====== تابع ()getenv و ()setenv - خواندن و نوشتن متغیر های محلی ======

----
==== تابع ()getenv ====

<code C>
// man 1 env

// man 3 getenv
/* getenv() returns a pointer to the value in the environment, or NULL if there is no match. */
#include <stdlib.h>
char *getenv(const char *name);
</code>

<code C>
/* env-var.c */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void)
{
   /* Using getenv() to fetch env. variables */
   printf("Your username is %s\n", getenv("USER"));
   printf("Your home directory is %s\n",
      getenv("HOME"));
   printf("Your preferred editor is %s\n",
      getenv("EDITOR"));
   printf("Your shell is %s\n", getenv("SHELL"));

   /* Check if the current terminal support colors*/
   if ( strstr(getenv("TERM"), "256color")  )
   {
      /* Color the output with \033 + colorcode */
      printf("\033[0;31mYour \033[0;32mterminal "
         "\033[0;35msupport "
         "\033[0;33mcolors\033[0m\n");
   }
   else
   {
      printf("Your terminal doesn't support"
         " colors\n");
   }
   return 0;
}
</code>

  $ make env-var
  cc     env-var.c   -o env-var

  $ ./env-var
  Your username is pejman
  Your home directory is /home/pejman
  Your preferred editor is (null)
  Your shell is /bin/bash
  Your terminal support colors

  $ echo $USER
  pejman

  $ echo $HOME
  /home/pejman

  $ echo $EDITOR

  $ echo $SHELL
  /bin/bash

  $ echo $TERM
  xterm-256color

  $ export TERM=xterm
  
  $ ./env-var
  Your username is pejman
  Your home directory is /home/pejman
  Your preferred editor is (null)
  Your shell is /bin/bash
  Your terminal doesn't support colors
  
  $ export TERM=xterm-256color

  $ echo $TERM
  xterm-256color
  
  $ TERM=xterm ./env-var
  Your username is pejman
  Your home directory is /home/pejman
  Your preferred editor is (null)
  Your shell is /bin/bash
  Your terminal doesn't support colors
  
  $ echo $TERM
  xterm-256color

----
==== تابع ()setenv ====

<code C>
// man 3 setenv
/* The  setenv()  function adds the variable name to the environment with the value value. 
 * if variable exists and overwrite is zero, then the value of name is not changed.
*/
#include <stdlib.h>
int setenv(const char *name, const char *value, int overwrite);
</code>

<code C>
/* env-var-set.c */
#define _POSIX_C_SOURCE 200112L
#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    setenv("FULLNAME", "Jack-Benny", 1);
    printf("Your full name is %s\n", getenv("FULLNAME"));
    return 0;
}
</code>

  $ make env-var-set
  cc     env-var-set.c   -o env-var-set

  $ ./env-var-set
  Your full name is Jack-Benny
  
  $ echo $FULLNAME