compat/bswap.h: fix endianness detection
[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 [verse]
11 'git update-ref' [-m <reason>] (-d <ref> [<oldvalue>] | [--no-deref] <ref> <newvalue> [<oldvalue>] | --stdin [-z])
12
13 DESCRIPTION
14 -----------
15 Given two arguments, stores the <newvalue> in the <ref>, possibly
16 dereferencing the symbolic refs. E.g. `git update-ref HEAD
17 <newvalue>` updates the current branch head to the new object.
18
19 Given three arguments, stores the <newvalue> in the <ref>,
20 possibly dereferencing the symbolic refs, after verifying that
21 the current value of the <ref> matches <oldvalue>.
22 E.g. `git update-ref refs/heads/master <newvalue> <oldvalue>`
23 updates the master branch head to <newvalue> only if its current
24 value is <oldvalue>. You can specify 40 "0" or an empty string
25 as <oldvalue> to make sure that the ref you are creating does
26 not exist.
27
28 It also allows a "ref" file to be a symbolic pointer to another
29 ref file by starting with the four-byte header sequence of
30 "ref:".
31
32 More importantly, it allows the update of a ref file to follow
33 these symbolic pointers, whether they are symlinks or these
34 "regular file symbolic refs". It follows *real* symlinks only
35 if they start with "refs/": otherwise it will just try to read
36 them and update them as a regular file (i.e. it will allow the
37 filesystem to follow them, but will overwrite such a symlink to
38 somewhere else with a regular filename).
39
40 If --no-deref is given, <ref> itself is overwritten, rather than
41 the result of following the symbolic pointers.
42
43 In general, using
44
45 git update-ref HEAD "$head"
46
47 should be a _lot_ safer than doing
48
49 echo "$head" > "$GIT_DIR/HEAD"
50
51 both from a symlink following standpoint *and* an error checking
52 standpoint. The "refs/" rule for symlinks means that symlinks
53 that point to "outside" the tree are safe: they'll be followed
54 for reading but not for writing (so we'll never write through a
55 ref symlink to some other tree, if you have copied a whole
56 archive by creating a symlink tree).
57
58 With `-d` flag, it deletes the named <ref> after verifying it
59 still contains <oldvalue>.
60
61 With `--stdin`, update-ref reads instructions from standard input and
62 performs all modifications together. Specify commands of the form:
63
64 update SP <ref> SP <newvalue> [SP <oldvalue>] LF
65 create SP <ref> SP <newvalue> LF
66 delete SP <ref> [SP <oldvalue>] LF
67 verify SP <ref> [SP <oldvalue>] LF
68 option SP <opt> LF
69
70 Quote fields containing whitespace as if they were strings in C source
71 code. Alternatively, use `-z` to specify commands without quoting:
72
73 update SP <ref> NUL <newvalue> NUL [<oldvalue>] NUL
74 create SP <ref> NUL <newvalue> NUL
75 delete SP <ref> NUL [<oldvalue>] NUL
76 verify SP <ref> NUL [<oldvalue>] NUL
77 option SP <opt> NUL
78
79 Lines of any other format or a repeated <ref> produce an error.
80 Command meanings are:
81
82 update::
83 Set <ref> to <newvalue> after verifying <oldvalue>, if given.
84 Specify a zero <newvalue> to ensure the ref does not exist
85 after the update and/or a zero <oldvalue> to make sure the
86 ref does not exist before the update.
87
88 create::
89 Create <ref> with <newvalue> after verifying it does not
90 exist. The given <newvalue> may not be zero.
91
92 delete::
93 Delete <ref> after verifying it exists with <oldvalue>, if
94 given. If given, <oldvalue> may not be zero.
95
96 verify::
97 Verify <ref> against <oldvalue> but do not change it. If
98 <oldvalue> zero or missing, the ref must not exist.
99
100 option::
101 Modify behavior of the next command naming a <ref>.
102 The only valid option is `no-deref` to avoid dereferencing
103 a symbolic ref.
104
105 Use 40 "0" or the empty string to specify a zero value, except that
106 with `-z` an empty <oldvalue> is considered missing.
107
108 If all <ref>s can be locked with matching <oldvalue>s
109 simultaneously, all modifications are performed. Otherwise, no
110 modifications are performed. Note that while each individual
111 <ref> is updated or deleted atomically, a concurrent reader may
112 still see a subset of the modifications.
113
114 Logging Updates
115 ---------------
116 If config parameter "core.logAllRefUpdates" is true and the ref is one under
117 "refs/heads/", "refs/remotes/", "refs/notes/", or the symbolic ref HEAD; or
118 the file "$GIT_DIR/logs/<ref>" exists then `git update-ref` will append
119 a line to the log file "$GIT_DIR/logs/<ref>" (dereferencing all
120 symbolic refs before creating the log name) describing the change
121 in ref value. Log lines are formatted as:
122
123 . oldsha1 SP newsha1 SP committer LF
124 +
125 Where "oldsha1" is the 40 character hexadecimal value previously
126 stored in <ref>, "newsha1" is the 40 character hexadecimal value of
127 <newvalue> and "committer" is the committer's name, email address
128 and date in the standard Git committer ident format.
129
130 Optionally with -m:
131
132 . oldsha1 SP newsha1 SP committer TAB message LF
133 +
134 Where all fields are as described above and "message" is the
135 value supplied to the -m option.
136
137 An update will fail (without changing <ref>) if the current user is
138 unable to create a new log file, append to the existing log file
139 or does not have committer information available.
140
141 GIT
142 ---
143 Part of the linkgit:git[1] suite