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以前遇到过一个问题，在用有些 Linux 发行版时，用 ssh-keygen 产生好了密钥对并上传到了目标服务器，但每次登录都要重新输入。
这与 ssh-agent 有关，看如下 man ssh-agent 的输出：
ssh-agent is a program to hold private keys used for public key authentication (RSA, DSA). The idea is
that ssh-agent is started in the beginning of an X-session or a login session, and all other windows or
programs are started as clients to the ssh-agent program. Through use of environment variables the agent
can be located and automatically used for authentication when logging in to other machines using ssh(1).
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原文地址：http://www.cnblogs.com/exclm/p/4340966.html
