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git-update-ref - Online in the Cloud

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This is the command git-update-ref that can be run in the OnWorks free hosting provider using one of our multiple free online workstations such as Ubuntu Online, Fedora Online, Windows online emulator or MAC OS online emulator

PROGRAM:

NAME


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

SYNOPSIS


git update-ref [-m <reason>] (-d <ref> [<oldvalue>] | [--no-deref] [--create-reflog] <ref> <newvalue> [<oldvalue>] | --stdin [-z])

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>.

With --stdin, update-ref reads instructions from standard input and performs all
modifications together. Specify commands of the form:

update SP <ref> SP <newvalue> [SP <oldvalue>] LF
create SP <ref> SP <newvalue> LF
delete SP <ref> [SP <oldvalue>] LF
verify SP <ref> [SP <oldvalue>] LF
option SP <opt> LF

With --create-reflog, update-ref will create a reflog for each ref even if one would not
ordinarily be created.

Quote fields containing whitespace as if they were strings in C source code; i.e.,
surrounded by double-quotes and with backslash escapes. Use 40 "0" characters or the empty
string to specify a zero value. To specify a missing value, omit the value and its
preceding SP entirely.

Alternatively, use -z to specify in NUL-terminated format, without quoting:

update SP <ref> NUL <newvalue> NUL [<oldvalue>] NUL
create SP <ref> NUL <newvalue> NUL
delete SP <ref> NUL [<oldvalue>] NUL
verify SP <ref> NUL [<oldvalue>] NUL
option SP <opt> NUL

In this format, use 40 "0" to specify a zero value, and use the empty string to specify a
missing value.

In either format, values can be specified in any form that Git recognizes as an object
name. Commands in any other format or a repeated <ref> produce an error. Command meanings
are:

update
Set <ref> to <newvalue> after verifying <oldvalue>, if given. Specify a zero
<newvalue> to ensure the ref does not exist after the update and/or a zero <oldvalue>
to make sure the ref does not exist before the update.

create
Create <ref> with <newvalue> after verifying it does not exist. The given <newvalue>
may not be zero.

delete
Delete <ref> after verifying it exists with <oldvalue>, if given. If given, <oldvalue>
may not be zero.

verify
Verify <ref> against <oldvalue> but do not change it. If <oldvalue> zero or missing,
the ref must not exist.

option
Modify behavior of the next command naming a <ref>. The only valid option is no-deref
to avoid dereferencing a symbolic ref.

If all <ref>s can be locked with matching <oldvalue>s simultaneously, all modifications
are performed. Otherwise, no modifications are performed. Note that while each individual
<ref> is updated or deleted atomically, a concurrent reader may still see a subset of the
modifications.

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

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