git update-ref HEAD "$head"
Update the object name stored in a ref safely
Man Page
Synopsis
git update-ref [-m <reason>] [--no-deref] (-d <ref> [<oldvalue>] | [--create-reflog] <ref> <newvalue> [<oldvalue>] | --stdin [-z])
Description
Given two arguments, stores the <newvalue> in the <ref>, possibly
dereferencing the symbolic refs. E.g. git update-ref HEAD
<newvalue> updates the current branch head to the new object.
Given three arguments, stores the <newvalue> in the <ref>,
possibly dereferencing the symbolic refs, after verifying that
the current value of the <ref> matches <oldvalue>.
E.g. git update-ref refs/heads/master <newvalue> <oldvalue>
updates the master branch head to <newvalue> only if its current
value is <oldvalue>. You can specify 40 "0" or an empty string
as <oldvalue> to make sure that the ref you are creating does
not exist.
It also allows a "ref" file to be a symbolic pointer to another ref file by starting with the four-byte header sequence of "ref:".
More importantly, it allows the update of a ref file to follow these symbolic pointers, whether they are symlinks or these "regular file symbolic refs". It follows real symlinks only if they start with "refs/": otherwise it will just try to read them and update them as a regular file (i.e. it will allow the filesystem to follow them, but will overwrite such a symlink to somewhere else with a regular filename).
If --no-deref is given, <ref> itself is overwritten, rather than the result of following the symbolic pointers.
In general, using
should be a lot safer than doing
echo "$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 start LF prepare LF commit LF abort 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 start NUL prepare NUL commit NUL abort 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> is zero or missing, the ref must not exist.
- option
-
Modify behavior of the next command naming a <ref>. The only valid option is
no-derefto avoid dereferencing a symbolic ref. - start
-
Start a transaction. In contrast to a non-transactional session, a transaction will automatically abort if the session ends without an explicit commit. This command may create a new empty transaction when the current one has been committed or aborted already.
- prepare
-
Prepare to commit the transaction. This will create lock files for all queued reference updates. If one reference could not be locked, the transaction will be aborted.
- commit
-
Commit all reference updates queued for the transaction, ending the transaction.
- abort
-
Abort the transaction, releasing all locks if the transaction is in prepared state.
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 a pseudoref
like HEAD or ORIG_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:
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:
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
Man Page
Synopsis
git update-ref [-m <reason>] [--no-deref] (-d <ref> [<oldvalue>] | [--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 start LF prepare LF commit LF abort 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 start NUL prepare NUL commit NUL abort 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> is zero or missing, the ref must not exist.
- option
-
Modify behavior of the next command naming a <ref>. The only valid option is
no-derefto avoid dereferencing a symbolic ref. - start
-
Start a transaction. In contrast to a non-transactional session, a transaction will automatically abort if the session ends without an explicit commit. This command may create a new empty transaction when the current one has been committed or aborted already.
- prepare
-
Prepare to commit the transaction. This will create lock files for all queued reference updates. If one reference could not be locked, the transaction will be aborted.
- commit
-
Commit all reference updates queued for the transaction, ending the transaction.
- abort
-
Abort the transaction, releasing all locks if the transaction is in prepared state.
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 a pseudoref
like HEAD or ORIG_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:
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:
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