Linux中的PATH环境变量

Question 1

您的问题的答案可以在INVOCATION部分找到man bash。以下是相关摘录：

   When  bash is invoked as an interactive login shell, or as a non-inter-
   active shell with the --login option, it first reads and executes  com-
   mands  from  the file /etc/profile, if that file exists.  After reading
   that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile,
   in  that order, and reads and executes commands from the first one that
   exists and is readable.  The --noprofile option may be  used  when  the
   shell is started to inhibit this behavior.

   When  a  login  shell  exits, bash reads and executes commands from the
   file ~/.bash_logout, if it exists.

   When an interactive shell that is not a login shell  is  started,  bash
   reads  and executes commands from ~/.bashrc, if that file exists.  This
   may be inhibited by using the --norc option.  The --rcfile file  option
   will  force  bash  to  read  and  execute commands from file instead of
   ~/.bashrc.

   When bash is started non-interactively, to  run  a  shell  script,  for
   example, it looks for the variable BASH_ENV in the environment, expands
   its value if it appears there, and uses the expanded value as the  name
   of  a  file to read and execute.  Bash behaves as if the following com-
   mand were executed:
          if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi
   but the value of the PATH variable is not used to search for  the  file
   name.

手册页中还有更多内容，我建议您阅读它。

Answer

您的问题的答案可以在INVOCATION部分找到man bash。以下是相关摘录：

   When  bash is invoked as an interactive login shell, or as a non-inter-
   active shell with the --login option, it first reads and executes  com-
   mands  from  the file /etc/profile, if that file exists.  After reading
   that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile,
   in  that order, and reads and executes commands from the first one that
   exists and is readable.  The --noprofile option may be  used  when  the
   shell is started to inhibit this behavior.

   When  a  login  shell  exits, bash reads and executes commands from the
   file ~/.bash_logout, if it exists.

   When an interactive shell that is not a login shell  is  started,  bash
   reads  and executes commands from ~/.bashrc, if that file exists.  This
   may be inhibited by using the --norc option.  The --rcfile file  option
   will  force  bash  to  read  and  execute commands from file instead of
   ~/.bashrc.

   When bash is started non-interactively, to  run  a  shell  script,  for
   example, it looks for the variable BASH_ENV in the environment, expands
   its value if it appears there, and uses the expanded value as the  name
   of  a  file to read and execute.  Bash behaves as if the following com-
   mand were executed:
          if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi
   but the value of the PATH variable is not used to search for  the  file
   name.

手册页中还有更多内容，我建议您阅读它。

Question 2

事情比 shell 提供的要复杂一些。

登录方式主要有以下三种：

从真正的终端登录（现在主要是控制台）
从伪终端登录（主要是网络连接）
从图形环境登录

它们都能够在执行您的 shell 之前设置环境，并且它们都可以（HOME、LOGNAME、TERM可能不是由您的 shell 设置的，即使您没有启动文件，您的 shell 通常也会继承PATH环境变量）。

当从真实终端登录时，处理连接的进程可能会设置TERM环境变量并将其余工作委托给程序login。该程序会执行一些验证（例如防止 root 在不安全的终端上登录），确保整个环境TERM是干净的，初始化HOME, PATH, SHELL, TERM, MAIL,LOGNAME然后启动您的登录 shell。然后您的 shell 将进行自己的初始化。

当从伪终端登录时，会发生同样的事情，但稍有不同。通常，环境从处理连接的进程中获得更多的初始化，而不仅仅是TERM环境变量（网络协议通常有一种从另一端传输环境的方法），因此login在不清理环境的模式下使用，清理工作是由处理连接的程序完成的。

图形环境通常不会委托，login但行为类似。验证您的凭据后，他们会创建一个干净的环境（使用常用的环境变量，并且至少DISPLAY设置正确；他们通常允许系统管理员提供的脚本添加内容），然后为您的桌面环境启动启动脚本；这些可能会尝试从您的登录 shell 获取环境，并且通常还提供提供脚本来完成设置的可能性。因此，当您从桌面启动任何程序时，其环境是图形登录程序、桌面环境和登录脚本设置的组合。

最后一件事：当您启动终端模拟器时，您可能会获得一个登录 shell（在这种情况下，会执行 shell 中的登录脚本，因此您可以看到其中最新更改的效果，但不会看到与其他程序相同的环境）或不同（在这种情况下，不执行 shell 中的登录脚本，您看不到最新的更改，但您获得与其他程序相同的环境 - 由交互式初始化修改shell 的脚本）。

Answer