[Q] Determing if a commit is reachable from the HEAD ?

Hello,

 Whilst I have found answers to my question on the Web,
 only one seems to do exactly what I want ....

                         x---Y---y---y---y  HEAD
                        /
  ...--o---o---C---o---S
                        \
                         n---n---N---*---*  other

 In a script, how can I determine commit Y is reachable
 from the current HEAD ?   And, much more importantly
 for my purposes, that commit N is _not_-reachable from
 the current HEAD ?  "A is reachable from B" meaning B
 is an ancestor of A (B could possibly part of a merge
 (not shown in the above diagram)):

  ✓ Commits o(all), C, S, x, y(all), and Y: YES  (reachable).
  ✗ Commits n(all), N, and *(all): NO  (not-reachable).

 The only readily script-able answer I've found is to use
 `git branch --contains=Y' and check to see if the current
 branch (conveniently marked `*') is/isn't included in the
 output.  That is probably Ok, but I'm wondering if there
 is some "better" method (with the IMPORTANT caveat it must
 work with GIT 1.5.x or later  ;-\ ).

 git-rev-list(1) may be a answer (see script below), but ....
 `git rev-list ^Y HEAD' lists all the y commits, which is Ok
 (correct for my purposes).  However, `git rev-list ^N HEAD'
 lists commits x, Y, and y(all) on branch "other", which is
 not-Ok:  As per above, the answer I want is "NO".

 Apologies if I've overlooked something totally obvious !

cheers,
	-blf-

=====(cut here and below)=====reach02.sh===== #!/bin/bash # # x---Y---y---y---y HEAD # / # ...--o---o---C---o---S # \ # n---n---N---*---* other #

: ${GIT:=git}	# Allow use of different installed GIT versions.

update_file() {
	date +"%c: $*" >>file
	$GIT add file  &&  $GIT commit -m "$*" file
}

set -e		# Terminate if problem creating diagrammed situation.

rm -rf -- reachable mkdir -- reachable cd reachable

$GIT version $GIT init

update_file o1 update_file o2 update_file "C (o3)"; $GIT tag C update_file o4 update_file "S (o5)"; $GIT tag S

$GIT checkout -b other update_file n1 update_file n2 update_file "N (n3)"; $GIT tag N update_file "*1" update_file "*2"

$GIT checkout master update_file x update_file "Y (y1)"; $GIT tag Y update_file y2 update_file y3 update_file y4

declare -i wrong=0

echo "Is Y reachable from HEAD?  Wanted answer: Yes."
$GIT log --oneline ^Y HEAD
if $GIT rev-list --quiet ^Y HEAD; then
	echo "rev-list: YES (wanted answer)!"
else
	echo "rev-list: No! *** not-wanted, oops... ***"
	wrong=$(( wrong + 1 ))
fi
if $GIT branch --contains=Y | grep -q -e '^\*'; then
	echo "contains: YES (wanted answer)!"
else
	echo "contains: No! *** not-wanted, oops... ***"
	wrong=$(( wrong + 1 ))
fi

echo "Is N reachable from HEAD?  Wanted answer: No."
$GIT log --oneline ^N HEAD
if $GIT rev-list --quiet ^N HEAD; then
	echo "rev-list: YES? *** not-wanted, oops... ***"
	wrong=$(( wrong + 1 ))
else
	echo "rev-list: No (wanted answer)!"
fi
if $GIT branch --contains=N | grep -q -e '^\*'; then
	echo "contains: YES? *** not-wanted, oops... ***"
	wrong=$(( wrong + 1 ))
else
	echo "contains: No (wanted answer)!"
fi

exit $wrong =====(cut here and above)=====reach02.sh=====