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+git-update-ref(1)
+=================
+
+NAME
+----
+git-update-ref - Update the object name stored in a ref safely
+
+SYNOPSIS
+--------
+'git update-ref' [-m <reason>] (-d <ref> [<oldvalue>] | [--no-deref] <ref> <newvalue> [<oldvalue>])
+
+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" > "$GIT_DIR/HEAD"
+
+both from a symlink following standpoint *and* an error checking
+standpoint.  The "refs/" rule for symlinks means that symlinks
+that point to "outside" the tree are safe: they'll be followed
+for reading but not for writing (so we'll never write through a
+ref symlink to some other tree, if you have copied a whole
+archive by creating a symlink tree).
+
+With `-d` flag, it deletes the named <ref> after verifying it
+still contains <oldvalue>.
+
+
+Logging Updates
+---------------
+If config parameter "core.logAllRefUpdates" is true or the file
+"$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:
+
+    . 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.
+
+Author
+------
+Written by Linus Torvalds <torvalds@osdl.org>.
+
+GIT
+---
+Part of the linkgit:git[1] suite