# echo $HOME
/root

# env
LC_PAPER=en_US.UTF-8
LC_ADDRESS=en_US.UTF-8
SSH_AGENT_PID=1768
LC_MONETARY=en_US.UTF-8
GPG_AGENT_INFO=/tmp/keyring-rB9csr/gpg:0:1
TERM=xterm
SHELL=/bin/bash
XDG_SESSION_COOKIE=619793d1806621208703bfca00000005-1489544306.787815-2133755939
WINDOWID=31457285
LC_NUMERIC=en_US.UTF-8
GNOME_KEYRING_CONTROL=/tmp/keyring-rB9csr
USER=root
.....

# env | grep SH
SSH_AGENT_PID=1768
SHELL=/bin/bash
SSH_AUTH_SOCK=/tmp/keyring-rB9csr/ssh
SHLVL=1

root@ubuntu:~# set
BASH=/bin/bash
BASHOPTS=checkwinsize:cmdhist:expand_aliases:extquote:force_fignore:histappend:hostcomplete:interactive_comments:progcomp:promptvars:sourcepath
BASH_ALIASES=()
BASH_ARGC=()
BASH_ARGV=()
BASH_CMDS=()
BASH_LINENO=()
BASH_SOURCE=()
BASH_VERSINFO=([0]="4" [1]="2" [2]="24" [3]="1" [4]="release" [5]="i686-pc-linux-gnu")
BASH_VERSION=‘4.2.24(1)-release‘
.....

# export HELLO="hello"
# echo $HELLO
hello

# unset HELLO
# echo $HELLO

#

# export HELLO="hello"
# readonly HELLO="hello"
# export HELLO="hello" world
bash: HELLO: 只读变量
# unset HELLO
bash: unset: HELLO: 无法反设定: 只读 variable

# echo $PATH
/usr/lib/lightdm/lightdm:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/home/tarena/workdir/toolchains/opt/S5PV210-crosstools/4.4.6/bin

# echo $HOME
/home/admin

# echo $HOME
/root

# echo $HISTSIZE
1000

echo $LOGNAME
root

echo $HOSTNAME
ubuntu

echo $SHELL
/bin/bash

echo $LANG
zh_CN.UTF-8

# echo $MAIL

# echo $PS1
\[\e]0;\u@\h: \w\a\]${debian_chroot:+($debian_chroot)}\u@\h:\w\$

# echo $PS2
>

gedit /etc/profile
添加 export PATH="/opt/hisi-linux/x86-arm/arm-hisiv300-linux/target/bin:$PATH"
执行 source /etc/profile

gedit .bashrc
添加 export PATH=$PATH:/opt/arm-2009q1-203/bin:
执行 source .bashrc

LIBRARY_PATH
The value of LIBRARY_PATH is a colon-separated list of directories, much like PATH.
When configured as a native compiler, GCC tries the directories thus specified when searching for special linker files, if it can‘t find them using GCC_EXEC_PREFIX.
Linking using GCC also uses these directories when searching for ordinary libraries for the -l option (but directories specified with -L come first). 

LD_LIBRARY_PATH
A colon-separated list of directories in which to search for
ELF libraries at execution-time.  Similar to the PATH
environment variable.  Ignored in set-user-ID and set-group-ID
programs.

LIBRARY_PATH is used by gcc before compilation to search for directories containing libraries that need to be linked to your program.

LD_LIBRARY_PATH is used by your program to search for directories containing the libraries after it has been successfully compiled and linked.

EDIT: As pointed below, your libraries can be static or shared.
If it is static then the code is copied over into your program and you don‘t need to search for the library after your program is compiled and linked.
If your library is shared then it needs to be dynamically linked to your program and that‘s when LD_LIBRARY_PATH comes into play.

SEARCH_DIR("/usr/i686-linux-gnu/lib32"); SEARCH_DIR("=/usr/local/lib32"); SEARCH_DIR("=/lib32"); SEARCH_DIR("=/usr/lib32"); SEARCH_DIR("=/usr/local/lib/i386-linux-gnu"); SEARCH_DIR("=/usr/local/lib"); SEARCH_DIR("=/lib/i386-linux-gnu"); SEARCH_DIR("=/lib"); SEARCH_DIR("=/usr/lib/i386-linux-gnu"); SEARCH_DIR("=/usr/lib");

//示例一
#include <stdio.h>

int main(void)
{
	//声明全局变量
	extern char** environ;
	//给环境表首地址指定替身
	char** p = environ;
	while(*p != NULL)
	{
		printf("%s\n",*p);
		//指向下一个
		p++;
	}
	return 0;
}

功能等同指令env，可自行尝试

//示例二
#include <stdio.h>
#include <string.h>

int main(void)
{
	//声明全局变量
	extern char** environ;
	//给环境表首地址指定替身
	char** p = environ;
	//练习：查找名字为SHELL的环境变量，获取SHELL的值存到buf的字符数组中，然后进行打印
	char buf[20] = {0};
	p = environ;
	while(*p != NULL)
	{
		//比较前6个字符是否相等
		if(!strncmp(*p,"SHELL=",6))
		{
			//跳过环境变量名=
			strcpy(buf,*p+6);
			break;
		}
		//比较下一个
		p++;
	}
	printf("SHELL=%s\n",buf);
	return 0;
}
输出结果：
SHELL=/bin/bash

功能等同指令echo $SHELL，可自行尝试

#include <stdio.h>
#include <stdlib.h>

int main (void)
{
	char *pc = getenv ("SHELL");
	if (NULL == pc)
		perror ("getenv"), exit (-1);
	printf ("SHELL = %s\n", pc);
	return 0;
}
输出结果：
SHELL = /bin/bash

使用echo指令
# echo $SHELL
/bin/bash

#include <stdio.h>
#include <stdlib.h>

int main (void)
{
	char *pc = getenv ("SHELL");
	if (NULL == pc)
		perror ("getenv"), exit (-1);
	printf ("SHELL = %s\n", pc);

	setenv ("SHELL", "ABC", 1);
	printf ("修改后的SHELL = %s\n", getenv ("SHELL"));
	return 0;
}
输出结果：
SHELL = /bin/bash
修改后的SHELL = ABC

#include <stdio.h>
#include <stdlib.h>

int main (void)
{
	char *pc = getenv ("SHELL");
	if (NULL == pc)
		perror ("getenv"), exit (-1);
	printf ("SHELL = %s\n", pc);

		
	int res = putenv ("SHELL=abc");
	if (res)
		perror("putenv"),exit(-1);
	printf ("修改后的SHELL = %s\n", getenv ("SHELL"));

	putenv("CSDN=666");
	printf("增加的环境值是：%s\n",getenv("CSDN"));//123
	return 0;
}
输出结果：
SHELL = /bin/bash
修改后的SHELL = abc
增加的环境值是：666

#include <stdio.h>
#include <stdlib.h>

int main (void)
{
	putenv("CSDN=666");
	printf("增加的环境值是：%s\n",getenv("CSDN"));//123

	unsetenv("CSDN");
	printf("删除的结果是：%s\n",getenv("CSDN"));
	return 0;
}
输出结果：
增加的环境值是：666
删除的结果是：(null)

#include <stdio.h>
#include <stdlib.h>

int main (void)
{
	int res;
	putenv("CSDN=666");
	printf("增加的环境值是：%s\n",getenv("CSDN"));//123

	res = clearenv ();
	if(res)
		perror("clearenv"), exit(-1);
	printf("清空整个环境表结束\n");
	printf("CSDN = %s\n",getenv("CSDN"));
	return 0;
}
输出结果：
增加的环境值是：666
清空整个环境表结束
CSDN = (null)

main函数原型：

int main (int argc, char * argv[], char * envp[]) {....}

第一个参数：命令行参数的个数

第二个参数：命令行参数的地址信息

第三个参数：环境表的首地址

参数argc表示输入参数的个数(含命令名)，如果有命令行参数，argc应不小于1；argv表示传入的参数的字符串，是一个字符串数组，argv[0]表示命令名，argv[1]指向第一个命令行参数；至于第三个参数env，它与全局变量environ相比也没有带来更多益处，所以POSIX.1也规定应使用environ而不使用第三个参数。通常用getenv和putenv函数来存取特定的环境变量，而不是直接使用environ变量。例如：

//main函数原型的使用  
#include <stdio.h>  
int main(int argc,char* argv[],char* envp[])  
{  
    printf("argc=%d\n",argc);  
    int i=0;  
    for(i=0;i<argc;i++)  
    {  
        printf("感谢%s\n",argv[i]);  
    }  
    //声明全局变量  
    extern char** environ;  
    printf("environ=%p,envp=%p\n",environ,envp);//直接访问环境表  
    return 0;  
}  
输出结果为：  
argc=1  
感谢./a.out  
environ=0xbfab74cc,envp=0xbfab74cc