Chapter 6

• I sort of lied, command line arguments (last set of notes) are from chapter 6.
• We will not cover this chapter in full
  • If you are in OS or care about memory, you should read it in full.
  • Otherwise read 6.1, 6.2, 6.6 and 6.7
• Discuss
  • pid_t getpid(void);
  • pid_t getppid(void);
  • Both need sys/types.h
  • Both need unistd.h
  • Neither can fail
• The Environment.
  • This is a global set of variables that the shell maintains.
  • Later when we see the exec family we will see that this is built into the system.
  • You normally get your environment from your parent, but this is not a requirement.
  • These variables control the behavior of programs.
• You can see environment variables with printenv
• Setting an environment variable in bash
  • varname=value for the current shell only
  • export varname=value for all children
  • Set the value in appropriate init file
    • See man bash on your system
    • Usually .bashrc, .bash_login
• In a bash script
  • echo $VARNAME
• Some special environment variables
  • $HOME is your home directory
  • $PATH is your executable path.
    • This lists where bash searches for programs to execute
    • The order given is the order searched.
    • whereis uses this to find where programs are
    • It is part of the exec family
    • If you add . to your path you don't have to type ./ any longer.
    • But be careful.
  • $MANPATH
• There are some special variables in bash
  • see the GUN Bash document.
  • PS1 is the prompt
  • You can change it to be many things.
• From a c/c++ program
  • All code in myenv.C
  • Old standard: int main(int argc, char * argv[], char * envp[])
    • This is a null terminated list

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

    • The entries will be in the form varname=value
    • Note, this is fixed at invocation
      • Variables added will not be present.
  • A global variable
    • extern char ** environ;
    • Null terminated list as well.
    • This is dynamic, as the environment changes, so will environ
    • see man 7 environ
    • Setting this to nullptr wipes out the environment.
  • With library functions
    • getenv
      • Needs stdlib.h
      • char *getenv(const char *name);
      • Returns a pointer or nullptr
      • You really should not change the value pointed to.
      • getenv will return nullptr on fail, but no error code.
    • setenv
      • stdlib.h as well.
      • int setenv(const char *name, const char *value, int overwrite);
        • If not set, sets it
        • If set, sets it if overwrite is not 0
        • This can fail with error codes.
      • The name can not contain a =
      • The name can not be null.
      • The name can not have a lenght of 0.
      • This allocates memory and copies the string.
    • unsetenv
      • int unsetenv(const char *name);
      • Error for bad variable names.
      • But not for an attempt to unset something that does not exist.
    • clearenv
      • int clearenv(void);
      • Wipes it out.
    • There is a putenv but I would avoid that.
  • Environment variables are used to control programs on a long term basis
    • Usually there is a default value if nothing is set.
    • Check a global configuration file for a value
    • Check a local configuration file for a value
    • Then check the environment for a variable
    • Finally check command line arguments.