1. Open fileC for writing.
  2. Open fileB for reading.
  3. Read in fileB up to and including the SEARCHPATTERN.
  4. Write everything you've read into fileC.
  5. Open fileA for reading.
  6. Read in fileA, write it to fileC.
  7. Close fileA.
  8. Read the rest of fileB, writing it to fileC.
  9. Close fileB.
  10. Close fileC.
  11. Rename fileC to fileB.
Or in an (untested) Perl/shell one-liner: 
perl -ne 'print; print `cat fileA` if /SEARCHPATTERN/' fileB > fileC &
+& mv fileC fileB

[download]

In reply to Re: Modifying File in the Middle by JavaFan
in thread Modifying File in the Middle by PyrexKidd

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.