sed insert line(s)

insert into empty file

sed operates on lines. If a file contains no lines, sed cannot insert a line into the file.

That means if we create a really empty file and try to insert some content into line 1, it won’t work and no lines are inserted:

sed insert into no-lines file
$ : > ./file.txt

$ du ./file.txt
0    ./file.txt

$ sed -f - ./file.txt <<EOF > ./new.txt
1 i\\

$ du  ./new.txt
0	    ./new.txt

As we see, new.txt is empty. It happens because there is no line 1 (the file has no lines at all), so the “line 1 address” cannot possibly match line 1.

One approach would be to create a one-empty-line file instead:

Empty Files

Here are some ways to create empty files (really zero bytes and zero lines). Note that the first echo example creates a file with one empty line (and that is not really an empty file):

$ echo > ./1line-echo.txt

$ echo -n > ./0line-echo.txt
$ : > ./0line-null.txt
$ touch ./0line-touch.txt

$ ls -1 ./*line*.txt

$ du -h ./*line*.txt
0	./0line-echo.txt
0	./0line-null.txt
0	./0line-touch.txt
4.0K	./1line-echo.txt

$ wc -l ./*line*.txt
0 ./0line-echo.txt
0 ./0line-null.txt
0 ./0line-touch.txt
1 ./1line-echo.txt
1 total

echo appends a newline to the output. Use -n to prevent that.

single empty line file
$ printf '\n' > ./file.txt

$ sed -f - ./file.txt <<EOF > ./new.txt
1 i\\

$ cat ./new.txt