How to insert a text at the beginning of a file?

If the file is only one line, you can use:

sed 's/^/insert this /' oldfile > newfile

If it's more than one line. one of:

sed '1s/^/insert this /' oldfile > newfile
sed '1,1s/^/insert this /' oldfile > newfile

I've included the latter so that you know how to do ranges of lines. Both of these "replace" the start line marker on their affected lines with the text you want to insert. You can also (assuming your sed is modern enough) use:

sed -i 'whatever command you choose' filename

to do in-place editing.

If you want to add a line at the beginning of a file, you need to add \n at the end of the string in the best solution above.

The best solution will add the string, but with the string, it will not add a line at the end of a file.

sed -i '1s/^/your text\n/' file

Use subshell:

echo "$(echo -n 'hello'; cat filename)" > filename

---

Unfortunately, command substitution will remove newlines at the end of file. So as to keep them one can use:

echo -n "hello" | cat - filename > /tmp/filename.tmp
mv /tmp/filename.tmp filename

Neither grouping nor command substitution is needed.

sed can operate on an address:

$ sed -i '1s/^/<added text> /' file

What is this magical 1s you see on every answer here? Line addressing!.

Want to add <added text> on the first 10 lines?

$ sed -i '1,10s/^/<added text> /' file

Or you can use Command Grouping:

$ { echo -n '<added text> '; cat file; } >file.new
$ mv file{.new,}