The Environment

• GNU/Linux provides each running program with an environment.The environment is a collection of variable/value pairs. Both environment variable names and their values are character strings.
• By convention, environment variable names are spelled in all capital letters.
  • USER contains your username.
  • HOME contains the path to your home directory.
  • PATH contains a colon-separated list of directories through which Linux searches for commands you invoke.
  • DISPLAY contains the name and display number of the X Window server on which windows from graphical X Window programs will appear.
• Your shell, like any other program, has an environment. Shells provide methods for examining and modifying the environment directly.
  • The shell automatically creates a shell variable for each environment variable that it finds, so you can access environment variable values using the $varname syntax. For instance:

    $ echo $USER
    $ echo $HOME
    

  • You can use the export command to export a shell variable into the environment. For example, to set the EDITOR environment variable, you would use this:

    $ export EDITOR=emacs
    

• In a program, you access an environment variable with the getenv function in $<stdlib.h>$.
• To set or clear environment variables, use the setenv and unsetenv functions, respectively. http://siber.cankaya.edu.tr/SystemsProgramming/cfiles/print_env.cprint_env.c

  #include <stdio.h>
  
  /* The ENVIRON variable contains the environment.  */
  extern char** environ;
  
  int main ()
  {
    char** var;
    for (var = environ; *var != NULL; ++var)
      printf ("%s\n", *var);
    return 0;
  }
  

  Don't modify environ yourself; use the setenv and unsetenv functions instead. http://siber.cankaya.edu.tr/SystemsProgramming/cfiles/client.cclient.c

  #include <stdio.h>
  #include <stdlib.h>
  
  int main ()
  {
    char* server_name = getenv ("SERVER_NAME");
    if (server_name == NULL) 
      /* The SERVER_NAME environment variable was not set.  Use the
         default.  */
      server_name = "server.my-company.com";
  
    printf ("accessing server %s\n", server_name);
    /* Access the server here...  */
  
    return 0;
  }
  
  $ client
  accessing server server.my-company.com
  $ export SERVER_NAME=backup-server.elsewhere.net
  $ client
  accessing server backup-server.elsewhere.net