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• In fact, the cd 'command' is not a command at all

• There is no /bin/cd

• cd must always be implemented as a shell built-in command

• Similarly a command can't set an environment variable in the parent shell

• This is true of almost everything in the process structure:

  • It is copied when the process forks

  • So the child process can change its own copy, but not the parent's

  • The program's memory is an important example of this:

        $X = "parent\n";
        if (fork() == 0) {   # I'm the child
          $X = "child\n";
        }
        print $X;

• Prints:

        parent
        child

• The child changed its copy of $X, leaving the parent's unaffected

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Copyright © 2002 M. J. Dominus