ex, which is specified by POSIX. Add a tab-newline sequence at the start of the file, then run
This works whether the file is empty or not, and should be fully portable to any UNIX or Linux system.
(Note that if the file begins as an empty file, the contents after this command will be a tab character and a newline. A file without a terminating newline is technically not actually a text file.)
printf '0a\n\t\n.\n1j!\nx\n' | ex myfile.txt
If you run the
printf command by itself you can see the instructions that are getting passed to
$ printf '0a\n\t\n.\n1j!\nx\n'
Note that there is a single tab character in the otherwise blank line above.