Deleting a file with LFTP using variables
USERNAME="theuser"
PASSWORD="verygoodpassword"
SERVER="example.com"
BACKUPDIR="thebackups"
FILETODELETE="uselessfile.obsolete"

lftp -e 'rm /${BACKUPDIR}/${FILETODELETE}; bye' -u $USERNAME,$PASSWORD $SERVER

lftp -e 'rm /thebackups/uselessfile.obsolete; bye' -u theuser,verygoodpassword example.com

lftp -e 'rm /${BACKUPDIR}/${FILETODELETE}; bye' -u theuser,verygoodpassword example.com

USERNAME="theuser"
PASSWORD="verygoodpassword"
SERVER="example.com"
BACKUPDIR="thebackups"
FILETODELETE="uselessfile.obsolete"

lftp -e "rm /${BACKUPDIR}/${FILETODELETE}; bye" -u $USERNAME,$PASSWORD $SERVER