Single command to create a file and set its permission
Solution 1:
install -m 777 /dev/null filename.txt
Solution 2:
For bash, simply use chmod with file redirection and history expansion:
chmod 777 filename.txt>>!#:2
For ksh and zsh you have to drop the history expansion (as shown above, there may be other ways) and use:
chmod 644 filename>>filename
For scripts of any shell you don't have (and really don't need) history expansion so use the above there as well.