How about:
echo "hello" >> <filename>
Using the >> operator will append data at the end of the file, while using the > will overwrite the contents of the file if already existing.
You could also use printf in the same way:
printf "hello" >> <filename>
Note that it can be dangerous to use the above. For instance if you already have a file and you need to append data to the end of the file and you forget to add the last > all data in the file will be destroyed. You can change this behavior by setting the noclobber variable in your .bashrc:
set -o noclobber
Now when you try to do echo "hello" > file.txt you will get a warning saying cannot overwrite existing file.
To force writing to the file you must now use the special syntax:
echo "hello" >| <filename>
You should also know that by default echo adds a trailing new-line character which can be suppressed by using the -n flag:
echo -n "hello" >> <filename>
References