Replace first few lines with first few lines from other file

Question

I am working on Linux. I have 2 files - file1.dat and file2.dat.

cat file1.dat
1
2
3
4
5
6
7
8
9
10

and for file2:

cat file         


        

Answer 1

Assuming GNU sed

$ sed '3q' f2 | sed -e '3r /dev/stdin' -e '1,4d' f1
1a
2a
3a
5
6
7
8
9
10

• sed '3q' f2 gives the first three lines from second file
• -e '3r /dev/stdin' use stdin data
• -e '1,4d' delete required lines
• order is important - first r then d

For small number of lines, you can also use

sed -e '3R f2' -e '3R f2' -e '3R f2' -e '1,4d' f1

R command reads one line at a time

With GNU coreutils, this would probably be better for all/most scenarios

head -n3 f2; tail -n +5 f1