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: theyll be followed for reading but not for writing (so well 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:
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 <ref> with <newvalue> after verifying it does not exist. The given <newvalue> may not be zero.
Delete <ref> after verifying it exists with <oldvalue>, if given. If given, <oldvalue> may not be zero.
Verify <ref> against <oldvalue> but do not change it. If <oldvalue> zero or missing, the ref must not exist.
Modify behavior of the next command naming a <ref>. The only valid option is
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.