Attempt to locate the SSH public and private key used for a host/user@host.

sshkey.bash

#!/usr/bin/env bash

HOST="$1"

[[ -z "$HOST" ]] && (>&2 echo "Usage: $0 [user@]hostname") && exit 1

OUT="$(ssh -o ConnectionAttempts=1 -o ConnectTimeout=2 -o StrictHostKeyChecking=no -o "PreferredAuthentications=publickey" -T "$HOST" -v  true 2>&1 | grep -B3 "Authentication succeeded (publickey)" )"
RET="$?"
if [[ "$RET" -ne "0" ]] ; then
    (>&2 echo "ERROR (errno: $RET): Unable to connect or authenticate to $HOST")
    exit 1
fi

PRIVKEY="$(echo "$OUT" | perl -ne 'if ( /^[^:]+: Offering \w+ public key: (.*)$/ ) { print $1 }')"
RET="$?"

if [[ "$RET" -ne "0" ]] ; then
    (>&2 echo "ERROR: Unable to find a matching SSH key $HOST")
    exit 1
else
    PRIVKEY="${PRIVKEY/%[$'\t\r\n']*}" ; # remove trailing tab, newline, carriage return
    PUBKEY="${PRIVKEY}.pub"
    echo "PRIVKEY: $PRIVKEY"
    [[ -f "$PUBKEY" ]] && echo "PUBKEY: $PUBKEY" || echo "Pubkey $PUBKEY was not found"
fi