Objectives

1. Understand environment variables

Notes

• From man 7 envrion
  • Probably in the book as well.
• This is the third "argument" passed to main.
  • int main(int argc, char * argc[], char * envp[])
  • int main(int argc, char * argc[], char ** envp)
  • It is null terminated array of c style strings.
  • By convention the strings are in the form "variable=value"
• Simple Processing

  • for(int i = 0; envp[i] != nullptr; ++i) { 

• As we have seen, it controls program behavior.
  • PATH is used by most shells to find programs to execute
    • If . is in your path the current directory will be searched.
    • But if . is in your path, security people will grump at your.
    • Examine with printenv PATH
      • By the way, printenv will print the entire environment.
      • And probably do other cool stuff, see man printenv
    • In bash echo $PATH
  • HOME is used by some programs to find your home directory.
  • TERM for your terminal type
  • SHELL for the name of your shell
• Also, see man bash and search for Shell Variables
• Setting environment variables
  • When a process is created, it normally receives its parent's environment.
  • Since your shell starts most of the programs you run, you can set the variable in the shell.
    • BASH: SILLY=YES
      • This will set the variable for this shell,
      • But will NOT pass it along to any subshells (or new programs started)
    • BASH: export SILLY
    • BASH: export SILLY="Yes I AM"
    • BASH: unset SILLY
  • Some programs, like bash, read configuration files
    • BASH,
    • This gets a bit complex
    • And, be careful
      • you can make it so you can't log in.
      • And then you are headed to the help desk with hat in hand.
      • Log in twice
        • One to edit
        • One to log out and log in again.
        • Never exit the one to edit until you test.
    • Bash looks at a number of files on initialization.
      • It depends on how it is started.
      • See man bash
      • But
        • /etc/profile system wide
        • ~/.bash_profile, ~/.bash_login, ~/.profile
        • ~/.bashrc
  • You could write a program to read a file if you want that level of customization.