Documentation / git-update-ref.txton commit Sync with maint (d2dbd39)
   1git-update-ref(1)
   2=================
   3
   4NAME
   5----
   6git-update-ref - Update the object name stored in a ref safely
   7
   8SYNOPSIS
   9--------
  10[verse]
  11'git update-ref' [-m <reason>] (-d <ref> [<oldvalue>] | [--no-deref] <ref> <newvalue> [<oldvalue>])
  12
  13DESCRIPTION
  14-----------
  15Given two arguments, stores the <newvalue> in the <ref>, possibly
  16dereferencing the symbolic refs.  E.g. `git update-ref HEAD
  17<newvalue>` updates the current branch head to the new object.
  18
  19Given three arguments, stores the <newvalue> in the <ref>,
  20possibly dereferencing the symbolic refs, after verifying that
  21the current value of the <ref> matches <oldvalue>.
  22E.g. `git update-ref refs/heads/master <newvalue> <oldvalue>`
  23updates the master branch head to <newvalue> only if its current
  24value is <oldvalue>.  You can specify 40 "0" or an empty string
  25as <oldvalue> to make sure that the ref you are creating does
  26not exist.
  27
  28It also allows a "ref" file to be a symbolic pointer to another
  29ref file by starting with the four-byte header sequence of
  30"ref:".
  31
  32More importantly, it allows the update of a ref file to follow
  33these symbolic pointers, whether they are symlinks or these
  34"regular file symbolic refs".  It follows *real* symlinks only
  35if they start with "refs/": otherwise it will just try to read
  36them and update them as a regular file (i.e. it will allow the
  37filesystem to follow them, but will overwrite such a symlink to
  38somewhere else with a regular filename).
  39
  40If --no-deref is given, <ref> itself is overwritten, rather than
  41the result of following the symbolic pointers.
  42
  43In general, using
  44
  45        git update-ref HEAD "$head"
  46
  47should be a _lot_ safer than doing
  48
  49        echo "$head" > "$GIT_DIR/HEAD"
  50
  51both from a symlink following standpoint *and* an error checking
  52standpoint.  The "refs/" rule for symlinks means that symlinks
  53that point to "outside" the tree are safe: they'll be followed
  54for reading but not for writing (so we'll never write through a
  55ref symlink to some other tree, if you have copied a whole
  56archive by creating a symlink tree).
  57
  58With `-d` flag, it deletes the named <ref> after verifying it
  59still contains <oldvalue>.
  60
  61
  62Logging Updates
  63---------------
  64If config parameter "core.logAllRefUpdates" is true and the ref is one under
  65"refs/heads/", "refs/remotes/", "refs/notes/", or the symbolic ref HEAD; or
  66the file "$GIT_DIR/logs/<ref>" exists then `git update-ref` will append
  67a line to the log file "$GIT_DIR/logs/<ref>" (dereferencing all
  68symbolic refs before creating the log name) describing the change
  69in ref value.  Log lines are formatted as:
  70
  71    . oldsha1 SP newsha1 SP committer LF
  72+
  73Where "oldsha1" is the 40 character hexadecimal value previously
  74stored in <ref>, "newsha1" is the 40 character hexadecimal value of
  75<newvalue> and "committer" is the committer's name, email address
  76and date in the standard Git committer ident format.
  77
  78Optionally with -m:
  79
  80    . oldsha1 SP newsha1 SP committer TAB message LF
  81+
  82Where all fields are as described above and "message" is the
  83value supplied to the -m option.
  84
  85An update will fail (without changing <ref>) if the current user is
  86unable to create a new log file, append to the existing log file
  87or does not have committer information available.
  88
  89GIT
  90---
  91Part of the linkgit:git[1] suite