(See git-config(1)).

<refspec>
Specifies which refs to fetch and which local refs to update. When no <refspec>s appear on the command
line, the refs to fetch are read from remote.<repository>.fetch variables instead (see CONFIGURED
REMOTE-TRACKING BRANCHES below).

The format of a <refspec> parameter is an optional plus +, followed by the source ref <src>, followed by
a colon :, followed by the destination ref <dst>. The colon can be omitted when <dst> is empty.

tag <tag> means the same as refs/tags/<tag>:refs/tags/<tag>; it requests fetching everything up to the
given tag.

The remote ref that matches <src> is fetched, and if <dst> is not empty string, the local ref that
matches it is fast-forwarded using <src>. If the optional plus + is used, the local ref is updated even
if it does not result in a fast-forward update.

Note

When the remote branch you want to fetch is known to be rewound and rebased regularly, it is expected
that its new tip will not be descendant of its previous tip (as stored in your remote-tracking branch
the last time you fetched). You would want to use the + sign to indicate non-fast-forward updates
will be needed for such branches. There is no way to determine or declare that a branch will be made
available in a repository with this behavior; the pulling user simply must know this is the expected
usage pattern for a branch.

GIT URLS

URL中同通常包含传输协议、远程服务器的地址、仓库路径。根据协议的不同,这些信息也可能没有。

Git支持ssh, git, http, https协议,但是也支持ftp、ftps,但是这两者的效率不高,不建议使用。

原始的传输(i.e. git:// URL)不做认证,并且在不安全的网络上要小心使用。

可以使用以下语法:

ssh协议也可以使用类似scp的语法:

This syntax is only recognized if there are no slashes before the first colon. This helps differentiate a
local path that contains a colon. For example the local path foo:bar could be specified as an absolute path
or ./foo:bar to avoid being misinterpreted as an ssh url.

The ssh and git protocols additionally support ~username expansion:

· ssh://[user@]host.xz[:port]/~[user]/path/to/repo.git/

· git://host.xz[:port]/~[user]/path/to/repo.git/

· [user@]host.xz:/~[user]/path/to/repo.git/

For local repositories, also supported by Git natively, the following syntaxes may be used:

· /path/to/repo.git/

· file:///path/to/repo.git/

These two syntaxes are mostly equivalent, except when cloning, when the former implies –local option. See
git-clone(1) for details.

When Git doesn’t know how to handle a certain transport protocol, it attempts to use the remote-<transport>
remote helper, if one exists. To explicitly request a remote helper, the following syntax may be used:

· <transport>::<address>

where <address> may be a path, a server and path, or an arbitrary URL-like string recognized by the specific
remote helper being invoked. See gitremote-helpers(1) for details.

If there are a large number of similarly-named remote repositories and you want to use a different format for
them (such that the URLs you use will be rewritten into URLs that work), you can create a configuration
section of the form:

[url “<actual url base>”]
insteadOf = <other url base>

For example, with this:

[url “git://git.host.xz/“]
insteadOf = host.xz:/path/to/
insteadOf = work:

a URL like “work:repo.git” or like “host.xz:/path/to/repo.git” will be rewritten in any context that takes a
URL to be “git://git.host.xz/repo.git“.

If you want to rewrite URLs for push only, you can create a configuration section of the form:

[url “<actual url base>”]
pushInsteadOf = <other url base>

For example, with this:

[url “ssh://example.org/“]
pushInsteadOf = git://example.org/

a URL like “git://example.org/path/to/repo.git” will be rewritten to “ssh://example.org/path/to/repo.git” for
pushes, but pulls will still use the original URL.

REMOTES

The name of one of the following can be used instead of a URL as <repository> argument:

· a remote in the Git configuration file: $GIT_DIR/config,

· a file in the $GIT_DIR/remotes directory, or

· a file in the $GIT_DIR/branches directory.

All of these also allow you to omit the refspec from the command line because they each contain a refspec
which git will use by default.

Named remote in configuration file
You can choose to provide the name of a remote which you had previously configured using git-remote(1), git-
config(1) or even by a manual edit to the $GIT_DIR/config file. The URL of this remote will be used to access
the repository. The refspec of this remote will be used by default when you do not provide a refspec on the
command line. The entry in the config file would appear like this:

[remote “<name>”]
url = <url>
pushurl = <pushurl>
push = <refspec>
fetch = <refspec>

The <pushurl> is used for pushes only. It is optional and defaults to <url>.

Named file in $GIT_DIR/remotes
You can choose to provide the name of a file in $GIT_DIR/remotes. The URL in this file will be used to access
the repository. The refspec in this file will be used as default when you do not provide a refspec on the
command line. This file should have the following format:

URL: one of the above URL format
Push: <refspec>
Pull: <refspec>

Push: lines are used by git push and Pull: lines are used by git pull and git fetch. Multiple Push: and Pull:
lines may be specified for additional branch mappings.

Named file in $GIT_DIR/branches
You can choose to provide the name of a file in $GIT_DIR/branches. The URL in this file will be used to
access the repository. This file should have the following format:

<url>#<head>

<url> is required; #<head> is optional.

Depending on the operation, git will use one of the following refspecs, if you don’t provide one on the
command line. <branch> is the name of this file in $GIT_DIR/branches and <head> defaults to master.

git fetch uses:

refs/heads/<head>:refs/heads/<branch>

git push uses:

HEAD:refs/heads/<head>

CONFIGURED REMOTE-TRACKING BRANCHES

You often interact with the same remote repository by regularly and repeatedly fetching from it. In order to
keep track of the progress of such a remote repository, git fetch allows you to configure
remote.<repository>.fetch configuration variables.

Typically such a variable may look like this:

[remote “origin”]
fetch = +refs/heads/*:refs/remotes/origin/*

This configuration is used in two ways:

· When git fetch is run without specifying what branches and/or tags to fetch on the command line, e.g.
git fetch origin or git fetch, remote.<repository>.fetch values are used as the refspecs—they specify
which refs to fetch and which local refs to update. The example above will fetch all branches that exist
in the origin (i.e. any ref that matches the left-hand side of the value, refs/heads/*) and update the
corresponding remote-tracking branches in the refs/remotes/origin/* hierarchy.

· When git fetch is run with explicit branches and/or tags to fetch on the command line, e.g. git fetch
origin master, the <refspec>s given on the command line determine what are to be fetched (e.g. master in
the example, which is a short-hand for master:, which in turn means “fetch the master branch but I do not
explicitly say what remote-tracking branch to update with it from the command line”), and the example
command will fetch only the master branch. The remote.<repository>.fetch values determine which
remote-tracking branch, if any, is updated. When used in this way, the remote.<repository>.fetch values
do not have any effect in deciding what gets fetched (i.e. the values are not used as refspecs when the
command-line lists refspecs); they are only used to decide where the refs that are fetched are stored by
acting as a mapping.

The latter use of the remote.<repository>.fetch values can be overridden by giving the –refmap=<refspec>
parameter(s) on the command line.

OUTPUT

The output of “git fetch” depends on the transport method used; this section describes the output when fetching over the Git protocol (either locally or via ssh) and Smart HTTP protocol.

The status of the fetch is output in tabular form, with each line representing the status of a single ref. Each line is of the form:

<flag> <summary> <from> -> <to> [<reason>]
The status of up-to-date refs is shown only if the –verbose option is used.

In compact output mode, specified with configuration variable fetch.output, if either entire <from> or <to> is found in the other string, it will be substituted with * in the other string. For example, master -> origin/master becomes master -> origin/*.

flag
A single character indicating the status of the ref:

(space)
for a successfully fetched fast-forward;

+
for a successful forced update;


for a successfully pruned ref;

t
for a successful tag update;

*
for a successfully fetched new ref;

!
for a ref that was rejected or failed to update; and

=
for a ref that was up to date and did not need fetching.

summary
For a successfully fetched ref, the summary shows the old and new values of the ref in a form suitable for using as an argument to git log (this is <old>..<new> in most cases, and <old>…<new> for forced non-fast-forward updates).

from
The name of the remote ref being fetched from, minus its refs/<type>/ prefix. In the case of deletion, the name of the remote ref is “(none)”.

to
The name of the local ref being updated, minus its refs/<type>/ prefix.

reason
A human-readable explanation. In the case of successfully fetched refs, no explanation is needed. For a failed ref, the reason for failure is described.

SECURITY

The fetch and push protocols are not designed to prevent one side from stealing data from the other repository that was not intended to be shared. If you have private data that you need to protect from a malicious peer, your best option is to store it in another repository. This applies to both clients and servers. In particular, namespaces on a server are not effective for read access control; you should only grant read access to a namespace to clients that you would trust with read access to the entire repository.

The known attack vectors are as follows:

The victim sends “have” lines advertising the IDs of objects it has that are not explicitly intended to be shared but can be used to optimize the transfer if the peer also has them. The attacker chooses an object ID X to steal and sends a ref to X, but isn’t required to send the content of X because the victim already has it. Now the victim believes that the attacker has X, and it sends the content of X back to the attacker later. (This attack is most straightforward for a client to perform on a server, by creating a ref to X in the namespace the client has access to and then fetching it. The most likely way for a server to perform it on a client is to “merge” X into a public branch and hope that the user does additional work on this branch and pushes it back to the server without noticing the merge.)

As in #1, the attacker chooses an object ID X to steal. The victim sends an object Y that the attacker already has, and the attacker falsely claims to have X and not Y, so the victim sends Y as a delta against X. The delta reveals regions of X that are similar to Y to the attacker.

BUGS

Using –recurse-submodules can only fetch new commits in already checked out submodules right now. When e.g.
upstream added a new submodule in the just fetched commits of the superproject the submodule itself can not
be fetched, making it impossible to check out that submodule later without having to do a fetch again. This
is expected to be fixed in a future Git version.

参考文献