71bcb7954f0c53a995ba56de6c99d9aebc6aa52a
[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>] (-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