git-update-ref <root
GIT-UPDATE-REF(1)                           Git Manual                          GIT-UPDATE-REF(1)

NAME
       git-update-ref - Update the object name stored in a ref safely

SYNOPSIS
       git update-ref [-m ] (-d  [] | [--no-deref] [--create-reflog]   [] | --stdin [-z])

DESCRIPTION
       Given two arguments, stores the  in the , 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  in the , possibly dereferencing the
       symbolic refs, after verifying that the current value of the  matches .
       E.g. git update-ref refs/heads/master   updates the master branch head
       to  only if its current value is . You can specify 40 "0" or an empty
       string as  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,  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  after verifying it still contains .

       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  produce an error. Command meanings
       are:

       update
           Set  to  after verifying , if given. Specify a zero
            to ensure the ref does not exist after the update and/or a zero 
           to make sure the ref does not exist before the update.

       create
           Create  with  after verifying it does not exist. The given 
           may not be zero.

       delete
           Delete  after verifying it exists with , if given. If given, 
           may not be zero.

       verify
           Verify  against  but do not change it. If  zero or missing,
           the ref must not exist.

       option
           Modify behavior of the next command naming a . The only valid option is no-deref
           to avoid dereferencing a symbolic ref.

       If all s can be locked with matching s simultaneously, all modifications
       are performed. Otherwise, no modifications are performed. Note that while each individual
        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
       "$GIT_DIR/logs/" exists then git update-ref will append a line to the log file
       "$GIT_DIR/logs/" (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 ,
           "newsha1" is the 40 character hexadecimal value of  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 ) 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 git(1) suite

Git 2.7.4                                   03/23/2016                          GIT-UPDATE-REF(1)

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