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git/Documentation/git-update-ref.txt
Elijah Newren d345e9fbe7 update-ref: allow --no-deref with --stdin
If passed both --no-deref and --stdin, update-ref would error out with a
general usage message that did not at all suggest these options were
incompatible.  The manpage for update-ref did suggest through its
synopsis line that --no-deref and --stdin were incompatible, but it sadly
also incorrectly suggested that -d and --no-deref were incompatible.  So
the help around the --no-deref option is buggy in a few ways.

The --stdin option did provide a different mechanism for avoiding
dereferencing symbolic-refs: adding a line reading
  option no-deref
before every other directive in the input.  (Technically, if the user
wants to do the extra work of first determining which refs they want to
update or delete are symbolic, then they only need to put the extra
"option no-deref" lines before the updates of those refs.  But in some
cases, that's more work than just adding the "option no-deref" before
every other directive.)

It's easier to allow the user to just pass --no-deref along with --stdin
in order to tell update-ref that the user doesn't want any symbolic ref
to be dereferenced.  It also makes the update-ref documentation simpler.
Implement that, and update the documentation to match.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2018-09-12 15:17:17 -07:00

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git-update-ref(1)
=================
NAME
----
git-update-ref - Update the object name stored in a ref safely
SYNOPSIS
--------
[verse]
'git update-ref' [-m <reason>] [--no-deref] (-d <ref> [<oldvalue>] | [--create-reflog] <ref> <newvalue> [<oldvalue>] | --stdin [-z])
DESCRIPTION
-----------
Given two arguments, stores the <newvalue> in the <ref>, possibly
dereferencing the symbolic refs. E.g. `git update-ref HEAD
<newvalue>` updates the current branch head to the new object.
Given three arguments, stores the <newvalue> in the <ref>,
possibly dereferencing the symbolic refs, after verifying that
the current value of the <ref> matches <oldvalue>.
E.g. `git update-ref refs/heads/master <newvalue> <oldvalue>`
updates the master branch head to <newvalue> only if its current
value is <oldvalue>. You can specify 40 "0" or an empty string
as <oldvalue> to make sure that the ref you are creating does
not exist.
It also allows a "ref" file to be a symbolic pointer to another
ref file by starting with the four-byte header sequence of
"ref:".
More importantly, it allows the update of a ref file to follow
these symbolic pointers, whether they are symlinks or these
"regular file symbolic refs". It follows *real* symlinks only
if they start with "refs/": otherwise it will just try to read
them and update them as a regular file (i.e. it will allow the
filesystem to follow them, but will overwrite such a symlink to
somewhere else with a regular filename).
If --no-deref is given, <ref> itself is overwritten, rather than
the result of following the symbolic pointers.
In general, using
git update-ref HEAD "$head"
should be a _lot_ safer than doing
echo "$head" > "$GIT_DIR/HEAD"
both from a symlink following standpoint *and* an error checking
standpoint. The "refs/" rule for symlinks means that symlinks
that point to "outside" the tree are safe: they'll be followed
for reading but not for writing (so we'll never write through a
ref symlink to some other tree, if you have copied a whole
archive by creating a symlink tree).
With `-d` flag, it deletes the named <ref> after verifying it
still contains <oldvalue>.
With `--stdin`, update-ref reads instructions from standard input and
performs all modifications together. Specify commands of the form:
update SP <ref> SP <newvalue> [SP <oldvalue>] LF
create SP <ref> SP <newvalue> LF
delete SP <ref> [SP <oldvalue>] LF
verify SP <ref> [SP <oldvalue>] LF
option SP <opt> LF
With `--create-reflog`, update-ref will create a reflog for each ref
even if one would not ordinarily be created.
Quote fields containing whitespace as if they were strings in C source
code; i.e., surrounded by double-quotes and with backslash escapes.
Use 40 "0" characters or the empty string to specify a zero value. To
specify a missing value, omit the value and its preceding SP entirely.
Alternatively, use `-z` to specify in NUL-terminated format, without
quoting:
update SP <ref> NUL <newvalue> NUL [<oldvalue>] NUL
create SP <ref> NUL <newvalue> NUL
delete SP <ref> NUL [<oldvalue>] NUL
verify SP <ref> NUL [<oldvalue>] NUL
option SP <opt> NUL
In this format, use 40 "0" to specify a zero value, and use the empty
string to specify a missing value.
In either format, values can be specified in any form that Git
recognizes as an object name. Commands in any other format or a
repeated <ref> produce an error. Command meanings are:
update::
Set <ref> to <newvalue> after verifying <oldvalue>, if given.
Specify a zero <newvalue> to ensure the ref does not exist
after the update and/or a zero <oldvalue> to make sure the
ref does not exist before the update.
create::
Create <ref> with <newvalue> after verifying it does not
exist. The given <newvalue> may not be zero.
delete::
Delete <ref> after verifying it exists with <oldvalue>, if
given. If given, <oldvalue> may not be zero.
verify::
Verify <ref> against <oldvalue> but do not change it. If
<oldvalue> zero or missing, the ref must not exist.
option::
Modify behavior of the next command naming a <ref>.
The only valid option is `no-deref` to avoid dereferencing
a symbolic ref.
If all <ref>s can be locked with matching <oldvalue>s
simultaneously, all modifications are performed. Otherwise, no
modifications are performed. Note that while each individual
<ref> is updated or deleted atomically, a concurrent reader may
still see a subset of the modifications.
LOGGING UPDATES
---------------
If config parameter "core.logAllRefUpdates" is true and the ref is one under
"refs/heads/", "refs/remotes/", "refs/notes/", or the symbolic ref HEAD; or
the file "$GIT_DIR/logs/<ref>" exists then `git update-ref` will append
a line to the log file "$GIT_DIR/logs/<ref>" (dereferencing all
symbolic refs before creating the log name) describing the change
in ref value. Log lines are formatted as:
. oldsha1 SP newsha1 SP committer LF
+
Where "oldsha1" is the 40 character hexadecimal value previously
stored in <ref>, "newsha1" is the 40 character hexadecimal value of
<newvalue> and "committer" is the committer's name, email address
and date in the standard Git committer ident format.
Optionally with -m:
. oldsha1 SP newsha1 SP committer TAB message LF
+
Where all fields are as described above and "message" is the
value supplied to the -m option.
An update will fail (without changing <ref>) if the current user is
unable to create a new log file, append to the existing log file
or does not have committer information available.
GIT
---
Part of the linkgit:git[1] suite