pg_dump -U datasync_user mobility > path/filename
You can then compress the backup to shrink the size (I recommend bzip2 for pretty tight compression).
This assumes that a file called .pgpass containing the password for datasync_user is in the home directory of the user running the script. For instance, if you run it as root, then the file should be in root's home directory. You can secure it a bit my changing the permissions to 600 which prevents anyone other than root from seeing the contents.