Add some more to readme

README.md

@@ -3,4 +3,68 @@
 Jarvis-util is a library which contains various utilities to aid with
 creating shell scripts within Python. This library contains wrappers
 for executing shell commands locally, SSH, SCP, MPI, argument parsing, 
-and various other random utilities.
+and various other random utilities.
+
+## Executing a program
+
+The following code will execute a command on the local machine.
+The output will NOT be collected into an in-memory buffer.
+The output will be printed to the terminal as it occurs.
+
+```python
+from jarvis_util.shell.exec import Exec
+from jarvis_util.shell.local_exec import LocalExecInfo 
+
+node = Exec('echo hello', LocalExecInfo(collect_output=False))
+```
+
+Programs can also be executed asynchronously:
+```python
+from jarvis_util.shell.exec import Exec
+from jarvis_util.shell.local_exec import LocalExecInfo 
+
+node = Exec('echo hello', LocalExecInfo(collect_output=False,
+                                        exec_async=True))
+node.wait()
+```
+
+This is useful if you have a program which runs using a daemon mode.
+
+## Executing an MPI program
+
+The following code will execute the "hostname" command on the local
+machine 24 times using MPI.
+
+```python
+from jarvis_util.shell.exec import Exec
+from jarvis_util.shell.mpi_exec import MpiExecInfo 
+
+node = Exec('hostname', MpiExecInfo(hostfile=None,
+                                    nprocs=24,
+                                    ppn=None,
+                                    collect_output=False))
+```
+
+## Executing an SSH program
+
+The following code will execute the "hostname" command on all machines
+in the hostfile
+
+```python
+from jarvis_util.shell.exec import Exec
+from jarvis_util.shell.pssh_exec import PsshExecInfo 
+
+node = Exec('hostname', PsshExecInfo(hostfile="/tmp/hostfile.txt",
+                                     collect_output=False))
+```
+
+## The contents of a hostfile
+
+A hostfile can have the following syntax:
+```
+ares-comp-01
+ares-comp-[02-04]
+ares-comp-[05-09,11,12-14]-40g
+```
+
+These will be expanded internally by PSSH and MPI.