최초 clone등의 작업을 할때 github.com에 대한 권한이 없어서 발생하는 문제로 SSH를 생성하여 github.com에 등록하여준다. 

1.public/private rsa key 생성
ssh-keygen -t rsa -b 4096 -C "gooper@gooper.com"

2. key를 ssh-agent에 추가
가. ssh-agent가 사용가능한지 확인
eval "$(ssh-agent -s)"

나. SSH 키를 ssh-agent에 추가
ssh-add ~/.ssh/id_rsa

3. SSH 키를 계정에 추가
가. xclip설치
apt-get install xclip

나. id_rsa.pub내용를 clipboard에 복사
xclip -sel clip < ~/.ssh/id_rsa.pub

4. 복사된 key를 GitHub에 등록
Settings->SSH and GPG keys->New SSH key->key는 clipboard에 있으므로 ctrl+v해서 빈칸에 값을 집어넣음->Add key

------------------clone을 시도했을때 발생하는 오류--------------------

gooper@gsda1:/home/gooper$ git clone git@github.com:lagom/online-auction-java.git
Cloning into 'online-auction-java'...
The authenticity of host 'github.com (XXX.XXX.XXX.XXX)' can't be established.
RSA key fingerprint is SHA256:.......
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 'github.com,XXX.XXX.XXX.XXX' (RSA) to the list of known hosts.
Permission denied (publickey).
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.