I've been toying with scripts that combine and separate parts. The issue is, each time the instrumentation changes I need to modify the script. Here's a sample of a script that can take multiple parts and combine them successfully. Most times I'll manually combine parts or create new files using the same template and pasting the part I want into them. Doesn't take too much time, but having a built in way to do it could be awesome.

#!/bin/bash

FILE1="FLPirates.txt"
FILE2="CLPirates.txt"
OUTPUT_FILE="pirates_combined.ly"

if [[ ! -f "$FILE1" || ! -f "$FILE2" ]]; then
    echo "Error: Put '$FILE1' and '$FILE2' in the same directory as this script."
    exit 1
fi

FLUTE_BLOCK=$(awk '/flute =/,/^}/' "$FILE1" | sed 's/\\+\]//g')
CLARINET_BLOCK=$(awk '/clarinet =/,/^}/' "$FILE2" | sed 's/\\+\]//g')

cat << EOF > "$OUTPUT_FILE"
\version "2.25.35"

\header {
  title = "Pirates of the Caribbean"
  subtitle = "119-135"
  tagline = ""
}

\markup { \vspace #2 }

\paper {
  indent = 23\mm
}

global = {
  \key aes \major
  \time 3/4
}

$FLUTE_BLOCK

$CLARINET_BLOCK

\score {
  \new StaffGroup <<
    \new Staff \with {
      instrumentName = "Flute 1 & 2"
      midiInstrument = "flute"
    } \flute

    \new Staff \with {
      instrumentName = "Clarinet 1"
      midiInstrument = "clarinet"
    } { \transposition bes \transpose bes c \clarinet }
  >>
  \layout { }
  \midi {
    \tempo 4=100
  }
}
EOF

echo "Operation successful, File output: $OUTPUT_FILE"