Xargs -r0 vs xargs -0

Yes, find ./work -print0 | xargs -0 rm will execute something like rm ./work/a "work/b c" .... You can check with echo, find ./work -print0 | xargs -0 echo rm will print the command that will be executed (except white space will be escaped appropriately, though the echo won't show that).

To get xargs to put the names in the middle, you need to add -I[string], where [string] is what you want to be replaced with the argument, in this case you'd use -I{}, e.g. <strings.txt xargs -I{} grep {} directory/*.

What you actually want to use is grep -F -f strings.txt:

-F, --fixed-strings
  Interpret PATTERN as a  list  of  fixed  strings,  separated  by
  newlines,  any  of  which is to be matched.  (-F is specified by
  POSIX.)
-f FILE, --file=FILE
  Obtain  patterns  from  FILE,  one  per  line.   The  empty file
  contains zero patterns, and therefore matches nothing.   (-f  is
  specified by POSIX.)

So grep -Ff strings.txt subdirectory/* will find all occurrences of any string in strings.txt as a literal, if you drop the -F option you can use regular expressions in the file. You could actually use grep -F "$(<strings.txt)" directory/* too. If you want to practice find, you can use the last two examples in the summary. If you want to do a recursive search instead of just the first level, you have a few options, also in the summary.

Summary:

# grep for each string individually.
<strings.txt xargs -I{} grep {} directory/*

# grep once for everything
grep -Ff strings.txt subdirectory/*
grep -F "$(<strings.txt)" directory/*

# Same, using file
find subdirectory -maxdepth 1 -type f -exec grep -Ff strings.txt {} +
find subdirectory -maxdepth 1 -type f -print0 | xargs -0 grep -Ff strings.txt

# Recursively
grep -rFf strings.txt subdirectory
find subdirectory -type f -exec grep -Ff strings.txt {} +
find subdirectory -type f -print0 | xargs -0 grep -Ff strings.txt

You may want to use the -l option to get just the name of each matching file if you don't need to see the actual line:

-l, --files-with-matches
  Suppress  normal  output;  instead  print the name of each input
  file from which output would normally have  been  printed.   The
  scanning  will  stop  on  the  first match.  (-l is specified by
  POSIX.)

(As this does not answer the question, this is should have been a comment, but is too long - so treat it as a comment).

As an alternative to FreeBSD's xargs you can use GNU Parallel which does not have this limitation. It even supports repeating the context:

seq 10 | parallel -Xj1 echo con{}text 
seq 10 | parallel -mj1 echo con{}text 

GNU Parallel is a general parallelizer and makes is easy to run jobs in parallel on the same machine or on multiple machines you have ssh access to. It can often replace a for loop.

If you have 32 different jobs you want to run on 4 CPUs, a straight forward way to parallelize is to run 8 jobs on each CPU:

GNU Parallel instead spawns a new process when one finishes - keeping the CPUs active and thus saving time:

Installation

If GNU Parallel is not packaged for your distribution, you can do a personal installation, which does not require root access. It can be done in 10 seconds by doing this:

(wget -O - pi.dk/3 || curl pi.dk/3/ || fetch -o - http://pi.dk/3) | bash

For other installation options see http://git.savannah.gnu.org/cgit/parallel.git/tree/README

Learn more

See more examples: http://www.gnu.org/software/parallel/man.html

Watch the intro videos: https://www.youtube.com/playlist?list=PL284C9FF2488BC6D1

Walk through the tutorial: http://www.gnu.org/software/parallel/parallel_tutorial.html

Sign up for the email list to get support: https://lists.gnu.org/mailman/listinfo/parallel