Shell Programming and Scripting

awk '/PATTERN/  { getline < "b.txt" }' a.txt

# insert fileB after line 10  and line 20
otherfile=$(< fileA )
cnt=0
while read line
do
 cnt=$(( $cnt + 1 ))
 echo "$line"
 if [[ $cnt -eq 10  || $cnt -eq 20 ]]; then
   echo "$otherfile"
 fi

done  < fileA > tmpfile

mv tmpfile fileA

awk '/To/{while(getline x<f)print x;close(f)}1' f=file1 file2

--------------
Text
to be inserted
--------------
To be or not to be– that is the question:
Whether 'tis nobler in the mind to suffer
The slings and arrows of outrageous fortune,
Or to take arms against a sea of troubles
--------------
Text
to be inserted
--------------
 And, by opposing, end them. To die, to sleep
 No more – and by a sleep to say we end
 The heartache and the thousand natural shocks
That flesh is heir to – ‘tis a consummation
--------------
Text
to be inserted
--------------
Devoutly to be wished. To die, to sleep
--------------
Text
to be inserted
--------------
To sleep, perchance to dream. Ay, there's the rub,
 For in that sleep of death what dreams may come,
When we have shuffled off this mortal coil,
 Must give us pause. There's the respect
That makes calamity of so long life.

sed '/To/rfile1' file2