Adding Existing SSH Key to git via command line

My environment

• windows 11
• using git extension with git bash
• ssh key was generated using PuttyGen

Problem

• cannot push because of permission issue

λ git push --set-upstream origin main
git@gitlab.com: Permission denied (publickey).
fatal: Could not read from remote repository.

Solution

• Add the existing key to git

• use ssh-add to add key, however, ssh-add requires ssh-agent to be running. (if in linux running eval "$(ssh-agent -s)" will allow next command ssh-agent to be executed directly, but on windows the command is not recognized)
  

λ  eval "$(ssh-agent -s)"
'eval' is not recognized as an internal or external command,
operable program or batch file.

• so we need to get the ssh-agent socket and pid info

λ ssh-agent
SSH_AUTH_SOCK=/tmp/ssh-YKsmNCSnWFXo/agent.837; export SSH_AUTH_SOCK;
SSH_AGENT_PID=838; export SSH_AGENT_PID;
echo Agent pid 838;

• next, set the environment. Note the SSH_AUTO_SOCK and SSH_AGENT_PID must be the same as above

λ set SSH_AUTH_SOCK=/tmp/ssh-YKsmNCSnWFXo/agent.837
λ set SSH_AGENT_PID=838

• now use the command ssh-add to add new key

λ ssh-add c:\Keys\newKey
Identity added: c:\Keys\newKey

• however, my other key is generated using puttygen with old openssh format. so I need to convert the key to new format.

• run puttygen and select the conversions menu -> export openssh key (force new format)

• then run the ssh-add again

λ ssh-add c:\Keys\gitprivatekey-new
Identity added: c:\Keys\gitprivatekey-new

• then run git push again

λ git push --set-upstream origin main
Enumerating objects: 2370, done.
Counting objects: 100% (2370/2370), done.
Delta compression using up to 12 threads

• use the command ssh-add -l to list existing identities

λ ssh-add -l
2048 SHA256:xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx user1 (RSA)
2048 SHA256:xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx rsa-key(RSA)

• done