locate -e0 '*/pg_type.h' | xargs -r0 cat
locate pg_type.h would find all the files with
pg_type.h in their path (so for instance if there was a
rpg_type.horn directory, you'd end up displaying all the files in there).
-0 the output of
locate can't be post-processed because the files are separated by newline characters while newline is a perfectly valid character in a file name.
cat without arguments writes to stdout what it reads from stdin, so
locate | cat would be the same as
cat would just pass the output of
locate along. What you need is to pass the list of files as arguments to
xargs is typically for: convert a stream of data into a list of arguments.
-r is to not call
cat if there's no input. Without
-0 (which like
-r is not standard but found on many implementations, at least those where xargs is useful to anything),
xargs would just look for words in its input to convert into arguments, where words are blank separated and where backslash, single and double quotes can be used to escape those separators, so typically not the format
locate uses to display file names.
That's why we use the
-0 option for both
xargs which uses the NUL character (which is the only character not allowed in a file path) to separate file names.
Also note that
locate is not a standard command and there exist a great number of different implementations with different versions thereof and different options and behaviours. The code above applies at least to relatively recent versions of the GNU
mlocate implementations which are the most common on Linux based operating systems at least.