UNIX再學習 -- 環境變數

# 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: 只讀變數
# unset HELLO
bash: unset: HELLO: 無法反設定: 只讀 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
新增 export PATH="/opt/hisi-linux/x86-arm/arm-hisiv300-linux/target/bin:$PATH"
執行 source /etc/profile

#gcc找到標頭檔案的路徑
$C_INCLUDE_PATH=/opt/example/include  
$export C_INCLUDE_PATH  

#g++找到標頭檔案的路徑  
$CPLUS_INCLUDE_PATH=/opt/example/include  
$export CPLUS_INCLUDE_PATH  

#找到靜態庫的路徑  
$LIBRARY_PATH=/opt/example/lib  
$export LIBRARY_PATH 

#找到動態連結庫的路徑
LD_LIBRARY_PATH=/opt/example/lib  
export LD_LIBRARY_PATH 

gedit .bashrc
新增 export PATH=$PATH:/opt/arm-2009q1-203/bin:
執行 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)
{
	//宣告全域性變數
	extern char** environ;
	//給環境表首地址指定替身
	char** p = environ;
	while(*p != NULL)
	{
		printf("%s\n",*p);
		//指向下一個
		p++;
	}
	return 0;
}

功能等同指令env，可自行嘗試

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

int main(void)
{
	//宣告全域性變數
	extern char** environ;
	//給環境表首地址指定替身
	char** p = environ;
	//練習：查詢名字為SHELL的環境變數，獲取SHELL的值存到buf的字元陣列中，然後進行列印
	char buf[20] = {0};
	p = environ;
	while(*p != NULL)
	{
		//比較前6個字元是否相等
		if(!strncmp(*p,"SHELL=",6))
		{
			//跳過環境變數名=
			strcpy(buf,*p+6);
			break;
		}
		//比較下一個
		p++;
	}
	printf("SHELL=%s\n",buf);
	return 0;
}
輸出結果：
SHELL=/bin/bash

功能等同指令echo $SHELL，可自行嘗試

#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;
}
輸出結果：
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 ("修改後的SHELL = %s\n", getenv ("SHELL"));
	return 0;
}
輸出結果：
SHELL = /bin/bash
修改後的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 ("修改後的SHELL = %s\n", getenv ("SHELL"));

	putenv("CSDN=666");
	printf("增加的環境值是：%s\n",getenv("CSDN"));//123
	return 0;
}
輸出結果：
SHELL = /bin/bash
修改後的SHELL = abc
增加的環境值是：666

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

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

	unsetenv("CSDN");
	printf("刪除的結果是：%s\n",getenv("CSDN"));
	return 0;
}
輸出結果：
增加的環境值是：666
刪除的結果是：(null)

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

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

	res = clearenv ();
	if(res)
		perror("clearenv"), exit(-1);
	printf("清空整個環境表結束\n");
	printf("CSDN = %s\n",getenv("CSDN"));
	return 0;
}
輸出結果：
增加的環境值是：666
清空整個環境表結束
CSDN = (null)

main函式原型：

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

第一個引數：命令列引數的個數

第二個引數：命令列引數的地址資訊

第三個引數：環境表的首地址

引數argc表示輸入引數的個數(含命令名)，如果有命令列引數，argc應不小於1；argv表示傳入的引數的字串，是一個字串陣列，argv[0]表示命令名，argv[1]指向第一個命令列引數；至於第三個引數env，它與全域性變數environ相比也沒有帶來更多益處，所以POSIX.1也規定應使用environ而不使用第三個引數。通常用getenv和putenv函式來存取特定的環境變數，而不是直接使用environ變數。例如：

//main函式原型的使用  
#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("感謝%s\n",argv[i]);  
    }  
    //宣告全域性變數  
    extern char** environ;  
    printf("environ=%p,envp=%p\n",environ,envp);//直接訪問環境表  
    return 0;  
}  
輸出結果為：  
argc=1  
感謝./a.out  
environ=0xbfab74cc,envp=0xbfab74cc