Documentation/git-update-ref.txt

   1 git-update-ref(1)
   2 =================
   3
   4 NAME
   5 ----
   6 git-update-ref - Update the object name stored in a ref safely
   7
   8 SYNOPSIS
   9 --------
  10 'git-update-ref' [-m <reason>] (-d <ref> <oldvalue> | <ref> <newvalue> [<oldvalue>])
  11
  12 DESCRIPTION
  13 -----------
  14 Given two arguments, stores the <newvalue> in the <ref>, possibly
  15 dereferencing the symbolic refs.  E.g. `git-update-ref HEAD
  16 <newvalue>` updates the current branch head to the new object.
  17
  18 Given three arguments, stores the <newvalue> in the <ref>,
  19 possibly dereferencing the symbolic refs, after verifying that
  20 the current value of the <ref> matches <oldvalue>.
  21 E.g. `git-update-ref refs/heads/master <newvalue> <oldvalue>`
  22 updates the master branch head to <newvalue> only if its current
  23 value is <oldvalue>.  You can specify 40 "0" or an empty string
  24 as <oldvalue> to make sure that the ref you are creating does
  25 not exist.
  26
  27 It also allows a "ref" file to be a symbolic pointer to another
  28 ref file by starting with the four-byte header sequence of
  29 "ref:".
  30
  31 More importantly, it allows the update of a ref file to follow
  32 these symbolic pointers, whether they are symlinks or these
  33 "regular file symbolic refs".  It follows *real* symlinks only
  34 if they start with "refs/": otherwise it will just try to read
  35 them and update them as a regular file (i.e. it will allow the
  36 filesystem to follow them, but will overwrite such a symlink to
  37 somewhere else with a regular filename).
  38
  39 In general, using
  40
  41         git-update-ref HEAD "$head"
  42
  43 should be a _lot_ safer than doing
  44
  45         echo "$head" > "$GIT_DIR/HEAD"
  46
  47 both from a symlink following standpoint *and* an error checking
  48 standpoint.  The "refs/" rule for symlinks means that symlinks
  49 that point to "outside" the tree are safe: they'll be followed
  50 for reading but not for writing (so we'll never write through a
  51 ref symlink to some other tree, if you have copied a whole
  52 archive by creating a symlink tree).
  53
  54 With `-d` flag, it deletes the named <ref> after verifying it
  55 still contains <oldvalue>.
  56
  57
  58 Logging Updates
  59 ---------------
  60 If config parameter "core.logAllRefUpdates" is true or the file
  61 "$GIT_DIR/logs/<ref>" exists then `git-update-ref` will append
  62 a line to the log file "$GIT_DIR/logs/<ref>" (dereferencing all
  63 symbolic refs before creating the log name) describing the change
  64 in ref value.  Log lines are formatted as:
  65
  66     . oldsha1 SP newsha1 SP committer LF
  67 +
  68 Where "oldsha1" is the 40 character hexadecimal value previously
  69 stored in <ref>, "newsha1" is the 40 character hexadecimal value of
  70 <newvalue> and "committer" is the committer's name, email address
  71 and date in the standard GIT committer ident format.
  72
  73 Optionally with -m:
  74
  75     . oldsha1 SP newsha1 SP committer TAB message LF
  76 +
  77 Where all fields are as described above and "message" is the
  78 value supplied to the -m option.
  79
  80 An update will fail (without changing <ref>) if the current user is
  81 unable to create a new log file, append to the existing log file
  82 or does not have committer information available.
  83
  84 Author
  85 ------
  86 Written by Linus Torvalds <torvalds@osdl.org>.
  87
  88 GIT
  89 ---
  90 Part of the gitlink:git[7] suite