replace file's content between specific line numbers with another file using BASH

Question

What is the most efficient way to replace file's content between specific line numbers with another file?

Here is a sample:

main.txt:

a b c
d e f
g h i
j k l

new.part.txt

x y z
p q r
s t u

The block of text starting at line 2 and ending at line 3 of main.txt is replaced with new.part.txt. The desired file is as follow:

a b c
x y z
p q r
s t u
j k l

This answer is for the case when range of the desired block is defined via marker strings. I need a solution that uses line numbers for defining range of the desired block.

Answer 1

You don't use bash at all.

The Bourne Again shell is a shell. The actual utility programs for doing this are not part of the shell, and are largely shell-agnostic. grawity's answer uses shell features, but mainly for parameterization.

Here's how to do it with ex, editing the file in place:

ex main.txt << EOT
2,3d
1r new.part.txt
w
EOT

Here's how one does it with sed, writing the changed data to standard output:

sed -e '1r new.part.txt' -e '2,3d' main.txt

Answer 2

start=2
end=3

{
  head -n $((start-1)) old.txt
  cat new.txt
  tail -n $((end+1)) old.txt
} > merged.txt