Insert a word in the first line of the file

With GNU sed:

sed -i '1s/^/insertedtext/' file

This replaces the beginning of the first line with the inserted text. -i replaces the text in file rather than sending the modified text to the standard output.

If portability across unices is a concern, use ed:

ed file <<END
1s/^/insertedtext/
w
q
END

POSIX one:

$ { printf %s insertedtext; cat <./input_file; } >/tmp/output_file
$ mv -- /tmp/output_file ./input_file