_learning perl_, ch14, 14.I.2002, nh

PROCESS MANAGEMENT

launch a new process: use the `system` function
                               ^^^^^^
        * hands a single string to a brand-new /bin/sh shell to be
          executed
        * STDIN, STDOUT, and STDERR are inherited from perl process
          (also inherits current umask, current directory, and uid)
        * NONZERO EXIT VALUES MEAN IT DID **NOT** WORK!
        * double-quoted strings variable-interpolated

    - also inherits current environment variables
    - can modify them through special hash %ENV
        keys are the names ov env variables -- PATH, USER, &c.  
    - `local` can be used to set an element of %ENV a temp value:
        {
          local $ENV{"PATH"} = "/bin:/usr/bin:/usr/ucb";
          # more commands here
        }

or use ``s -- backquotes

      * put a /bin/sh command between `s`
          $now = "the time is now " . `date`;
      
      * can evaluate backquoted commands within a list context:
          foreach $_ (`who`) {
            ($who,$where,$when) = /(\S+)\s+\S+)\s+(.*)/;
            print "$who on $where at $when\n";
          }

      normally, ou just get the standard output of the commands
      within the backquotes as the value of the backqouted-string:
        die "rm spoke!" if `rm fred 2>&1`;
      --process dies if rm says anything, to STDOUT *or* to STDERR

      
using processes as filehandles
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  can create a process-filehandle that either caputres the output
  from *or* provides input to the process.  (but not both.)

      open(WHOPROC,"who|");   # open who for reading

  - `|` on the right side of 'who' means that this `open` is not
    about a filename but a command to be started

  - all normal file I/O operators apply to this process-filehandle
    
  - can also use | to executed a command that expects input:

      open (LPR,"|lpr -Psnoriprint");
      print LPR @report;
      close(LPR);   # waits until process ends
                    # if you don't close it, the process can keep
                    # running beyond the execution of the perl script

  - to discard error messages:
      open(LPR,"|lpr -Psnoriprint >/dev/null 2>&1");

  - can do multiple commands at once:
      open(WHOPR, "ls | tail -r |");

fork  | | |
~~~~  | | |  --clones current perl process ("child")
       \|/
        |   -- return value is zero for the child; nonzero (the
        |      child's process ID) for parent (or "undef" if the
        |      system fails)
        
            exec
            ~~~~
              just like `system` except it *replaces* the current
              perl process with the shell

            wait
            ~~~~
              waits for the child to complete 

            waitpid
            ~~~~~~~
              waits for a specific child to complete
              waitpid($kidpid,0);  # cleans up after dead kid

            exit
            ~~~~
              - causes an immediate exite from the current perl process
              - takes an optional parameter, which serves as the
                numeric exit value that can be noticed by the parent
                process.  default is 0.


sending & receiving signals
~~~~~~~~~~~~~~~~~~~~~~~~~~~
  - a small number, one-bit message
  - respone to a signal is the signal's "action"