Environment Variable

In Unix-like systems, we have to deal with environment variables such as PATH, MANPATH etc. Here are some tips for reference.

1. Order of directory

   PATH=$PATH:~/opt/bin
   PATH=~/opt/bin:$PATH
   

   The diffeence of the two lines is the order when searching for commands. The 2nd line make directory ~/opt/bin searched before those ones in original PATH.

2. export or not

   PATH=$PATH:~/opt/bin
   export PATH=$PATH:~/opt/bin
   

   export is a shell builtin for bash, ksh and zsh. The first line omit the export keyword which is called assignment. We don't need export if the variable is already in the environment: any change of the value of the variable is reflected in the environment for modern shells. PATH is pretty much always in the environment; all Unix-like system sets it in the very beginning (usually in the very first process, in fact).

   But if the variable is not in the environment, you must add export. For example, some environment variable is for specific program newly installed. You could test by command:

   ```bash ~ $ declare -p VAR_NAME

   Use export if not sure.

3. $HOME or ~

   export PATH="$PATH:$HOME/bin"
   PATH="$PATH:$HOME/bin"
   PATH=$PATH:$HOME/bin
   PATH=$PATH:~/bin
   PATH="$PATH:~/bin"
   

   The last line does not work*. The ~ character is only expanded to your home directory when it's the first character of a word and it's unquoted. For the last line, the ~ is between double quotes and therefore not expanded. Even if you wrote export "PATH=$PATH:"~/Unix/homebrew/bin, the ~ would not be expanded because it is not at the beginning of a shell word.

   Use $HOME instead of ~.

4. Double quotes or not

   If you have ~ in variable settings, omit double quotes. In all other cases, this does not matter.

5. ~/.profile is the place to put stuff that applies to your whole session of a login shell (like zsh), such as programs that you want to start when you log in (but not graphical programs, they go into a different file, i.e. ~/.xinitrc), and environment variable definitions. For Bash shell, we have a counterpart .bash_profile.
6. ~/.bashrc is the place to put stuff that applies only to Bash only, such as alias and function definitions, shell options, and prompt settings. (You could also put key bindings there, but for bash they normally go into ~/.inputrc.)

7. ~/.bash_profile can be used instead of ~/.profile, but it is Bash's alternative of ~/.profile, not used by any other shell. (This is mostly a concern if you want your initialization files to work on multiple machines and your login shell isn't bash on all of them.) This is a logical place to include ~/.bashrc if the shell is interactive.

   Set up your environment (PATH, LANG, EDITOR, …) in "~/.bash_profile". Set up your bash shell (PS1, functions, shopts, …) in "~/.bashrc". Then source ".bashrc" from ".bash_profile".

   Check dotfiles.

8. You may see here and there recommendations to either put environment variable definitions in ~/.bashrc or always launch login shells in terminals. Both are bad ideas. The most common problem with either of these ideas is that your environment variables will only be set in programs launched via the terminal, not in programs started directly with an icon or desktop menu or keyboard shortcut.

   But in Gentoo, the official Wiki recommends per-user setting in ~/.bashrc which is sourced by ~/.bash_profile.

9. ~/.bash_profile is executed for login shells, while ~/.bashrc is executed for interactive non-login shells. When you login (eg: type username and password) via console, either physically sitting at the machine when booting, or remotely via ssh: .bash_profile is executed to configure things before the initial command prompt. But, if you've already logged into your machine and open a new terminal window (xterm) inside Gnome or KDE, then .bashrc is executed before the window command prompt. ~/.bashrc is also run when you start a new bash instance by typing /bin/bash in a terminal.

10. Execution order:

    Login shell: /etc/profile -> /etc/env.d/* -> /etc/bash/bashrc -> /etc/profile.d/*.sh -> ~/.bash_profile -> ~/.bashrc

    Interactive shell: /etc/bash/bashrc -> ~/.bashrc

References: 1, 2 and 3.