Validate an IP before using scp - bash

Question

I'm writing a file transfer script and it gets pretty complex. So in the beginning when I generate my IP address to transfer from, I want to validate that I can indeed connect to it. The code in the area looks like this:
USER_ID=$1
if [[ $GROUP == "A" ]]; then
ADDRESS="${USER_ID}#morgan.company.gov"
elif [[ $GROUP == "B" ]]; then
ADDRESS="${USER_ID}#mendel.company.gov"
else
log_msg fatal "Couldn't resolve group $GROUP. Exiting"
exit 1;
fi
// HERE I want to test that $ADDRESS exists, and I can connect right now I
// have what is below. I just think there is a better way to do it
ssh -q $ADDRESS exit
if [ $? != 0 ]; then
log_msg fatal "Couldn't resolve host, do you have login privileges with $ADDRESS"
fi
... // lots of other things happen
scp $ADDRESS:$INCOMING_FILE $NEW_FILE
What I have works, but it doesn't seem like its an elegant solution. I'd prefer not to actually ssh and exit the server, just test the connection.

Answer 1

You can use this shell function that tests wether the host has the ssh port open or not:
#!/bin/bash
function isUp(){
local ip=$1
local sshport=22
if [[ $ip =~ ^[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+$ ]]; then
if [[ $(nmap -P0 $ip -p$sshport | grep ^$sshport | cut -d' ' -f2) == "open" ]]; then
return 0
else
return 1
fi
fi
}
if isUp $1; then
ssh $1 uptime
else
echo "Host $1 is not available"
fi
or make use of this (new to me) bash functionality:
#!/bin/bash
function isUp(){
local ip=$1
if echo > /dev/tcp/$ip/22 >/dev/null 2>&1; then
return 0
else
return 1
fi
}

Answer 2

If you want to thoroughly check for the connection then you are better off analyzing the return codes from an scp connection. This will allow you to understand the reason why your connection failed and act in consequence (there may be various reasons at different levels, from a general lack of connectivity to issues with the keys for instance).
If you are just interested in a binary answer ("is my connection OK as it is?") then your code is fine, though I would directly use scp.

Answer 3

There is nothing wrong with your line:
ssh -q $ADDRESS exit
This is the best/fastest way to test a connection.