COMMAND(2)                                             COMMAND(2)

          command - command interface

          include "sh.m";
          cmd := load Command path;

          PATH: con "/dis/sh.dis";
          init: fn(ctxt: ref Draw->Context, args: list of string);

          Command defines the module interface for programs started by
          the Inferno shells sh(1) and mash(1), and the window manager
          wm(1). Applications to be run as commands must adhere to it,
          and any application wishing to start another command may use

          Every command must have an init function with the signature
          shown above.  Note that Limbo rules allow a module to expose
          a larger interface for use by other applications (see for
          instance sh(2)); provided it includes the form of init shown
          above, the module can also be invoked as a command.

          Ctxt provides the graphics context for a windowing applica-
          tion, which typically passes it to wmlib(2) (eg, to
          titlebar) or directly to tk(2). It is nil for commands
          started by the shells.

          The arguments to the command are passed as a simple list of
          strings, args.  By convention, the name of the command or
          the name of its .dis file heads the list.

          PATH names the file containing sh(1), (on small systems,
          this might actually name an instance of tiny(1)) but usually
          the path name of another command will be given to the Limbo
          load operator:

               include "sh.m";

               cmd := load Command "/dis/date.dis";
               cmd->init(nil, "date" :: nil);

          In practice more care must be taken when invoking programs.
          In the example above, the current process executes the body
          of cmd->init and if that executes exit, raises an exception,
          or otherwise modifies the state of the process, the caller
          is affected.  The following is more prudent:

               child(file: string, args: list of string, pidc: chan of int)

     Page 1                       Plan 9             (printed 12/2/22)

     COMMAND(2)                                             COMMAND(2)

                     pidc <-= sys->pctl(Sys->NEWFD|Sys->FORKNS|Sys->NEWPGRP,
                           list of {0, 1, 2});
                     cmd := load Command file;
                     if(cmd == nil){
                           sys->print("can't load %s: %r\n", file);
                     cmd->init(nil, args);
                     pidc := chan of int;
                     spawn child(disfile, args, pidc);
                     pid := <-pidc;

          A new child process runs the command only after using sys-
          pctl to insulate the caller from untoward changes to its
          environment.  Note the idiomatic use of a channel to return
          the child's process ID to its parent, which can then if
          desired use the wait file of prog(3) to watch over the child
          and wait for its demise.  or use the ctl file of prog(3) to
          dispose of it.  Furthermore, any state shared between parent
          and child can safely be accessed by the child before it
          sends the ID because the parent is blocked on the receive.

          mash(1), sh(1), wm(1), sh(2), sys-pctl(2)

     Page 2                       Plan 9             (printed 12/2/22)