How to insert all lines from one file to another file?

Another awk solution:

awk '{print; getline < "file1"; print}' file2

Using awk (gawk, nawk, mawk):

awk 'NR==FNR {x[FNR]=$0;next} {print x[FNR]"\n"$0}' file2 file1 > outputfile

• NR==FNR {x[FNR]=$0;next}: NR==FNR is matched only if the current record number is equal to the current file record number (hence it's matched only while processing the first file): stores the currently processed record into the array x at an index equal to the current file record number and skips the current record
• {print x[FNR]"\n"$0}: prints the content of the array x at an index equal to the current file record number followed by a newline and by the content of the current record

~/tmp$ cat file1
.tid.setnr := 1123
.tid.setnr := 3345
.tid.setnr := 5431
.tid.setnr := 89323
~/tmp$ cat file2
.tid.info := 12
.tid.info := 3
.tid.info := 44
.tid.info := 60
~/tmp$ awk 'NR==FNR {x[FNR]=$0;next} {print x[FNR]"\n"$0}' file2 file1
.tid.info := 12
.tid.setnr := 1123
.tid.info := 3
.tid.setnr := 3345
.tid.info := 44
.tid.setnr := 5431
.tid.info := 60
.tid.setnr := 89323

The paste solution is the most portable and most efficient. I'm only mentioning this alternative in case you prefer its behaviour in the case where the two files don't have the same number of lines:

With GNU sed:

sed Rfile1 file2

If file1 has fewer lines than file2, then when file1 is exhausted, sed will not output anything for it (as opposed to empty lines for paste).

If file1 has more lines than file2, then those extra lines will be discarded (as opposed to printing empty lines for file2 with paste).

$ paste a b
1       a
2       b
3
4
$ paste -d \\n a b
1
a
2
b
3

4

$ sed Rb a
1
a
2
b
3
4
$ sed Ra b
a
1
b
2