EDX Linux Foundation Ch 9：User Environment Section 2 Environment Variables

##Environment Variables
Environment variables are simply named quantities that have specific values and are understood by the command shell, such as bash. Some of these are pre-set (built-in) by the system, and others are set by the user either at the command line or within startup and other scripts.

環境變數可以幫我們達到很多功能～包括家目錄的變換啊、提示字元的顯示啊、執行檔搜尋的路徑啊等等的， 還有很多很多啦！那麼，既然環境變數有那麼多的功能，問一下，目前我的 shell 環境中， 有多少預設的環境變數啊？我們可以利用兩個指令來查閱，分別是 env 與 export 呢！

###env和export
env是environment的簡寫，可以列出來所有的環境變數。

$ env
SSH_AGENT_PID=1892
GPG_AGENT_INFO=/run/user/me/keyring-Ilf3vt/gpg:0:1
TERM=xterm
SHELL=/bin/bash
...

如果使用 export 也會是一樣的內容～ 只不過， export 還有其他額外的功能就是了。（下面會講到）

$ export
declare -x COLORTERM=gnome-terminal
declare -x COMPIZ_BIN_PATH=/usr/bin /
declare -x COMPIZ_CONFIG_PROFILE=ubuntu
...

###set
用 set 觀察所有變數 (含環境變數與自訂變數)

$ set
BASH=/bin/bash
BASHOPTS=checkwinsize:cmdhist:expand_aliases:extglob:extquote:force_fignore
BASH_ALIASES=()
...

###環境變數與自定變數
By default, variables created within a script are only available to the current shell; child processes (sub-shells) will not have access to values that have been set or modified. Allowing child processes to see the values, requires use of the export command.

環境變數與自訂變數這兩者之間有啥差異呢？其實這兩者的差異在於『 該變數是否會被子程式所繼續引用』
子程式僅會繼承父程式的環境變數， 子程式不會繼承父程式的自訂變數啦！所以你在原本 bash 的自訂變數在進入了子程式後就會消失不見， 一直到你離開子程式並回到原本的父程式後，這個變數才會又出現！

##Setting Environment Variables

###Show the value of a specific variable

echo $SHELL

###Export a new variable value
export除了可以查看以外還可以用來設定環境變數

export VARIABLE=value (or VARIABLE=value; export VARIABLE)

###Add a variable permanently

Edit ~/.bashrc and add the line export VARIABLE=value

Type source ~/.bashrc or just . ~/.bashrc (dot ~/.bashrc); or just start a new shell by typing bash

##The HOME Variable
HOME is an environment variable that represents the home (or login) directory of the user.
輸入echo $HOME會顯示家目錄的路徑
輸入cd後面不加目錄名稱會切換到家目錄

##The PATH Variable
PATH is an ordered list of directories (the path) which is scanned when a command is given to find the appropriate program or script to run. Each directory in the path is separated by colons (:). A null (empty) directory name (or ./) indicates the current directory at any given time.

###show $Path
echo $PATH

###To prefix a private bin directory to your path:

$ export PATH=$HOME/bin:$PATH
$ echo $PATH
/home/me/bin:/usr/local/bin:/usr/bin:/bin/usr

延伸閱讀：鳥哥：$Path

##The PS1 Variable
Prompt Statement (PS) is used to customize your prompt string in your terminal windows to display the information you want.
用來改變終端機外觀的變數。

##The SHELL Variable
The environment variable SHELL points to the user’s default command shell and contains the full pathname to the shell:

$ echo $SHELL
/bin/bash