.\" ** You probably do not want to edit this file directly **
.\" It was generated using the DocBook XSL Stylesheets (version 1.69.1).
.\" Instead of manually editing it, you probably should edit the DocBook XML
.\" source for it and then use the DocBook XSL Stylesheets to regenerate it.
.TH "GIT\-UPDATE\-REF" "1" "09/19/2007" "Git 1.5.3.2" "Git Manual"
.\" disable hyphenation
.nh
.\" disable justification (adjust text to left margin only)
.ad l
.SH "NAME"
git\-update\-ref \- Update the object name stored in a ref safely
.SH "SYNOPSIS"
\fIgit\-update\-ref\fR [\-m <reason>] (\-d <ref> <oldvalue> | [\-\-no\-deref] <ref> <newvalue> [<oldvalue>])
.SH "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 \fBreal\fR 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
.sp
.nf
git\-update\-ref HEAD "$head"
.fi
should be a _lot_ safer than doing
.sp
.nf
echo "$head" > "$GIT_DIR/HEAD"
.fi
both from a symlink following standpoint \fBand\fR 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>.
.SH "LOGGING UPDATES"
If config parameter "core.logAllRefUpdates" is true 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:
.TP 3
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:
.TP 3
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.
.SH "AUTHOR"
Written by Linus Torvalds <torvalds@osdl.org>.
.SH "GIT"
Part of the \fBgit\fR(7) suite