git-update-ref(1)

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NAME

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git-update-ref - update the object name stored in a ref safely

SYNOPSIS

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`git-update-ref` <ref> <newvalue> [<oldvalue>]

DESCRIPTION

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Given two arguments, stores the <newvalue> in the <ref>, possibly

dereferencing the symbolic refs. E.g. `git-update-ref HEAD

<newvalue>` updates the current branch head to the new object.

Given three arguments, stores the <newvalue> in the <ref>,

possibly dereferencing the symbolic refs, after verifying that

the current value of the <ref> matches <oldvalue>.

E.g. `git-update-ref refs/heads/master <newvalue> <oldvalue>`

updates the master branch head to <newvalue> only if its current

value is <oldvalue>.

It also allows a "ref" file to be a symbolic pointer to another

ref file by starting with the four-byte header sequence of

"ref:".

More importantly, it allows the update of a ref file to follow

these symbolic pointers, whether they are symlinks or these

"regular file symbolic refs". It follows *real* symlinks only

if they start with "refs/": otherwise it will just try to read

them and update them as a regular file (i.e. it will allow the

filesystem to follow them, but will overwrite such a symlink to

somewhere else with a regular filename).

In general, using

git-update-ref HEAD "$head"

should be a _lot_ safer than doing

echo "$head" > "$GIT_DIR/HEAD"

both from a symlink following standpoint *and* an error checking

standpoint. The "refs/" rule for symlinks means that symlinks

that point to "outside" the tree are safe: they'll be followed

for reading but not for writing (so we'll never write through a

ref symlink to some other tree, if you have copied a whole

archive by creating a symlink tree).

Author

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Written by Linus Torvalds <torvalds@osdl.org>.

GIT

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Part of the gitlink:git[7] suite