Some enhancements to the script at https://exchange.nagios.org/directory/Plugins/Network-Protocols/SFTP/Powerful-shell-scripts--2D-sftp/details

check_sftp_file.sh

#!/bin/bash
#############################
# check_sftp-file.sh script #
#############################
# Author : Vittorio Memmo
# Ver.1.0.0 - Mar 2012
# Author : aseques
# Ver.1.0.1 - Oct 2017
#############################
#
E_NOARGS=0
if [ -z $1 ]; then
        echo "Hostname missing!"
        E_NOARGS=1
fi
if [ -z $2 ]; then
        echo "Username missing!"
        E_NOARGS=2
fi
if [ -z $3 ]; then
        echo "You need a password to login into the FTP server!"
        E_NOARGS=3
fi
if [ -z $4 ]; then
        echo "You must specify the path to search file for!"
        E_NOARGS=4
fi
if [ -z $5 ]; then
        echo "You must specify the file name to search for!"
             E_NOARGS=5
fi
if [ $E_NOARGS != 0 ]; then
        echo "Usage : check_sftp-file.sh <Hostname> <Username> <Password> <Path> <Filename> <Filename date ext format: none|%Y%m%d> <any|Number of lines in the file>"
        exit $E_NOARGS
fi
HOST=$1
USER=$2
PSWD=$3
PTH1=$4
filenm=$5
filend=$6 # date format
lines_num=$7
if [ -z $filend ] || [ $filend = "none" ]
        then
                file_name=$filenm
else
   filewsf=$(echo "$filenm" | cut -d '.' -f1)
   filesff=$(echo "$filenm" | cut -d '.' -f2)
   filendate=$(date +$filend)
   file_name=$filewsf$filendate'.'$filesff
fi

if [ "${lines_num}" = "" -o "${lines_num}" = "any" ] ; then
        lines_num=-1
fi
#Copies the specified file on local path
/usr/bin/lftp -u ${USER},${PSWD} -p22222 sftp://${HOST} <<EOF 2>/dev/null
cd $PTH1
get -O /tmp $file_name
bye
EOF
#
if [  -f /tmp/$file_name ] ; then
        lines=$(cat /tmp/$file_name 2> /dev/null | wc -l)
        rm -f /tmp/$file_name
        if [ $lines -eq $lines_num ] || [ $lines_num -eq -1 ] ; then
            echo "OK: $file_name found with $lines rows"
            exit 0
        else
            echo "CRITICAL: $file_name does not contain $lines_num rows!"
            exit 2
        fi
    else
        echo "CRITICAL: $file_name not found!"
        exit 2
fi