git/Documentation/revisions.txt

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

A revision parameter typically, but not necessarily, names a
commit object.  They use what is called an 'extended SHA1'
syntax.  Here are various ways to spell object names.  The
ones listed near the end of this list are to name trees and
blobs contained in a commit.

* The full SHA1 object name (40-byte hexadecimal string), or
  a substring of such that is unique within the repository.
  E.g. dae86e1950b1277e545cee180551750029cfe735 and dae86e both
  name the same commit object if there are no other object in
  your repository whose object name starts with dae86e.

* An 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.

* 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 `<name>` is disambiguated by taking the
  first match in the following rules:

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

  . otherwise, `refs/<name>` if exists;

  . otherwise, `refs/tags/<name>` if exists;

  . otherwise, `refs/heads/<name>` if exists;

  . otherwise, `refs/remotes/<name>` if exists;

  . otherwise, `refs/remotes/<name>/HEAD` if exists.
+
HEAD names the commit your changes in the working tree is based on.
FETCH_HEAD records the branch you fetched from a remote repository
with your last 'git fetch' invocation.
ORIG_HEAD is created by commands that moves your HEAD in a drastic
way, to record the position of the HEAD before their operation, so that
you can change the tip of the branch back to the state before you ran
them easily.
MERGE_HEAD records the commit(s) you are merging into your branch
when you run 'git merge'.
+
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.

* 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\}') to specify 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`.

* A ref followed by the suffix '@' with an ordinal specification
  enclosed in a brace pair (e.g. '\{1\}', '\{15\}') to specify
  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/<ref>).

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

* The special construct '@\{-<n>\}' means the <n>th branch checked out
  before the current one.

* The suffix '@\{upstream\}' to a ref (short form 'ref@\{u\}') refers to
  the branch the ref is set to build on top of.  Missing ref defaults
  to the current branch.

* A suffix '{caret}' to a revision parameter (e.g. 'HEAD{caret}') 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.

* A suffix '{tilde}<n>' to a revision parameter means the commit
  object that is the <n>th generation grand-parent of the named
  commit object, following only the first parent.  I.e. rev~3 is
  equivalent to rev{caret}{caret}{caret} which is equivalent to
  rev{caret}1{caret}1{caret}1.  See below for a illustration of
  the usage of this form.

* A suffix '{caret}' followed by an object type name enclosed in
  brace pair (e.g. `v0.99.8{caret}\{commit\}`) means the object
  could be a tag, and dereference the tag recursively until an
  object of that type is found or the object cannot be
  dereferenced anymore (in which case, barf).  `rev{caret}0`
  introduced earlier is a short-hand for `rev{caret}\{commit\}`.

* A suffix '{caret}' followed by an empty brace pair
  (e.g. `v0.99.8{caret}\{\}`) means the object could be a tag,
  and dereference the tag recursively until a non-tag object is
  found.

* A colon, followed by a slash, followed by a text (e.g. `:/fix nasty bug`): this names
  a commit whose commit message starts with the specified text.
  This name returns the youngest matching commit which is
  reachable from any ref.  If the commit message starts with a
  '!', you have to repeat that;  the special sequence ':/!',
  followed by something else than '!' is reserved for now.

* A suffix ':' followed by a path (e.g. `HEAD:README`); this names the blob or tree
  at the given path in the tree-ish object named by the part
  before the colon.
  ':path' (with an empty part before the colon, e.g. `:README`)
  is a special case of the syntax described next: content
  recorded in the index at the given path.

* A colon, optionally followed by a stage number (0 to 3) and a
  colon, followed by a path (e.g. `:0:README`); this names a blob object in the
  index at the given path. Missing stage number (and the colon
  that follows it, e.g. `:README`) 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 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.  To these commands,
specifying a single revision with the notation described in the
previous section means the set of commits reachable from that
commit, following the commit ancestry chain.

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`.

This 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`.

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` or `r2` but not from both.

Two other shorthands for naming a set that is formed by a commit
and its parent commits exist.  The `r1{caret}@` notation means all
parents of `r1`.  `r1{caret}!` includes commit `r1` but excludes
all of its parents.

Here are a handful of examples:

   D                G H D
   D F              G H I J D F
   ^G D             H D
   ^D B             E I J F B
   B...C            G H D E B C
   ^D B C           E I J F B C
   C^@              I J F
   F^! D            G H D F