A modular shell

posh is a modular shell. It uses separate processes to perform the different
functions of the shell.

The core of posh is the main shell program named 'posh'. This program simply
reads command lines one after the other and prints a prompt. Commands are
executed one after the other, until an empty command is encountered.

A command is just a list of strings. posh has a special syntax for parsing lists
of strings. It would be best to give an example: the input

   asdf jkl; (abc def) (foo (bar))

is parsed as the four strings:
"asdf"
"jkl;"
"abc def"
"foo (bar)"

The point of the grouped parentheses is to escape the spaces passed as part
of the command list of a sub-command. Let me explain that. Like bash, the first
string in the command is used as the name of the program to execute. The
following strings in the command are passed as arguments. Most posh processes
expect one of their arguments to be an entire command. This enables each
process to do something to its execution environment, like duplicate a file
descriptor, then start up a child process by interpreting that command
argument.

Here is a list of the available processes:

posh
This is the main shell program. It expects no arguments.

dup
Usage: dup source dest command
dup redirects I/O by calling dup2(), so that the file descriptor dest refers
to the same file descriptor to which source refers. The following example prints
"Hello, world!" on stderr:

dup 2 1 (echo Hello, world!)


read
Usage: read file fd command
read opens a file for reading on the given file descriptor. The common use is
to open a file for reading on stdin. Example:

read file 0 cat


write
Usage: write file fd command
write opens (or creates) a file for writing on the given file descriptor.
Example:

write file 1 (echo Hello, world!)


pipe
Usage: pipe writePipe readPipe writeCommand readCommand
pipe creates a pipe between two processes. One process gets the write end of
the pipe, and the other process gets the read end. By assigning the read end to
file descriptor 0 and the write end to file descriptor 1, we can mimic the behavior of pipes in bash. The following example prints "Hello, world!" on stderr:

dup 2 1 (pipe 1 0 (echo Hello, world!) cat)


setenv
Usage: setenv variable value command
setenv sets an environment variable in the child process.

unsetenv
Usage: unsetenv variable command
unsetenv removes the value of an environment variable in the child process.


clearenv
Usage: clearenv command
clearenv clears the entire environment for the child process.


seq
Usage: seq (commands)
This process executes each of its arguments as a command, in order. Example:

seq (echo Hello,) (echo world!)


sub
Think of sub as a minimal subshell. Its purpose is to allow for command
substitution. The following example is equivalent to 'ls `pwd`' in bash:

pipe 1 0 (seq (concat (ls )) pwd) sub

sub simply runs one command line from stdin without printing any prompt. The
concat program is provided for convenience.


Compiling:
To compile the posh suite, simply run make. You will want to put the posh/bin
directory in your PATH, otherwise all posh programs have to be referred to
by prefixing them with './'.