Bash script to insert item in a alphabetical list

Question

I would like to use a bash script to insert a new "source" file into a list of sources which occurs within another file.
The file in question is quite long and contains many other things. However the list of sources abides by the format below.
[snip]
SOURCES = \
_add_datasource_.m \
_add_line_series_.m \
ylabel.m \
ylim.m \
zlabel.m \
zlim.m
[snip]
How might I insert a new source file in alphabetical order. I'm assuming there is a fairly simple script to do such.
TiA

Answer 1

#!/usr/bin/env bash
if [[ $# != 1 ]]; then
echo 1>&2 "Usage: ${0##*/} fileto_insert_inSOURCE"
exit 1
fi
MAKEFILE=Makefile.in
TMP=/tmp/tmp.$$
awk -v new="$1" '
/^SOURCES =/ { in_src = 1 }
in_src && ! done && $1 > new {
# New file goes here
printf(" %s \
", new)
in_src = 0
done = 1
in_src && ! done && ! /\$/ {
# New file must go after the last entry.
# Put  on old last entry.
printf("%s \
", $0)
# replace current $0 with the new file.
$0 = sprintf(" %s
", new)
done = 1
! /\$/ { in_src = 0 } # NOT in SOURCE
{ print } # print current line
' $MAKEFILE >$TMP
mv $TMP $MAKEFILE