t4034: abstract away SHA-1-specific constants

[thirdparty/git.git] / Documentation / revisions.txt

SPECIFYING REVISIONS
--------------------

A revision parameter '<rev>' typically, but not necessarily, names a
commit object.  It uses what is called an 'extended SHA-1'
syntax.  Here are various ways to spell object names.  The
ones listed near the end of this list name trees and
blobs contained in a commit.

NOTE: This document shows the "raw" syntax as seen by git. The shell
and other UIs might require additional quoting to protect special
characters and to avoid word splitting.

'<sha1>', e.g. 'dae86e1950b1277e545cee180551750029cfe735', 'dae86e'::
  The full SHA-1 object name (40-byte hexadecimal string), or
  a leading substring that is unique within the repository.
  E.g. dae86e1950b1277e545cee180551750029cfe735 and dae86e both
  name the same commit object if there is no other object in
  your repository whose object name starts with dae86e.

'<describeOutput>', e.g. 'v1.7.4.2-679-g3bee7fb'::
  Output from `git describe`; i.e. a closest tag, optionally
  followed by a dash and a number of commits, followed by a dash, a
  'g', and an abbreviated object name.

'<refname>', e.g. 'master', 'heads/master', 'refs/heads/master'::
  A symbolic ref name.  E.g. 'master' typically means the commit
  object referenced by 'refs/heads/master'.  If you
  happen to have both 'heads/master' and 'tags/master', you can
  explicitly say 'heads/master' to tell Git which one you mean.
  When ambiguous, a '<refname>' is disambiguated by taking the
  first match in the following rules:

  . If '$GIT_DIR/<refname>' exists, that is what you mean (this is usually
    useful only for `HEAD`, `FETCH_HEAD`, `ORIG_HEAD`, `MERGE_HEAD`
    and `CHERRY_PICK_HEAD`);

  . otherwise, 'refs/<refname>' if it exists;

  . otherwise, 'refs/tags/<refname>' if it exists;

  . otherwise, 'refs/heads/<refname>' if it exists;

  . otherwise, 'refs/remotes/<refname>' if it exists;

  . otherwise, 'refs/remotes/<refname>/HEAD' if it exists.
+
`HEAD` names the commit on which you based the changes in the working tree.
`FETCH_HEAD` records the branch which you fetched from a remote repository
with your last `git fetch` invocation.
`ORIG_HEAD` is created by commands that move your `HEAD` in a drastic
way, to record the position of the `HEAD` before their operation, so that
you can easily change the tip of the branch back to the state before you ran
them.
`MERGE_HEAD` records the commit(s) which you are merging into your branch
when you run `git merge`.
`CHERRY_PICK_HEAD` records the commit which you are cherry-picking
when you run `git cherry-pick`.
+
Note that any of the 'refs/*' cases above may come either from
the `$GIT_DIR/refs` directory or from the `$GIT_DIR/packed-refs` file.
While the ref name encoding is unspecified, UTF-8 is preferred as
some output processing may assume ref names in UTF-8.

'@'::
  '@' alone is a shortcut for `HEAD`.

'[<refname>]@{<date>}', e.g. 'master@\{yesterday\}', 'HEAD@{5 minutes ago}'::
  A ref followed by the suffix '@' with a date specification
  enclosed in a brace
  pair (e.g. '\{yesterday\}', '{1 month 2 weeks 3 days 1 hour 1
  second ago}' or '{1979-02-26 18:30:00}') specifies the value
  of the ref at a prior point in time.  This suffix may only be
  used immediately following a ref name and the ref must have an
  existing log ('$GIT_DIR/logs/<ref>'). Note that this looks up the state
  of your *local* ref at a given time; e.g., what was in your local
  'master' branch last week. If you want to look at commits made during
  certain times, see `--since` and `--until`.

'<refname>@{<n>}', e.g. 'master@\{1\}'::
  A ref followed by the suffix '@' with an ordinal specification
  enclosed in a brace pair (e.g. '\{1\}', '\{15\}') specifies
  the n-th prior value of that ref.  For example 'master@\{1\}'
  is the immediate prior value of 'master' while 'master@\{5\}'
  is the 5th prior value of 'master'. This suffix may only be used
  immediately following a ref name and the ref must have an existing
  log ('$GIT_DIR/logs/<refname>').

'@{<n>}', e.g. '@\{1\}'::
  You can use the '@' construct with an empty ref part to get at a
  reflog entry of the current branch. For example, if you are on
  branch 'blabla' then '@\{1\}' means the same as 'blabla@\{1\}'.

'@{-<n>}', e.g. '@{-1}'::
  The construct '@{-<n>}' means the <n>th branch/commit checked out
  before the current one.

'[<branchname>]@\{upstream\}', e.g. 'master@\{upstream\}', '@\{u\}'::
  The suffix '@\{upstream\}' to a branchname (short form '<branchname>@\{u\}')
  refers to the branch that the branch specified by branchname is set to build on
  top of (configured with `branch.<name>.remote` and
  `branch.<name>.merge`).  A missing branchname defaults to the
  current one. These suffixes are also accepted when spelled in uppercase, and
  they mean the same thing no matter the case.

'[<branchname>]@\{push\}', e.g. 'master@\{push\}', '@\{push\}'::
  The suffix '@\{push}' reports the branch "where we would push to" if
  `git push` were run while `branchname` was checked out (or the current
  `HEAD` if no branchname is specified). Since our push destination is
  in a remote repository, of course, we report the local tracking branch
  that corresponds to that branch (i.e., something in `refs/remotes/`).
+
Here's an example to make it more clear:
+
------------------------------
$ git config push.default current
$ git config remote.pushdefault myfork
$ git switch -c mybranch origin/master

$ git rev-parse --symbolic-full-name @{upstream}
refs/remotes/origin/master

$ git rev-parse --symbolic-full-name @{push}
refs/remotes/myfork/mybranch
------------------------------
+
Note in the example that we set up a triangular workflow, where we pull
from one location and push to another. In a non-triangular workflow,
'@\{push}' is the same as '@\{upstream}', and there is no need for it.
+
This suffix is also accepted when spelled in uppercase, and means the same
thing no matter the case.

'<rev>{caret}[<n>]', e.g. 'HEAD{caret}, v1.5.1{caret}0'::
  A suffix '{caret}' to a revision parameter means the first parent of
  that commit object.  '{caret}<n>' means the <n>th parent (i.e.
  '<rev>{caret}'
  is equivalent to '<rev>{caret}1').  As a special rule,
  '<rev>{caret}0' means the commit itself and is used when '<rev>' is the
  object name of a tag object that refers to a commit object.

'<rev>{tilde}[<n>]', e.g. 'HEAD{tilde}, master{tilde}3'::
  A suffix '{tilde}' to a revision parameter means the first parent of
  that commit object.
  A suffix '{tilde}<n>' to a revision parameter means the commit
  object that is the <n>th generation ancestor of the named
  commit object, following only the first parents.  I.e. '<rev>{tilde}3' is
  equivalent to '<rev>{caret}{caret}{caret}' which is equivalent to
  '<rev>{caret}1{caret}1{caret}1'.  See below for an illustration of
  the usage of this form.

'<rev>{caret}{<type>}', e.g. 'v0.99.8{caret}\{commit\}'::
  A suffix '{caret}' followed by an object type name enclosed in
  brace pair means dereference the object at '<rev>' recursively until
  an object of type '<type>' is found or the object cannot be
  dereferenced anymore (in which case, barf).
  For example, if '<rev>' is a commit-ish, '<rev>{caret}\{commit\}'
  describes the corresponding commit object.
  Similarly, if '<rev>' is a tree-ish, '<rev>{caret}\{tree\}'
  describes the corresponding tree object.
  '<rev>{caret}0'
  is a short-hand for '<rev>{caret}\{commit\}'.
+
'<rev>{caret}\{object\}' can be used to make sure '<rev>' names an
object that exists, without requiring '<rev>' to be a tag, and
without dereferencing '<rev>'; because a tag is already an object,
it does not have to be dereferenced even once to get to an object.
+
'<rev>{caret}\{tag\}' can be used to ensure that '<rev>' identifies an
existing tag object.

'<rev>{caret}{}', e.g. 'v0.99.8{caret}{}'::
  A suffix '{caret}' followed by an empty brace pair
  means the object could be a tag,
  and dereference the tag recursively until a non-tag object is
  found.

'<rev>{caret}{/<text>}', e.g. 'HEAD^{/fix nasty bug}'::
  A suffix '{caret}' to a revision parameter, followed by a brace
  pair that contains a text led by a slash,
  is the same as the ':/fix nasty bug' syntax below except that
  it returns the youngest matching commit which is reachable from
  the '<rev>' before '{caret}'.

':/<text>', e.g. ':/fix nasty bug'::
  A colon, followed by a slash, followed by a text, names
  a commit whose commit message matches the specified regular expression.
  This name returns the youngest matching commit which is
  reachable from any ref, including HEAD.
  The regular expression can match any part of the
  commit message. To match messages starting with a string, one can use
  e.g. ':/^foo'. The special sequence ':/!' is reserved for modifiers to what
  is matched. ':/!-foo' performs a negative match, while ':/!!foo' matches a
  literal '!' character, followed by 'foo'. Any other sequence beginning with
  ':/!' is reserved for now.
  Depending on the given text, the shell's word splitting rules might
  require additional quoting.

'<rev>:<path>', e.g. 'HEAD:README', 'master:./README'::
  A suffix ':' followed by a path names the blob or tree
  at the given path in the tree-ish object named by the part
  before the colon.
  A path starting with './' or '../' is relative to the current working directory.
  The given path will be converted to be relative to the working tree's root directory.
  This is most useful to address a blob or tree from a commit or tree that has
  the same tree structure as the working tree.

':[<n>:]<path>', e.g. ':0:README', ':README'::
  A colon, optionally followed by a stage number (0 to 3) and a
  colon, followed by a path, names a blob object in the
  index at the given path. A missing stage number (and the colon
  that follows it) names a stage 0 entry. During a merge, stage
  1 is the common ancestor, stage 2 is the target branch's version
  (typically the current branch), and stage 3 is the version from
  the branch which is being merged.

Here is an illustration, by Jon Loeliger.  Both commit nodes B
and C are parents of commit node A.  Parent commits are ordered
left-to-right.

........................................
G   H   I   J
 \ /     \ /
  D   E   F
   \  |  / \
    \ | /   |
     \|/    |
      B     C
       \   /
        \ /
         A
........................................

    A =      = A^0
    B = A^   = A^1     = A~1
    C = A^2  = A^2
    D = A^^  = A^1^1   = A~2
    E = B^2  = A^^2
    F = B^3  = A^^3
    G = A^^^ = A^1^1^1 = A~3
    H = D^2  = B^^2    = A^^^2  = A~2^2
    I = F^   = B^3^    = A^^3^
    J = F^2  = B^3^2   = A^^3^2


SPECIFYING RANGES
-----------------

History traversing commands such as `git log` operate on a set
of commits, not just a single commit.

For these commands,
specifying a single revision, using the notation described in the
previous section, means the set of commits `reachable` from the given
commit.

A commit's reachable set is the commit itself and the commits in
its ancestry chain.


Commit Exclusions
~~~~~~~~~~~~~~~~~

'{caret}<rev>' (caret) Notation::
 To exclude commits reachable from a commit, a prefix '{caret}'
 notation is used.  E.g. '{caret}r1 r2' means commits reachable
 from 'r2' but exclude the ones reachable from 'r1' (i.e. 'r1' and
 its ancestors).

Dotted Range Notations
~~~~~~~~~~~~~~~~~~~~~~

The '..' (two-dot) Range Notation::
 The '{caret}r1 r2' set operation appears so often that there is a shorthand
 for it.  When you have two commits 'r1' and 'r2' (named according
 to the syntax explained in SPECIFYING REVISIONS above), you can ask
 for commits that are reachable from r2 excluding those that are reachable
 from r1 by '{caret}r1 r2' and it can be written as 'r1..r2'.

The '...' (three-dot) Symmetric Difference Notation::
 A similar notation 'r1\...r2' is called symmetric difference
 of 'r1' and 'r2' and is defined as
 'r1 r2 --not $(git merge-base --all r1 r2)'.
 It is the set of commits that are reachable from either one of
 'r1' (left side) or 'r2' (right side) but not from both.

In these two shorthand notations, you can omit one end and let it default to HEAD.
For example, 'origin..' is a shorthand for 'origin..HEAD' and asks "What
did I do since I forked from the origin branch?"  Similarly, '..origin'
is a shorthand for 'HEAD..origin' and asks "What did the origin do since
I forked from them?"  Note that '..' would mean 'HEAD..HEAD' which is an
empty range that is both reachable and unreachable from HEAD.

Other <rev>{caret} Parent Shorthand Notations
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Three other shorthands exist, particularly useful for merge commits,
for naming a set that is formed by a commit and its parent commits.

The 'r1{caret}@' notation means all parents of 'r1'.

The 'r1{caret}!' notation includes commit 'r1' but excludes all of its parents.
By itself, this notation denotes the single commit 'r1'.

The '<rev>{caret}-[<n>]' notation includes '<rev>' but excludes the <n>th
parent (i.e. a shorthand for '<rev>{caret}<n>..<rev>'), with '<n>' = 1 if
not given. This is typically useful for merge commits where you
can just pass '<commit>{caret}-' to get all the commits in the branch
that was merged in merge commit '<commit>' (including '<commit>'
itself).

While '<rev>{caret}<n>' was about specifying a single commit parent, these
three notations also consider its parents. For example you can say
'HEAD{caret}2{caret}@', however you cannot say 'HEAD{caret}@{caret}2'.

Revision Range Summary
----------------------

'<rev>'::
	Include commits that are reachable from <rev> (i.e. <rev> and its
	ancestors).

'{caret}<rev>'::
	Exclude commits that are reachable from <rev> (i.e. <rev> and its
	ancestors).

'<rev1>..<rev2>'::
	Include commits that are reachable from <rev2> but exclude
	those that are reachable from <rev1>.  When either <rev1> or
	<rev2> is omitted, it defaults to `HEAD`.

'<rev1>\...<rev2>'::
	Include commits that are reachable from either <rev1> or
	<rev2> but exclude those that are reachable from both.  When
	either <rev1> or <rev2> is omitted, it defaults to `HEAD`.

'<rev>{caret}@', e.g. 'HEAD{caret}@'::
  A suffix '{caret}' followed by an at sign is the same as listing
  all parents of '<rev>' (meaning, include anything reachable from
  its parents, but not the commit itself).

'<rev>{caret}!', e.g. 'HEAD{caret}!'::
  A suffix '{caret}' followed by an exclamation mark is the same
  as giving commit '<rev>' and then all its parents prefixed with
  '{caret}' to exclude them (and their ancestors).

'<rev>{caret}-<n>', e.g. 'HEAD{caret}-, HEAD{caret}-2'::
	Equivalent to '<rev>{caret}<n>..<rev>', with '<n>' = 1 if not
	given.

Here are a handful of examples using the Loeliger illustration above,
with each step in the notation's expansion and selection carefully
spelt out:

....
   Args   Expanded arguments    Selected commits
   D                            G H D
   D F                          G H I J D F
   ^G D                         H D
   ^D B                         E I J F B
   ^D B C                       E I J F B C
   C                            I J F C
   B..C   = ^B C                C
   B...C  = B ^F C              G H D E B C
   B^-    = B^..B
	  = ^B^1 B              E I J F B
   C^@    = C^1
	  = F                   I J F
   B^@    = B^1 B^2 B^3
	  = D E F               D G H E F I J
   C^!    = C ^C^@
	  = C ^C^1
	  = C ^F                C
   B^!    = B ^B^@
	  = B ^B^1 ^B^2 ^B^3
	  = B ^D ^E ^F          B
   F^! D  = F ^I ^J D           G H D F
....