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CentOS 7.0 - man page for git-update-ref (centos section 1)

GIT-UPDATE-REF(1)				      Git Manual				    GIT-UPDATE-REF(1)

NAME
git-update-ref - Update the object name stored in a ref safely
SYNOPSIS
git update-ref [-m <reason>] (-d <ref> [<oldvalue>] | [--no-deref] <ref> <newvalue> [<oldvalue>])
DESCRIPTION
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>. You can specify 40 "0" or an empty string as <oldvalue> to make sure that the ref you are creating does not exist. 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). If --no-deref is given, <ref> itself is overwritten, rather than the result of following the symbolic pointers. In general, using git update-ref HEAD "$head" should be a lot safer than doing echo "$head" > "$GIT_DIR/HEAD" both from a symlink following standpoint and an error checking standpoint. The "refs/" rule for symlinks means that symlinks that point to "outside" the tree are safe: they'll be followed for reading but not for writing (so we'll never write through a ref symlink to some other tree, if you have copied a whole archive by creating a symlink tree). With -d flag, it deletes the named <ref> after verifying it still contains <oldvalue>.
LOGGING UPDATES
If config parameter "core.logAllRefUpdates" is true and the ref is one under "refs/heads/", "refs/remotes/", "refs/notes/", or the symbolic ref HEAD; or the file "$GIT_DIR/logs/<ref>" exists then git update-ref will append a line to the log file "$GIT_DIR/logs/<ref>" (dereferencing all symbolic refs before creating the log name) describing the change in ref value. Log lines are formatted as: 1. oldsha1 SP newsha1 SP committer LF Where "oldsha1" is the 40 character hexadecimal value previously stored in <ref>, "newsha1" is the 40 character hexadecimal value of <newvalue> and "committer" is the committer's name, email address and date in the standard Git committer ident format. Optionally with -m: 1. oldsha1 SP newsha1 SP committer TAB message LF Where all fields are as described above and "message" is the value supplied to the -m option. An update will fail (without changing <ref>) if the current user is unable to create a new log file, append to the existing log file or does not have committer information available.
GIT
Part of the git(1) suite Git 1.8.3.1 06/10/2014 GIT-UPDATE-REF(1)


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