Care has been taken that MIDAS and the Host Operating System (DCL for VMS and
Bourne or C-shell for Unix) co--exist
smoothly and complement each other.
Migration from one environment to the other is simple:
If you are in the MIDAS environment, type BYE
to switch back to the Host System.
If you have returned to the host environment from a MIDAS session,
(indicated by the $--prompt in VMS, and by $ or % in Unix), type
GOMIDAS (in VMS) or gomidas (in Unix)
to revive MIDAS.
The status of the keywords and the
command buffer of the stopped MIDAS session are
preserved - if you want to start afresh, use INMIDAS (VMS) or
inmidas (Unix) again.
You may also use host commands directly inside MIDAS
by preceding them with ` $'. For instance,
Midas 027> $DIR (in VMS) or
Midas 027> $ls (in Unix)
will display the contents of the current directory.
Please, note, that currently this mode of operation will only invoke Bourne
shell commands in Unix, not C-shell or Kornshell commands.
To execute C-shell (or any other Shell) commands you have to insert them
in a Bourne shell script which has as the first line: 
! /bin/csh,
or: 
! /bin/ksh, etc.
Note you work on a VMS system, beware of DCL command procedures:
DCL modifies command I/O streams when executing a procedure. This causes problems for the interprocess communication inside MIDAS. When executing a DCL procedure via $ @ `procedure' the correct settings will be maintained inside MIDAS.
However assigning a symbol MIMI to the command above and then executing the DCL procedure by just typing $ MIMI will lead to disaster from which only a BYE and subsequent GOMIDAS will get you going again.
Since images, tables, etc. are standard disk files, all host commands related to
file operations can be employed. However, if a MIDAS catalog is used, care has
to be taken that the information in the catalog is not invalidated, when e.g.
renaming or deleting data files outside MIDAS (i.e. using commands of the host
file system directly).
The output from MIDAS commands can be redirected to ASCII files
enabling easy combination of MIDAS and host commands. E.g.
Midas 123> READ/DESCR myimage * >dsc.dat
will send all the output from the READ/DESCR command to the ASCII
file dsc.dat (created in your current work directory)
which can then be used by any host command. For example,
Midas 124> $EDIT dsc.dat (in VMS) or
Midas 124> $vi dsc.dat (in Unix)
Note mechanism is pretty much like the one used in Unix with the exception that there should be no space between the > and the output file name. Furthermore, this output redirection also works on VMS.
Midas 125> STATISTICS/IMAGE myimage >dsc.dat
always creates a new file dsc.dat, if you want to append data to an
existing ASCII file use
Midas 126> STATISTICS/IMAGE myimage >>dsc.dat
instead.
As you may have guessed already, there is also input redirection. E.g.
Midas 127> $ls a*.bdf >dscin.dat
Midas 128> READ/DESCR <dscin.dat
will display the standard descriptors of all Midas images with names beginning
with the letter `a' in the current directory.
Again there should be no space between < and the file name.
See also the subsection 
for more info about I/O
redirection.
On a Unix system you can connect MIDAS and Unix command via the pipe
symbol , e.g.
Midas 129> READ/DESCR myimage * | $grep NGC425
Midas 130> $ls a*.bdf | read/descr