Read from and append to file at the same time while preserving end-of-line

Conceptually I have an easy task… I have [loosely] structured data in a file:

Testing: debug, default CXXFLAGS
<100's of additional output lines>
Testing: release, default CXXFLAGS
<100's of additional output lines>
...

I try to summarize it in a log file:

echo "Configurations tested:" | tee -a "$TEST_RESULTS"
echo $($GREP 'Testing: ' "$TEST_RESULTS" | $SED 's/Testing: /    * /g') | tee -a "$TEST_RESULTS"

Instead of:

Configurations tested:
    * debug, default CXXFLAGS
    * release, default CXXFLAGS

I get:

Configurations tested:
1 3way.cpp 3way.h CMakeLists.txt CMakeLists.txt.diff Doxyfile Filelist.txt GNUmakefile
GNUmakefile-cross Install.txt License.txt Readme.txt TestData TestVectors adhoc.cpp.proto
adler32.cpp adler32.h aes.h algebra.cpp algebra.h ...

I think I am wreaking havoc on the file buffer $TEST_RESULTS because its being read from in the grep, and written to with the tee.

When I attempt to put the result of $GREP 'Testing: ' "$TEST_RESULTS" | $SED 's/Testing: / * /g' in a shell variable, I loose the line endings which results in one big concatenation:

* debug, default CXXFLAGS    * release, default CXXFLAGS ... <30 additional configs>

How do I read from and append to a file at the same time while preserving the end-of-lines?

---

I’ve made some progress with:

ESCAPED=$($GREP 'Testing: ' "$TEST_RESULTS" | $AWK -F ": " '{print "  -" $2 "$"}')
echo $ESCAPED | tr $ 'n' | tee -a "$TEST_RESULTS"

However, it can’t use * as a bullet point, and it seems to drop leading space:

Configurations tested:
-debug, default CXXFLAGS
 -release, default CXXFLAGS

I’m not using sed because swapping-in a new line is an absolute pain across platforms. Platforms include BSD, Cygwin, Linux, OS X, Solaris.

tmpfile="$( mktemp )"
echo "Configurations tested:" >"$tmpfile"
sed -n 's/^Testing: (.*)/  * 1/p' "$TEST_RESULTS" >>"$tmpfile"

cat "$tmpfile" >>"$TEST_RESULTS"
rm -f "$tmpfile"

tmpfile="$TEST_RESULTS.tmp"

tmpfile="/tmp/$( basename "$TEST_RESULTS" ).tmp"

awk -v pattern="Testing:" '$0 ~ pattern { sub(pattern, "  *"); print }'