The twentieth batch

[alt-git.git] / Documentation / githooks.txt

githooks(5)
===========

NAME
----
githooks - Hooks used by Git

SYNOPSIS
--------
$GIT_DIR/hooks/* (or \`git config core.hooksPath`/*)


DESCRIPTION
-----------

Hooks are programs you can place in a hooks directory to trigger
actions at certain points in git's execution. Hooks that don't have
the executable bit set are ignored.

By default the hooks directory is `$GIT_DIR/hooks`, but that can be
changed via the `core.hooksPath` configuration variable (see
linkgit:git-config[1]).

Before Git invokes a hook, it changes its working directory to either
$GIT_DIR in a bare repository or the root of the working tree in a non-bare
repository. An exception are hooks triggered during a push ('pre-receive',
'update', 'post-receive', 'post-update', 'push-to-checkout') which are always
executed in $GIT_DIR.

Environment variables, such as `GIT_DIR`, `GIT_WORK_TREE`, etc., are exported
so that Git commands run by the hook can correctly locate the repository.  If
your hook needs to invoke Git commands in a foreign repository or in a
different working tree of the same repository, then it should clear these
environment variables so they do not interfere with Git operations at the
foreign location.  For example:

------------
local_desc=$(git describe)
foreign_desc=$(unset $(git rev-parse --local-env-vars); git -C ../foreign-repo describe)
------------

Hooks can get their arguments via the environment, command-line
arguments, and stdin. See the documentation for each hook below for
details.

`git init` may copy hooks to the new repository, depending on its
configuration. See the "TEMPLATE DIRECTORY" section in
linkgit:git-init[1] for details. When the rest of this document refers
to "default hooks" it's talking about the default template shipped
with Git.

The currently supported hooks are described below.

HOOKS
-----

applypatch-msg
~~~~~~~~~~~~~~

This hook is invoked by linkgit:git-am[1].  It takes a single
parameter, the name of the file that holds the proposed commit
log message.  Exiting with a non-zero status causes `git am` to abort
before applying the patch.

The hook is allowed to edit the message file in place, and can
be used to normalize the message into some project standard
format. It can also be used to refuse the commit after inspecting
the message file.

The default 'applypatch-msg' hook, when enabled, runs the
'commit-msg' hook, if the latter is enabled.

pre-applypatch
~~~~~~~~~~~~~~

This hook is invoked by linkgit:git-am[1].  It takes no parameter, and is
invoked after the patch is applied, but before a commit is made.

If it exits with non-zero status, then the working tree will not be
committed after applying the patch.

It can be used to inspect the current working tree and refuse to
make a commit if it does not pass certain tests.

The default 'pre-applypatch' hook, when enabled, runs the
'pre-commit' hook, if the latter is enabled.

post-applypatch
~~~~~~~~~~~~~~~

This hook is invoked by linkgit:git-am[1].  It takes no parameter,
and is invoked after the patch is applied and a commit is made.

This hook is meant primarily for notification, and cannot affect
the outcome of `git am`.

pre-commit
~~~~~~~~~~

This hook is invoked by linkgit:git-commit[1], and can be bypassed
with the `--no-verify` option.  It takes no parameters, and is
invoked before obtaining the proposed commit log message and
making a commit.  Exiting with a non-zero status from this script
causes the `git commit` command to abort before creating a commit.

The default 'pre-commit' hook, when enabled, catches introduction
of lines with trailing whitespaces and aborts the commit when
such a line is found.

All the `git commit` hooks are invoked with the environment
variable `GIT_EDITOR=:` if the command will not bring up an editor
to modify the commit message.

The default 'pre-commit' hook, when enabled--and with the
`hooks.allownonascii` config option unset or set to false--prevents
the use of non-ASCII filenames.

pre-merge-commit
~~~~~~~~~~~~~~~~

This hook is invoked by linkgit:git-merge[1], and can be bypassed
with the `--no-verify` option.  It takes no parameters, and is
invoked after the merge has been carried out successfully and before
obtaining the proposed commit log message to
make a commit.  Exiting with a non-zero status from this script
causes the `git merge` command to abort before creating a commit.

The default 'pre-merge-commit' hook, when enabled, runs the
'pre-commit' hook, if the latter is enabled.

This hook is invoked with the environment variable
`GIT_EDITOR=:` if the command will not bring up an editor
to modify the commit message.

If the merge cannot be carried out automatically, the conflicts
need to be resolved and the result committed separately (see
linkgit:git-merge[1]). At that point, this hook will not be executed,
but the 'pre-commit' hook will, if it is enabled.

prepare-commit-msg
~~~~~~~~~~~~~~~~~~

This hook is invoked by linkgit:git-commit[1] right after preparing the
default log message, and before the editor is started.

It takes one to three parameters.  The first is the name of the file
that contains the commit log message.  The second is the source of the commit
message, and can be: `message` (if a `-m` or `-F` option was
given); `template` (if a `-t` option was given or the
configuration option `commit.template` is set); `merge` (if the
commit is a merge or a `.git/MERGE_MSG` file exists); `squash`
(if a `.git/SQUASH_MSG` file exists); or `commit`, followed by
a commit object name (if a `-c`, `-C` or `--amend` option was given).

If the exit status is non-zero, `git commit` will abort.

The purpose of the hook is to edit the message file in place, and
it is not suppressed by the `--no-verify` option.  A non-zero exit
means a failure of the hook and aborts the commit.  It should not
be used as a replacement for the pre-commit hook.

The sample `prepare-commit-msg` hook that comes with Git removes the
help message found in the commented portion of the commit template.

commit-msg
~~~~~~~~~~

This hook is invoked by linkgit:git-commit[1] and linkgit:git-merge[1], and can be
bypassed with the `--no-verify` option.  It takes a single parameter,
the name of the file that holds the proposed commit log message.
Exiting with a non-zero status causes the command to abort.

The hook is allowed to edit the message file in place, and can be used
to normalize the message into some project standard format. It
can also be used to refuse the commit after inspecting the message
file.

The default 'commit-msg' hook, when enabled, detects duplicate
`Signed-off-by` trailers, and aborts the commit if one is found.

post-commit
~~~~~~~~~~~

This hook is invoked by linkgit:git-commit[1]. It takes no parameters, and is
invoked after a commit is made.

This hook is meant primarily for notification, and cannot affect
the outcome of `git commit`.

pre-rebase
~~~~~~~~~~

This hook is called by linkgit:git-rebase[1] and can be used to prevent a
branch from getting rebased.  The hook may be called with one or
two parameters.  The first parameter is the upstream from which
the series was forked.  The second parameter is the branch being
rebased, and is not set when rebasing the current branch.

post-checkout
~~~~~~~~~~~~~

This hook is invoked when a linkgit:git-checkout[1] or
linkgit:git-switch[1] is run after having updated the
worktree.  The hook is given three parameters: the ref of the previous HEAD,
the ref of the new HEAD (which may or may not have changed), and a flag
indicating whether the checkout was a branch checkout (changing branches,
flag=1) or a file checkout (retrieving a file from the index, flag=0).
This hook cannot affect the outcome of `git switch` or `git checkout`,
other than that the hook's exit status becomes the exit status of
these two commands.

It is also run after linkgit:git-clone[1], unless the `--no-checkout` (`-n`) option is
used. The first parameter given to the hook is the null-ref, the second the
ref of the new HEAD and the flag is always 1. Likewise for `git worktree add`
unless `--no-checkout` is used.

This hook can be used to perform repository validity checks, auto-display
differences from the previous HEAD if different, or set working dir metadata
properties.

post-merge
~~~~~~~~~~

This hook is invoked by linkgit:git-merge[1], which happens when a `git pull`
is done on a local repository.  The hook takes a single parameter, a status
flag specifying whether or not the merge being done was a squash merge.
This hook cannot affect the outcome of `git merge` and is not executed,
if the merge failed due to conflicts.

This hook can be used in conjunction with a corresponding pre-commit hook to
save and restore any form of metadata associated with the working tree
(e.g.: permissions/ownership, ACLS, etc).  See contrib/hooks/setgitperms.perl
for an example of how to do this.

pre-push
~~~~~~~~

This hook is called by linkgit:git-push[1] and can be used to prevent
a push from taking place.  The hook is called with two parameters
which provide the name and location of the destination remote, if a
named remote is not being used both values will be the same.

Information about what is to be pushed is provided on the hook's standard
input with lines of the form:

  <local-ref> SP <local-object-name> SP <remote-ref> SP <remote-object-name> LF

For instance, if the command +git push origin master:foreign+ were run the
hook would receive a line like the following:

  refs/heads/master 67890 refs/heads/foreign 12345

although the full object name would be supplied.  If the foreign ref does not
yet exist the `<remote-object-name>` will be the all-zeroes object name.  If a
ref is to be deleted, the `<local-ref>` will be supplied as `(delete)` and the
`<local-object-name>` will be the all-zeroes object name.  If the local commit
was specified by something other than a name which could be expanded (such as
`HEAD~`, or an object name) it will be supplied as it was originally given.

If this hook exits with a non-zero status, `git push` will abort without
pushing anything.  Information about why the push is rejected may be sent
to the user by writing to standard error.

[[pre-receive]]
pre-receive
~~~~~~~~~~~

This hook is invoked by linkgit:git-receive-pack[1] when it reacts to
`git push` and updates reference(s) in its repository.
Just before starting to update refs on the remote repository, the
pre-receive hook is invoked.  Its exit status determines the success
or failure of the update.

This hook executes once for the receive operation. It takes no
arguments, but for each ref to be updated it receives on standard
input a line of the format:

  <old-oid> SP <new-oid> SP <ref-name> LF

where `<old-oid>` is the old object name stored in the ref,
`<new-oid>` is the new object name to be stored in the ref and
`<ref-name>` is the full name of the ref.
When creating a new ref, `<old-oid>` is the all-zeroes object name.

If the hook exits with non-zero status, none of the refs will be
updated. If the hook exits with zero, updating of individual refs can
still be prevented by the <<update,'update'>> hook.

Both standard output and standard error output are forwarded to
`git send-pack` on the other end, so you can simply `echo` messages
for the user.

The number of push options given on the command line of
`git push --push-option=...` can be read from the environment
variable `GIT_PUSH_OPTION_COUNT`, and the options themselves are
found in `GIT_PUSH_OPTION_0`, `GIT_PUSH_OPTION_1`,...
If it is negotiated to not use the push options phase, the
environment variables will not be set. If the client selects
to use push options, but doesn't transmit any, the count variable
will be set to zero, `GIT_PUSH_OPTION_COUNT=0`.

See the section on "Quarantine Environment" in
linkgit:git-receive-pack[1] for some caveats.

[[update]]
update
~~~~~~

This hook is invoked by linkgit:git-receive-pack[1] when it reacts to
`git push` and updates reference(s) in its repository.
Just before updating the ref on the remote repository, the update hook
is invoked.  Its exit status determines the success or failure of
the ref update.

The hook executes once for each ref to be updated, and takes
three parameters:

 - the name of the ref being updated,
 - the old object name stored in the ref,
 - and the new object name to be stored in the ref.

A zero exit from the update hook allows the ref to be updated.
Exiting with a non-zero status prevents `git receive-pack`
from updating that ref.

This hook can be used to prevent 'forced' update on certain refs by
making sure that the object name is a commit object that is a
descendant of the commit object named by the old object name.
That is, to enforce a "fast-forward only" policy.

It could also be used to log the old..new status.  However, it
does not know the entire set of branches, so it would end up
firing one e-mail per ref when used naively, though.  The
<<post-receive,'post-receive'>> hook is more suited to that.

In an environment that restricts the users' access only to git
commands over the wire, this hook can be used to implement access
control without relying on filesystem ownership and group
membership. See linkgit:git-shell[1] for how you might use the login
shell to restrict the user's access to only git commands.

Both standard output and standard error output are forwarded to
`git send-pack` on the other end, so you can simply `echo` messages
for the user.

The default 'update' hook, when enabled--and with
`hooks.allowunannotated` config option unset or set to false--prevents
unannotated tags from being pushed.

[[proc-receive]]
proc-receive
~~~~~~~~~~~~

This hook is invoked by linkgit:git-receive-pack[1].  If the server has
set the multi-valued config variable `receive.procReceiveRefs`, and the
commands sent to 'receive-pack' have matching reference names, these
commands will be executed by this hook, instead of by the internal
`execute_commands()` function.  This hook is responsible for updating
the relevant references and reporting the results back to 'receive-pack'.

This hook executes once for the receive operation.  It takes no
arguments, but uses a pkt-line format protocol to communicate with
'receive-pack' to read commands, push-options and send results.  In the
following example for the protocol, the letter 'S' stands for
'receive-pack' and the letter 'H' stands for this hook.

    # Version and features negotiation.
    S: PKT-LINE(version=1\0push-options atomic...)
    S: flush-pkt
    H: PKT-LINE(version=1\0push-options...)
    H: flush-pkt

    # Send commands from server to the hook.
    S: PKT-LINE(<old-oid> <new-oid> <ref>)
    S: ... ...
    S: flush-pkt
    # Send push-options only if the 'push-options' feature is enabled.
    S: PKT-LINE(push-option)
    S: ... ...
    S: flush-pkt

    # Receive results from the hook.
    # OK, run this command successfully.
    H: PKT-LINE(ok <ref>)
    # NO, I reject it.
    H: PKT-LINE(ng <ref> <reason>)
    # Fall through, let 'receive-pack' execute it.
    H: PKT-LINE(ok <ref>)
    H: PKT-LINE(option fall-through)
    # OK, but has an alternate reference.  The alternate reference name
    # and other status can be given in option directives.
    H: PKT-LINE(ok <ref>)
    H: PKT-LINE(option refname <refname>)
    H: PKT-LINE(option old-oid <old-oid>)
    H: PKT-LINE(option new-oid <new-oid>)
    H: PKT-LINE(option forced-update)
    H: ... ...
    H: flush-pkt

Each command for the 'proc-receive' hook may point to a pseudo-reference
and always has a zero-old as its old-oid, while the 'proc-receive' hook
may update an alternate reference and the alternate reference may exist
already with a non-zero old-oid.  For this case, this hook will use
"option" directives to report extended attributes for the reference given
by the leading "ok" directive.

The report of the commands of this hook should have the same order as
the input.  The exit status of the 'proc-receive' hook only determines
the success or failure of the group of commands sent to it, unless
atomic push is in use.

[[post-receive]]
post-receive
~~~~~~~~~~~~

This hook is invoked by linkgit:git-receive-pack[1] when it reacts to
`git push` and updates reference(s) in its repository.
It executes on the remote repository once after all the refs have
been updated.

This hook executes once for the receive operation.  It takes no
arguments, but gets the same information as the
<<pre-receive,'pre-receive'>>
hook does on its standard input.

This hook does not affect the outcome of `git receive-pack`, as it
is called after the real work is done.

This supersedes the <<post-update,'post-update'>> hook in that it gets
both old and new values of all the refs in addition to their
names.

Both standard output and standard error output are forwarded to
`git send-pack` on the other end, so you can simply `echo` messages
for the user.

The default 'post-receive' hook is empty, but there is
a sample script `post-receive-email` provided in the `contrib/hooks`
directory in Git distribution, which implements sending commit
emails.

The number of push options given on the command line of
`git push --push-option=...` can be read from the environment
variable `GIT_PUSH_OPTION_COUNT`, and the options themselves are
found in `GIT_PUSH_OPTION_0`, `GIT_PUSH_OPTION_1`,...
If it is negotiated to not use the push options phase, the
environment variables will not be set. If the client selects
to use push options, but doesn't transmit any, the count variable
will be set to zero, `GIT_PUSH_OPTION_COUNT=0`.

[[post-update]]
post-update
~~~~~~~~~~~

This hook is invoked by linkgit:git-receive-pack[1] when it reacts to
`git push` and updates reference(s) in its repository.
It executes on the remote repository once after all the refs have
been updated.

It takes a variable number of parameters, each of which is the
name of ref that was actually updated.

This hook is meant primarily for notification, and cannot affect
the outcome of `git receive-pack`.

The 'post-update' hook can tell what are the heads that were pushed,
but it does not know what their original and updated values are,
so it is a poor place to do log old..new. The
<<post-receive,'post-receive'>> hook does get both original and
updated values of the refs. You might consider it instead if you need
them.

When enabled, the default 'post-update' hook runs
`git update-server-info` to keep the information used by dumb
transports (e.g., HTTP) up to date.  If you are publishing
a Git repository that is accessible via HTTP, you should
probably enable this hook.

Both standard output and standard error output are forwarded to
`git send-pack` on the other end, so you can simply `echo` messages
for the user.

reference-transaction
~~~~~~~~~~~~~~~~~~~~~

This hook is invoked by any Git command that performs reference
updates. It executes whenever a reference transaction is prepared,
committed or aborted and may thus get called multiple times. The hook
does not cover symbolic references (but that may change in the future).

The hook takes exactly one argument, which is the current state the
given reference transaction is in:

    - "prepared": All reference updates have been queued to the
      transaction and references were locked on disk.

    - "committed": The reference transaction was committed and all
      references now have their respective new value.

    - "aborted": The reference transaction was aborted, no changes
      were performed and the locks have been released.

For each reference update that was added to the transaction, the hook
receives on standard input a line of the format:

  <old-oid> SP <new-oid> SP <ref-name> LF

where `<old-oid>` is the old object name passed into the reference
transaction, `<new-oid>` is the new object name to be stored in the
ref and `<ref-name>` is the full name of the ref. When force updating
the reference regardless of its current value or when the reference is
to be created anew, `<old-oid>` is the all-zeroes object name. To
distinguish these cases, you can inspect the current value of
`<ref-name>` via `git rev-parse`.

The exit status of the hook is ignored for any state except for the
"prepared" state. In the "prepared" state, a non-zero exit status will
cause the transaction to be aborted. The hook will not be called with
"aborted" state in that case.

push-to-checkout
~~~~~~~~~~~~~~~~

This hook is invoked by linkgit:git-receive-pack[1] when it reacts to
`git push` and updates reference(s) in its repository, and when
the push tries to update the branch that is currently checked out
and the `receive.denyCurrentBranch` configuration variable is set to
`updateInstead`.  Such a push by default is refused if the working
tree and the index of the remote repository has any difference from
the currently checked out commit; when both the working tree and the
index match the current commit, they are updated to match the newly
pushed tip of the branch.  This hook is to be used to override the
default behaviour.

The hook receives the commit with which the tip of the current
branch is going to be updated.  It can exit with a non-zero status
to refuse the push (when it does so, it must not modify the index or
the working tree).  Or it can make any necessary changes to the
working tree and to the index to bring them to the desired state
when the tip of the current branch is updated to the new commit, and
exit with a zero status.

For example, the hook can simply run `git read-tree -u -m HEAD "$1"`
in order to emulate `git fetch` that is run in the reverse direction
with `git push`, as the two-tree form of `git read-tree -u -m` is
essentially the same as `git switch` or `git checkout`
that switches branches while
keeping the local changes in the working tree that do not interfere
with the difference between the branches.


pre-auto-gc
~~~~~~~~~~~

This hook is invoked by `git gc --auto` (see linkgit:git-gc[1]). It
takes no parameter, and exiting with non-zero status from this script
causes the `git gc --auto` to abort.

post-rewrite
~~~~~~~~~~~~

This hook is invoked by commands that rewrite commits
(linkgit:git-commit[1] when called with `--amend` and
linkgit:git-rebase[1]; however, full-history (re)writing tools like
linkgit:git-fast-import[1] or
https://github.com/newren/git-filter-repo[git-filter-repo] typically
do not call it!).  Its first argument denotes the command it was
invoked by: currently one of `amend` or `rebase`.  Further
command-dependent arguments may be passed in the future.

The hook receives a list of the rewritten commits on stdin, in the
format

  <old-object-name> SP <new-object-name> [ SP <extra-info> ] LF

The 'extra-info' is again command-dependent.  If it is empty, the
preceding SP is also omitted.  Currently, no commands pass any
'extra-info'.

The hook always runs after the automatic note copying (see
"notes.rewrite.<command>" in linkgit:git-config[1]) has happened, and
thus has access to these notes.

The following command-specific comments apply:

rebase::
        For the 'squash' and 'fixup' operation, all commits that were
        squashed are listed as being rewritten to the squashed commit.
        This means that there will be several lines sharing the same
        'new-object-name'.
+
The commits are guaranteed to be listed in the order that they were
processed by rebase.

sendemail-validate
~~~~~~~~~~~~~~~~~~

This hook is invoked by linkgit:git-send-email[1].

It takes these command line arguments. They are,
1. the name of the file which holds the contents of the email to be sent.
2. The name of the file which holds the SMTP headers of the email.

The SMTP headers are passed in the exact same way as they are passed to the
user's Mail Transport Agent (MTA). In effect, the email given to the user's
MTA, is the contents of $2 followed by the contents of $1.

An example of a few common headers is shown below. Take notice of the
capitalization and multi-line tab structure.

  From: Example <from@example.com>
  To: to@example.com
  Cc: cc@example.com,
          A <author@example.com>,
          One <one@example.com>,
          two@example.com
  Subject: PATCH-STRING

Exiting with a non-zero status causes `git send-email` to abort
before sending any e-mails.

The following environment variables are set when executing the hook.

`GIT_SENDEMAIL_FILE_COUNTER`::
        A 1-based counter incremented by one for every file holding an e-mail
        to be sent (excluding any FIFOs). This counter does not follow the
        patch series counter scheme. It will always start at 1 and will end at
        GIT_SENDEMAIL_FILE_TOTAL.

`GIT_SENDEMAIL_FILE_TOTAL`::
        The total number of files that will be sent (excluding any FIFOs). This
        counter does not follow the patch series counter scheme. It will always
        be equal to the number of files being sent, whether there is a cover
        letter or not.

These variables may for instance be used to validate patch series.

The sample `sendemail-validate` hook that comes with Git checks that all sent
patches (excluding the cover letter) can be applied on top of the upstream
repository default branch without conflicts. Some placeholders are left for
additional validation steps to be performed after all patches of a given series
have been applied.

fsmonitor-watchman
~~~~~~~~~~~~~~~~~~

This hook is invoked when the configuration option `core.fsmonitor` is
set to `.git/hooks/fsmonitor-watchman` or `.git/hooks/fsmonitor-watchmanv2`
depending on the version of the hook to use.

Version 1 takes two arguments, a version (1) and the time in elapsed
nanoseconds since midnight, January 1, 1970.

Version 2 takes two arguments, a version (2) and a token that is used
for identifying changes since the token. For watchman this would be
a clock id. This version must output to stdout the new token followed
by a NUL before the list of files.

The hook should output to stdout the list of all files in the working
directory that may have changed since the requested time.  The logic
should be inclusive so that it does not miss any potential changes.
The paths should be relative to the root of the working directory
and be separated by a single NUL.

It is OK to include files which have not actually changed.  All changes
including newly-created and deleted files should be included. When
files are renamed, both the old and the new name should be included.

Git will limit what files it checks for changes as well as which
directories are checked for untracked files based on the path names
given.

An optimized way to tell git "all files have changed" is to return
the filename `/`.

The exit status determines whether git will use the data from the
hook to limit its search.  On error, it will fall back to verifying
all files and folders.

p4-changelist
~~~~~~~~~~~~~

This hook is invoked by `git-p4 submit`.

The `p4-changelist` hook is executed after the changelist
message has been edited by the user. It can be bypassed with the
`--no-verify` option. It takes a single parameter, the name
of the file that holds the proposed changelist text. Exiting
with a non-zero status causes the command to abort.

The hook is allowed to edit the changelist file and can be used
to normalize the text into some project standard format. It can
also be used to refuse the Submit after inspect the message file.

Run `git-p4 submit --help` for details.

p4-prepare-changelist
~~~~~~~~~~~~~~~~~~~~~

This hook is invoked by `git-p4 submit`.

The `p4-prepare-changelist` hook is executed right after preparing
the default changelist message and before the editor is started.
It takes one parameter, the name of the file that contains the
changelist text. Exiting with a non-zero status from the script
will abort the process.

The purpose of the hook is to edit the message file in place,
and it is not suppressed by the `--no-verify` option. This hook
is called even if `--prepare-p4-only` is set.

Run `git-p4 submit --help` for details.

p4-post-changelist
~~~~~~~~~~~~~~~~~~

This hook is invoked by `git-p4 submit`.

The `p4-post-changelist` hook is invoked after the submit has
successfully occurred in P4. It takes no parameters and is meant
primarily for notification and cannot affect the outcome of the
git p4 submit action.

Run `git-p4 submit --help` for details.

p4-pre-submit
~~~~~~~~~~~~~

This hook is invoked by `git-p4 submit`. It takes no parameters and nothing
from standard input. Exiting with non-zero status from this script prevent
`git-p4 submit` from launching. It can be bypassed with the `--no-verify`
command line option. Run `git-p4 submit --help` for details.



post-index-change
~~~~~~~~~~~~~~~~~

This hook is invoked when the index is written in read-cache.c
do_write_locked_index.

The first parameter passed to the hook is the indicator for the
working directory being updated.  "1" meaning working directory
was updated or "0" when the working directory was not updated.

The second parameter passed to the hook is the indicator for whether
or not the index was updated and the skip-worktree bit could have
changed.  "1" meaning skip-worktree bits could have been updated
and "0" meaning they were not.

Only one parameter should be set to "1" when the hook runs.  The hook
running passing "1", "1" should not be possible.

SEE ALSO
--------
linkgit:git-hook[1]

GIT
---
Part of the linkgit:git[1] suite