How to extract lines between same patterns from a file

sed '/P1/,/P2/!d;/P2/q'

...would do the job portably by deleting all lines which do !not fall within the range, then quitting the first time it encounters the end of the range. It does not fail for P2 preceding P1, and it does not require GNU specific syntax to write simply.

You can always do:

tac < fileName | sed  '/EndPattern/,$!d;/StartPattern/q' | tac

If your system doesn't have GNU tac, you may be able to use tail -r instead.

You can also do it like:

awk '
  inside {
    text = text $0 RS
    if (/EndPattern/) inside=0
    next
  }
  /StartPattern/ {
    inside = 1
    text = $0 RS
  }
  END {printf "%s", text}' < filename

But that means reading the whole file.

Note that it may give different results if there's another StartPattern in between a StartPattern and the next EndPattern or if the last StartPattern does not have an ending EndPattern or if there are lines matching both StartPattern and EndPattern.

awk '
  /StartPattern/ {
    inside = 1
    text = ""
  }
  inside {text = text $0 RS}
  /EndPattern/ {inside = 0} 
  END {printf "%s", text}' < filename

Would make it behave more like the tac+sed+tac approach (except for the unclosed trailing StartPattern case).

That last one seems to be the closest to your edited requirements. To add the warning would simply be:

awk '
  /StartPattern/ {
    inside = 1
    text = ""
  }
  inside {text = text $0 RS}
  /EndPattern/ {inside = 0} 
  END {
    printf "%s", text
    if (inside)
      print "Warning: EOF reached without seeing the end pattern" > "/dev/stderr"
  }' < filename

To avoid reading the whole file:

tac < filename | awk '
  /StartPattern/ {
    printf "%s", $0 RS text
    if (!inside)
      print "Warning: EOF reached without seeing the end pattern" > "/dev/stderr"
    exit
  }
  /EndPattern/ {inside = 1; text = ""}
  {text = $0 RS text}'

Portability note: for /dev/stderr, you need either a system with such a special file (beware that on Linux if stderr is open on a seekable file that will write the text at the beginning of the file instead of the current position within the file) or an awk implementation that emulates it like gawk, mawk or busybox awk (those work around the Linux issue mentioned above).

On other systems, you can replace print ... > "/dev/stderr" with print ... | "cat>&2".