The FTP protocol is simple to use. In a console, type ftp <host> to connect to your host. Enter your login and password when prompted and use get to download and put to upload a file.
But what about doing that in a bash script ?
Here is a working example of an FTP upload within a bash script:
ftp -v -n $HOST_FTP <<SCRIPT
quote USER $LOGIN_FTP
quote PASS $PASS_FTP
Extra tip: if you need to download more than one file, use mget instead of get. The same works for mput instead of put.