Merge branch 'lt/tree-2'
[git/git.git] / 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>] <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>.
24
25 It also allows a "ref" file to be a symbolic pointer to another
26 ref file by starting with the four-byte header sequence of
27 "ref:".
28
29 More importantly, it allows the update of a ref file to follow
30 these symbolic pointers, whether they are symlinks or these
31 "regular file symbolic refs". It follows *real* symlinks only
32 if they start with "refs/": otherwise it will just try to read
33 them and update them as a regular file (i.e. it will allow the
34 filesystem to follow them, but will overwrite such a symlink to
35 somewhere else with a regular filename).
36
37 In general, using
38
39 git-update-ref HEAD "$head"
40
41 should be a _lot_ safer than doing
42
43 echo "$head" > "$GIT_DIR/HEAD"
44
45 both from a symlink following standpoint *and* an error checking
46 standpoint. The "refs/" rule for symlinks means that symlinks
47 that point to "outside" the tree are safe: they'll be followed
48 for reading but not for writing (so we'll never write through a
49 ref symlink to some other tree, if you have copied a whole
50 archive by creating a symlink tree).
51
52 Logging Updates
53 ---------------
54 If config parameter "core.logAllRefUpdates" is true or the file
55 "$GIT_DIR/logs/<ref>" exists then `git-update-ref` will append
56 a line to the log file "$GIT_DIR/logs/<ref>" (dereferencing all
57 symbolic refs before creating the log name) describing the change
58 in ref value. Log lines are formatted as:
59
60 . oldsha1 SP newsha1 SP committer LF
61 +
62 Where "oldsha1" is the 40 character hexadecimal value previously
63 stored in <ref>, "newsha1" is the 40 character hexadecimal value of
64 <newvalue> and "committer" is the committer's name, email address
65 and date in the standard GIT committer ident format.
66
67 Optionally with -m:
68
69 . oldsha1 SP newsha1 SP committer TAB message LF
70 +
71 Where all fields are as described above and "message" is the
72 value supplied to the -m option.
73
74 An update will fail (without changing <ref>) if the current user is
75 unable to create a new log file, append to the existing log file
76 or does not have committer information available.
77
78 Author
79 ------
80 Written by Linus Torvalds <torvalds@osdl.org>.
81
82 GIT
83 ---
84 Part of the gitlink:git[7] suite