git-update-ref
(draft)
NAME
git-update-ref - Update the object name stored in a ref safely
SYNOPSIS
git update-ref [-m <reason>] (-d <ref> [<oldvalue>] | [--no-deref] [--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
` 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 ` 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 [ SP [SP ] LF
create SP ][ SP LF
delete SP ][ [SP ] LF
verify SP ][ [SP ] LF
option SP 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 ][ NUL NUL [] NUL
create SP ][ NUL NUL
delete SP ][ NUL [] NUL
verify SP ][ NUL [] NUL
option SP 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:
1. 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:
1. 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
]
