Update the object name stored in a ref safely


Delete a ref, useful for soft resetting the first commit

$ git update-ref -d [HEAD]

Update ref with a message

$ git update-ref -m [message] [HEAD] [4e95e05]


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


Given two arguments, stores the <newvalue> in the <ref>, possibly dereferencing the symbolic refs . E .g . gitupdate -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 . gitupdate -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
.RS 4
git update -ref HEAD "$head" .RE
should be a lot safer than doing
.RS 4
echo "$head" > "$GIT_DIR/HEAD" .RE
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 be followed for reading but not for writing (so we 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:
.RS 4
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 .RE
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:
.RS 4
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 .RE
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 .


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 "$GIT_DIR/logs/<ref>" exists then gitupdate -ref will append a line to the log file "$GIT_DIR/logs/<ref>" (dereferencing all symbolic refs before creating the log name) describing the change in ref value . Log lines are formatted as:
.RS 4
oldsha1 SP newsha1 SP committer LF .RE
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 name, email address and date in the standard Git committer ident format .
Optionally with -m:
.RS 4
oldsha1 SP newsha1 SP committer TAB message LF .RE
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 .


Part of the git (1)suite

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