开机过程中会执行 /etc/profile ~/.profile 和 shell 的 rc 脚本(比如 zsh 的~/.zshrc )，我查看这些脚本里面没有对$PATH 设置的内容，之前在安装 JDK 的时候有在 /etc/profile 里面添加 export PATH=${JAVA_HOME}/bin:$PATH ，在这里已经出现$PATH 了，那么这个$PATH 是在什么时候设置的呢

Ubuntu 可能是 /etc/enviroment

cat: /etc/enviroment: No such file or directory

INVOCATION
A login shell is one whose first character of argument zero is a -, or one started with the --login option.
An interactive shell is one started without non-option arguments and without the -c option whose standard input and error are both connected
to terminals (as determined by isatty (3 )), or one started with the -i option. PS1 is set and $- includes i if bash is interactive, allowing a
shell script or a startup file to test this state.
The following paragraphs describe how bash executes its startup files. If any of the files exist but cannot be read, bash reports an error.
Tildes are expanded in filenames as described below under Tilde Expansion in the EXPANSION section.
When bash is invoked as an interactive login shell, or as a non-interactive shell with the --login option, it first reads and executes com ‐
mands from the file /etc/profile, if that file exists. After reading that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile,
in that order, and reads and executes commands from the first one that exists and is readable. The --noprofile option may be used when the
shell is started to inhibit this behavior.
When a login shell exits, bash reads and executes commands from the file ~/.bash_logout, if it exists.
When an interactive shell that is not a login shell is started, bash reads and executes commands from ~/.bashrc, if that file exists. This
may be inhibited by using the --norc option. The --rcfile file option will force bash to read and execute commands from file instead of
~/.bashrc.
When bash is started non-interactively, to run a shell script, for example, it looks for the variable BASH_ENV in the environment, expands its
value if it appears there, and uses the expanded value as the name of a file to read and execute. Bash behaves as if the following command
were executed:
if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi
but the value of the PATH variable is not used to search for the filename.
If bash is invoked with the name sh, it tries to mimic the startup behavior of historical versions of sh as closely as possible, while con ‐
forming to the POSIX standard as well. When invoked as an interactive login shell, or a non-interactive shell with the --login option, it
first attempts to read and execute commands from /etc/profile and ~/.profile, in that order. The --noprofile option may be used to inhibit
this behavior. When invoked as an interactive shell with the name sh, bash looks for the variable ENV, expands its value if it is defined,
and uses the expanded value as the name of a file to read and execute. Since a shell invoked as sh does not attempt to read and execute com ‐
mands from any other startup files, the --rcfile option has no effect. A non-interactive shell invoked with the name sh does not attempt to
read any other startup files. When invoked as sh, bash enters posix mode after the startup files are read.

/etc/security/pam_env.conf

etc 下是全局设置， home 下是自己用户的设置，最好只设置后者。
java 在安装的时候向 /etc/profile 添加了一句 PATH 呗。

在我这里这个文件是全被注释了的

貌似是在执行 /etc/profile 之前 $PATH 就已经存在了

读读你用的 shell 的源代码

其实 1L 已经给出基本答案了，只不过不是 /etc/enviroment ，而是 /etc/environment

忘了说，公司的内网服务器也是 ubuntu

原来如此！那么还有一个问题，这个 /etc/environment 是在启动的哪个阶段执行的呢

没深究过呢，猜测是用户登录成功之后就会被用到

A suitable file for environment variable settings that affect the system as a whole (rather than just a particular user ) is /etc/environment.
/etc/environment 是一个系统级别（而非用户级别）的环境变量配置文件
出处： https://help.ubuntu.com/community/EnvironmentVariables

这个得看 distro, 不过都差不多。比方说在 osx 上，/etc/zshenv 里面有个 path finder 会设置 path.记得把它删掉，是个 bug