View methods to control environment variables under Linux

View environment variables

View an environment variable: For example, we need to view the environment variable HOME, we can directly enter echo $HOME under shell, we can print out all the environment variables and the value of the environment variable

Print environment variable

The global variable environ defined in libc refers to the environment variable scale. environ is not included in any header file and is declared in extern when used.

A program print environment variable is provided below

#include<stdio.h>
 int main()
 {
   //libc Global variables defined in environ Directional environmental variability scale ,environ Not included in any header ⽂ In the file , so 
   // In making ⽤ when   to ⽤ with extern The statement. 
   extern char** environ;
 //  while(environ)
 //  {
 //   printf("%s\n",*environ++);
 //  }
 //  printf("hah");
   int i = 0;
   for(i = 0;environ[i]!=NULL;i++)
   {
     printf("%s\n",environ[i]);
   }
   return 0;
 }

Controls the environment variable interface getenv . setenv . unsetenv

getenv () function

#include <stdlib.h>
char *getenv(const char *name);

The getenv() function's search environment list finds the environment variable name and returns a pointer to the corresponding value string. If not, return NULL

The setenv() function and unsetenv

#include <stdlib.h>
int setenv(const char *name, const char *value, int overwrite);
int unsetenv(const char *name);

setenv() sets the environment variable to return 0 on success or non-0 on failure

unsetenv() removes 1 environment variable

The following provides a section of procedures you can test 1

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

int main(void) 
{ 
  char* val; 
  const char* name ="ABC"; 

  // To obtain ABC The value of an environment variable  
  val = getenv(name); 
  printf("No.1 %s=%s\n", name, val); 

  // Override the write environment variable  
  setenv(name, "I amsure of that I will get it", 1); 
  printf("No.2%s=%s\n", name, val); 

  val = getenv(name); 
  printf("No.3%s=%s\n", name, val); 

  // delete 1 Environment variables  
  int ret =unsetenv("ABC"); 
  printf("ret =%d\n",ret); 

  val = getenv(name); 
  printf("No.3 %s=%s\n",name, val); 

  return 0; 
}