The man page of Debian 8's find command says:
If the whole expression contains no actions other than -prune or -print, -print is performed on all files for which the whole expression is true.
So why do these outputs differ:
$ mkdir -p test/foo test/bar && cd test && touch foo/bar bar/foo
$ # Test 1
$ find . -name foo -type d -prune -o -name foo
./foo
./bar/foo
$ # Test 2
$ find . -name foo -type d -prune -o -name foo -print
./bar/foo
So test 1: does the expression contain "no actions other than -prune or -print?" Well, excluding the prune, yes that statement is true, there are no actions. So these results are expected since for ./foo the expression before the -o option returns True, so it's printed.
But test 2: does the expression contain "no actions other than -prune or -print?" Well, excluding the prune and the print, yes that statement is true again, there are no other actions. So I would expect the same results.
But I don't get ./foo. Why?
It's as if the man page should read: "If the whole expression contains no actions other than -prune or -print, -print is performed on all files for which the whole expression is true."
I'm going with the simpler explanation, the man page is wrong. It should instead say
If the whole expression contains no actions other than -prune
or -print, -print is performed on all files for which the whole expression is true.
It should also maybe contain a caveat for -quit, which is an action, but it causes -find to exit immediately. So even though an implicit -print is added for the whole expression it is never actually executed.
The posix find man page contains a clearer explanation, though it doesn't have quite as many actions as the expanded gnu version.
If no expression is present, -print shall be used as the expression. Otherwise, if the given expression does not contain any of the primaries -exec, -ok, or -print, the given expression shall be effectively replaced by:
( given_expression ) -print
Out of what gnu calls actions, posix only defines -exec, -ok, -print, and -prune. It does not have any of the expanded actions -delete, -ls, etc... So the definition matches the corrected gnu one by only omitting -prune.
Here are some examples using all the gnu find actions which prove the point. For all consider the following file structure
$ tree
.
└── file
-delete
$ find -name file -delete
$
-exec command ;
$ find -name file -exec echo '-exec is an action so an implicit -print is not applied' \;
-exec is an action so an implicit -print is not applied
$
-execdir command {} +
$ find -name file -exec echo 'This should print the filename twice if an implicit -print is applied: ' {} +
This should print the filename twice if an implicit -print is applied: ./file
$
-fls
$ find -name file -fls file
$
-fprint
$ find -name file -fprint file
$
-ls
$ find -name file -ls
1127767338 0 -rw-rw-r-- 1 user user 0 May 6 07:15 ./file
$
-ok command ;
$ find -name file -ok echo '-ok is an action so an implicit -print is not applied' \;
< echo ... ./file > ? y
-ok is an action so an implicit -print is not applied
$
-okdir command ;
$ find -name file -okdir echo '-okdir is an action so an implicit -print is not applied' \;
< echo ... ./file > ? y
-okdir is an action so an implicit -print is not applied
$
#./file would be printed twice if an implicit `-print was applied`
$ find -name file -print
./file
$
-print0
#./file would be printed twice if an implicit `-print was applied`
$ find -name file -print0
./file$
-printf
$ find -name file -printf 'Since -printf is an action the implicit -print is not applied\n'
Since -printf is an action the implicit -print is not applied
$
-prune
$ find -name file -prune
./file
$
-quit
$ find -name file -quit
$ find -D opt -name file -quit
...
Optimized command line:
( -name file [0.1] -a [0.1] -quit [1] ) -a [0.1] -print [1]