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README - Shell: Pipelines and redirections
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==============================================================================
Abstract
==============================================================================

Shell provides some of the functionality of a Unix shell; in particular the 
library supports starting simple commands, executing pipelined commands, and 
arranging redirections.  

==============================================================================
Documentation
==============================================================================

Sorry, there is no manual. The mli files describe each function in detail. 
Furthermore, the following additional information may be useful.

------------------------------------------------------------------------------
Module "Shell": The comfortable layer
------------------------------------------------------------------------------

This module is designed for the average user who does not know very much about 
the Unix process model, and just wants to start external commands. A simple 
command can be executed by a statement like 

call [ cmd "ls" [ "/dir1"; "/dir2" ]]

This statement searches "ls" in the current search PATH, launches the new 
process, and passes the arguments "/dir1" and /dir2" to the process. Nothing 
special is done with the file descriptors; the new process shares stdin, stdout 
and stderr with the current process (all other descriptors are automatically 
closed). 

A pipeline can be constructed by "call" as well. For example: 

call [ cmd "find" [ "/dir" ]; cmd "grep" [ "something" ] ]

The output of "find" is redirected to the input of "grep".

You can redirect stdin, stdout, stderr (and every other descriptor) by 
assigning them to other descriptors, or by opening and reading from or writing 
to them. In the latter case, multiple descriptors can be served parallely. For 
example: 

let s = "d\na\nc\nb\n" in
let b = Buffer.create 20 in
call ~stdin:(from_string s) ~stdout:(to_buffer b) [ cmd "sort" ]

Here, "sort" reads the contents of s and writes the result to b. 

Unlike the Unix shell, this module reports errors from all components of a 
pipeline. For example: 

call [ cmd "cat" [ "/notfound" ]; cmd "ls" [ "/notfound.too" ] ]

This will raise an exception 

Subprocess_error [ "/bin/cat", Unix.WEXITED 1; "/bin/ls", Unix.WEXITED 1 ]

There is another subtle difference to many Unix shells (and normally also the 
"system" function in libc). This module reports errors occuring between "fork" 
and "exec"; for instance if the file "fail" refers to a non-existing 
interpreter 

#! /not/found

but is executable, this special error can only be detected by the "exec" call. 
Unix shells print an error message to stderr, and return an exit code of 127 
(which is reserved for this case): 

Sys.command "fail";;
sh: ./fail: No such file or directory
- : int = 127

However, the true reason isn't reported. In contrast to this, the Shell module 
is able to pass the real error condition back to the calling program: 

call [ command "fail" ];;
Uncaught exception: Unix.Unix_error(20, "execve", "./fail").



------------------------------------------------------------------------------
Module "Shell_sys": The fundamental layer
------------------------------------------------------------------------------

The module Shell is a simple application of the functions defined in Shell_sys, 
the more fundamental module. Shell_sys allows a more fine-grained control of 
the execution of external commands; however, it is more difficult to use.

Shell_sys allows it to run processes both synchronously and asynchronously, to 
form pipelines with arbitrary topology, to create user-defined handlers serving 
file descriptors, and to control signal handling.

------------------------------------------------------------------------------
Module "Unix_exts": The missing system calls
------------------------------------------------------------------------------

This module contains some system calls missing in the Unix library distributed 
with O'Caml. 

==============================================================================
Author, Copying
==============================================================================

Shell has been written by Gerd Stolpmann [1]. You may copy it as you like, you 
may use it even for commercial purposes as long as the license conditions are 
respected, see the file LICENSE coming with the distribution. It allows almost 
everything. 


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[1]   see mailto:gerd@gerd-stolpmann.de