Introduction to Linux environment variables and process address space

//View the process pid and parent process [dy@VM-12-10-centos jincheng_12_5]$ ps ajx | head -1 && ps axj | grep 3669470
   PPID PID PGID SID TTY TPGID STAT UID TIME COMMAND
3669469 3669470 3669470 3669439 pts/0 3681645 S 1001 0:00 -bash
3669470 3681645 3681645 3669439 pts/0 3681645 R+ 1001 0:00 ps axj
3669470 3681646 3681645 3669439 pts/0 3681645 R+ 1001 0:00 grep --color=auto 3669470


//The first method to run the program without adding a path [dy@VM-12-10-centos jincheng_12_5]$ ll
total 28
-rw-rw-r-- 1 dy dy 73 Dec 5 16:46 Makefile
-rwxrwxr-x 1 dy dy 17432 Dec 5 16:47 proc
-rw-rw-r-- 1 dy dy 75 Dec 5 16:47 proc.c
[dy@VM-12-10-centos jincheng_12_5]$ sudo cp proc /usr/bin/                  
//Add your own executable program to the system path [sudo] password for dy: 
[dy@VM-12-10-centos jincheng_12_5]$ proc //Add your own executable program to the system path, and you don't need to add the ./ path. Hello world!
[dy@VM-12-10-centos jincheng_12_5]$ ./proc
hello world!
[dy@VM-12-10-centos jincheng_12_5]$ sudo rm /usr/bin/proc //Delete from the system path [dy@VM-12-10-centos jincheng_12_5]$ proc
-bash: /usr/bin/proc: No such file or directory

//The second type [dy@VM-12-10-centos jincheng_12_5]$ $PATH
-bash: /home/dy/.local/bin:/home/dy/bin:/usr/local/bin:/usr/bin:/usr/local/sbin:/usr/sbin: No such file or directory
[dy@VM-12-10-centos jincheng_12_5]$ pwd
/home/dy/jincheng_12_5
//Add the current path to the environment variable PATH [dy@VM-12-10-centos jincheng_12_5]$ PATH=$PATH:/home/dy/jincheng_12_5
[dy@VM-12-10-centos jincheng_12_5]$ proc
hello world!

//Only save 3000 commands [dy@VM-12-10-centos jincheng_12_5]$ echo $HISTSIZE
3000
//Show how many commands were run before [dy@VM-12-10-centos jincheng_12_5]$ history |wc -l
636
[dy@VM-12-10-centos jincheng_12_5]$ history
// Output all previous commands to the display // Display all environment variables [dy@VM-12-10-centos jincheng_12_5]$ env
LS_COLORS=rs=0:di=01;34:ln=01;36:mh=00:pi=40;33:so=01;35:do=01;35:bd=40;33;01:cd=40;33;01:or=40;31;01:mi=01;05;37;41:su=37;41:sg=30;43:ca=30;41:tw=30;42:ow=34;42:st=37;44:ex=01;32:*.tar=01;31:*.tgz=01;31:*.arc=01;31:*.arj=01;31:*.taz=01;31:*.lha=01;31:*.lz4=01;31:*.lzh=01;31:*.lzma=01;31:*.tlz=01;31:*.txz=01;31:*.tzo=01;31:*.t7z=01;31:*.zip=01;31:*.z=01;31:*.dz=01;31:*.gz=01;31:*.lrz=01;31:*.lz=01;31:*.lzo=01;31:*.xz=01;31:*.zst=01;31:*.tzst=01;31:*.bz2=01;31:*.bz=01;31:*.tbz=01;31:*.tbz2=01;31:*.tz=01;31:*.deb=01;31:*.rpm=01;31:*.jar=01;31:*.war=01;31:*.ear=01;31:*.sar=01;31:*.rar=01;31:*.alz=01;31:*.ace=01;31:*.zoo=01;31:*.cpio=01;31:*.7z=01;31:*.rz=01;31:*.cab=01;31:*.wim=01;31:*.swm=01;31:*.dwm=01;31:*.esd=01;31:*.jpg=01;35:*.jpeg=01;35:*.mjpg=01;35:*.mjpeg=01;35:*.gif=01;35:*.bmp=01;35:*.pbm=01;35:*.pgm=01;35:*.ppm=01;35:*.tga=01;35:*.xbm=01;35:*.xpm=01;35:*.tif=01;35:*.tiff=01;35:*.png=01;35:*.svg=01;35:*.svgz=01;35:*.mng=01;35:*.pcx=01;35:*.mov=01;35:*.mpg=01;35:*.mpeg=01;35:*.m2v=01;35:*.mkv=01;35:*.webm=01;35:*.ogm=01;35:*.mp4=01;35:*.m4v=01;35:*.mp4v=01;35:*.vob=01;35:*.qt=01;35:*.nuv=01;35:*.wmv=01;35:*.asf=01;35:*.rm=01;35:*.rmvb=01;35:*.flc=01;35:*.avi=01;35:*.fli=01;35:*.flv=01;35:*.gl=01;35:*.dl=01;35:*.xcf=01;35:*.xwd=01;35:*.yuv=01;35:*.cgm=01;35:*.emf=01;35:*.ogv=01;35:*.ogx=01;35:*.aac=01;36:*.au=01;36:*.flac=01;36:*.m4a=01;36:*.mid=01;36:*.midi=01;36:*.mka=01;36:*.mp3=01;36:*.mpc=01;36:*.ogg=01;36:*.ra=01;36:*.wav=01;36:*.oga=01;36:*.opus=01;36:*.spx=01;36:*.xspf=01;36:
LANG=en_US.utf8
HISTTIMEFORMAT=%F %T 
HOSTNAME=VM-12-10-centos
OLDPWD=/home/dy
USER=dy
PWD=/home/dy/jincheng_12_5
HOME=/home/dy
MAIL=/var/spool/mail/dy
SHELL=/bin/bash
TERM=xterm
SHLVL=1
PROMPT_COMMAND=history -a; history -a; printf "\033]0;%s@%s:%s\007" "${USER}" "${HOSTNAME%%.*}" "${PWD/#$HOME/\~}"
LOGNAME=dy
PATH=/home/dy/.local/bin:/home/dy/bin:/usr/local/bin:/usr/bin:/usr/local/sbin:/usr/sbin:/home/dy/jincheng_12_5
HISTSIZE=3000
LESSOPEN=||/usr/bin/lesspipe.sh %s
_=/usr/bin/env

[dy@VM-12-10-centos jincheng_12_5]$ ./proc //Local variables do not exist I am a proc : pid:3700079 ppid:3669470
Segmentation fault (core dumped)
[dy@VM-12-10-centos jincheng_12_5]$ export MY_VAL
[dy@VM-12-10-centos jincheng_12_5]$ ./proc
I am a proc : pid:3700163 ppid:3669470
hello dy
/home/dy/.local/bin:/home/dy/bin:/usr/local/bin:/usr/bin:/usr/local/sbin:/usr/sbin:/home/dy/jincheng_12_5
/home/dy
/bin/bash

env environment variable

//Set environment variables [dy@VM-12-10-centos jincheng_12_5]$ hello=1234
//Remember to add $ to output environment variables [dy@VM-12-10-centos jincheng_12_5]$ echo $hello
1234
[dy@VM-12-10-centos jincheng_12_5]$ set | grep hello
hello=1234

#include <stdio.h>
#include<stdlib.h>
#include <unistd.h>
int main(int argc, char *argv[], char *env[])
{
        for(int i = 0; argv[i]; ++i)//Interpret the command line parameters//for(int i = 0; i < argc; ++i)
        {
            printf("argv[%d]：%s\n",i,argv[i]);
        }
        for(int i = 0; env[i]; ++i) //You can get the environment variable through the code {
           printf("env[%d]：%s\n",i,env[i]);
        }
   return 0;
}

int x = atoi(argv[2]);
int y = atoi(argv[3]);
if (strcmp(argv[1],"-a") == 0)
{
    printf("%d + %d = %d\n",x,y,x+y);
}
else
{
    printf("%d - %d = %d\n",x,y,xy);
}

#include <stdio.h>
int main(int argc, char *argv[])
{
    extern char **environ;
    int i = 0;
    for(; environ[i]; i++){
    	printf("%s\n", environ[i]);
    }
	return 0;
}

#include <stdio.h>
#include <unistd.h>
int g_val = 0;
int main()
{
   pid_t id = fork();
   if(id < 0)
   {
       perror("fork() fail\n");
   }
   else if(id == 0)
   {
       printf("I am a child:%d，%d，%p\n",getpid(),g_val,&g_val);
   }else
   {
       printf("I am a fatcher: %d, %d, %p\n",getppid(),g_val,&g_val);
   }
   return 0;
}

#include <stdio.h>
#include <unistd.h>
int g_val;
int main()
{
   pid_t id = fork();
   if(id < 0)
   {
       perror("fork() fail");
   }
   else if(id == 0)
   {
       g_val = 100;
       printf("I am a child:%d，%d，%p\n",getpid(),g_val,&g_val);
   }else
   {
       sleep(3);
       printf("I am a fatcher: %d, %d, %p\n",getppid(),g_val,&g_val);
   }
   return 0;
}