2 added workaround if null termination is not available
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grep -rlZI "bananas" . | xargs -0 cat > out.txt

The -lZ outputs a null-separated list of the names of matching files:

-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.

-Z, --null
       Output a zero byte (the ASCII  NUL  character)  instead  of  the
       character  that normally follows a file name.  For example, grep
       -lZ outputs a zero byte after each  file  name  instead  of  the
       usual  newline.   This option makes the output unambiguous, even
       in the presence of file names containing unusual characters like
       newlines.   This  option  can  be  used  with commands like find
       -print0, perl -0, sort -z, and xargs  -0  to  process  arbitrary
       file names, even those that contain newline characters.

If your version of grep doen't provide the -Z option, then you can fall back to plain -l which will still handle filenames containing whitespace (excluding newlines obviously) provided you set the xargs delimiter to newline as well:

grep -rlI "bananas" . | xargs -d '\n' cat > out.txt
grep -rlZI "bananas" . | xargs -0 cat > out.txt

The -lZ outputs a null-separated list of the names of matching files:

-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.

-Z, --null
       Output a zero byte (the ASCII  NUL  character)  instead  of  the
       character  that normally follows a file name.  For example, grep
       -lZ outputs a zero byte after each  file  name  instead  of  the
       usual  newline.   This option makes the output unambiguous, even
       in the presence of file names containing unusual characters like
       newlines.   This  option  can  be  used  with commands like find
       -print0, perl -0, sort -z, and xargs  -0  to  process  arbitrary
       file names, even those that contain newline characters.
grep -rlZI "bananas" . | xargs -0 cat > out.txt

The -lZ outputs a null-separated list of the names of matching files:

-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.

-Z, --null
       Output a zero byte (the ASCII  NUL  character)  instead  of  the
       character  that normally follows a file name.  For example, grep
       -lZ outputs a zero byte after each  file  name  instead  of  the
       usual  newline.   This option makes the output unambiguous, even
       in the presence of file names containing unusual characters like
       newlines.   This  option  can  be  used  with commands like find
       -print0, perl -0, sort -z, and xargs  -0  to  process  arbitrary
       file names, even those that contain newline characters.

If your version of grep doen't provide the -Z option, then you can fall back to plain -l which will still handle filenames containing whitespace (excluding newlines obviously) provided you set the xargs delimiter to newline as well:

grep -rlI "bananas" . | xargs -d '\n' cat > out.txt
1
source | link

grep -rlZI "bananas" . | xargs -0 cat > out.txt

The -lZ outputs a null-separated list of the names of matching files:

-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.

-Z, --null
       Output a zero byte (the ASCII  NUL  character)  instead  of  the
       character  that normally follows a file name.  For example, grep
       -lZ outputs a zero byte after each  file  name  instead  of  the
       usual  newline.   This option makes the output unambiguous, even
       in the presence of file names containing unusual characters like
       newlines.   This  option  can  be  used  with commands like find
       -print0, perl -0, sort -z, and xargs  -0  to  process  arbitrary
       file names, even those that contain newline characters.