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+git-update-ref - update the object name stored in a ref safely
+`git-update-ref` <ref> <newvalue> [<oldvalue>]
+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>.
+It also allows a "ref" file to be a symbolic pointer to another
+ref file by starting with the four-byte header sequence of
+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).
+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).
+Written by Linus Torvalds <>.
+Part of the gitlink:git[7] suite