Add to file if exists and create if not

Use two angles: echo $some_line >> /path/to/file

> creates the file if it doesn't exist; if it exists, overwrites it.

>> creates the file if it doesn't exist; if it exists, appends to it.

if [ ! -e /path/to/file ]; then
   echo $some_line > /path/to/file
else
   echo $some_line >> /path/to/file
fi