How to specify the private SSH-key to use when executing shell command on Git?

None of these solutions worked for me.

Instead, I elaborate on @Martin v. Löwis 's mention of setting a config file for SSH.

SSH will look for the user's ~/.ssh/config file. I have mine setup as:

Host gitserv
    Hostname remote.server.com
    IdentityFile ~/.ssh/id_rsa.github
    IdentitiesOnly yes # see NOTES below

And I add a remote git repository:

git remote add origin git@gitserv:myrepo.git

And then git commands work normally for me.

git push -v origin master

NOTES

• The IdentitiesOnly yes is required to prevent the SSH default behavior of sending the identity file matching the default filename for each protocol. If you have a file named ~/.ssh/id_rsa that will get tried BEFORE your ~/.ssh/id_rsa.github without this option.

References

• Best way to use multiple SSH private keys on one client
• How could I stop ssh offering a wrong key

Something like this should work (suggested by orip):

ssh-agent bash -c 'ssh-add /somewhere/yourkey; git clone [email protected]:user/project.git'

if you prefer subshells, you could try the following (though it is more fragile):

ssh-agent $(ssh-add /somewhere/yourkey; git clone [email protected]:user/project.git)

Git will invoke SSH which will find its agent by environment variable; this will, in turn, have the key loaded.

Alternatively, setting HOME may also do the trick, provided you are willing to setup a directory that contains only a .ssh directory as HOME; this may either contain an identity.pub, or a config file setting IdentityFile.

Starting from Git 2.3.0 we also have the simple command (no config file needed):

GIT_SSH_COMMAND='ssh -i private_key_file -o IdentitiesOnly=yes' git clone user@host:repo.git

Note the -o IdentitiesOnly=yes is required to prevent the SSH default behavior of sending the identity file matching the default filename for each protocol as noted in the answer above.

Other people's suggestions about ~/.ssh/config are extra complicated. It can be as simple as:

Host github.com
  IdentityFile ~/.ssh/github_rsa