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    Return the unit number of a FITS file positioned at specified extension


    The FITS file will be ready to be read at the beginning of the
    specified extension. Either an extension number or extension name
    can be specified. Called by headfits.pro, mrdfits.pro
    Modified in March 2009 to set the /SWAP_IF_LITTLE_ENDIAN keyword
    when opening a file, and **may not be compatible with earlier versions**

Calling Sequence

                      UNIXPIPE=, ERRMSG= , EXTNUM= , UNIT=, /SILENT
                        /FPACK, /NO_FPACK

Input Parameters

    FILE = FITS file name, scalar string. If an empty string is supplied
              then the user will be prompted for the file name. The user
              will also be prompted if a wild card is supplied, and more than
              one file matches the wildcard.
    EXT_NO_OR_NAME = Either the extension to be moved to (scalar
              nonnegative integer) or the name of the extension to read
              (scalar string)


    Unit number of file or -1 if an error is detected.

Optional Input Keyword Parameter

    COMPRESS - If this keyword is set and non-zero, then then treat
                the file as compressed. If 1 assume a gzipped file.
                and use IDLs internal decompression facility. For Unix
                compressed or bzip2 compressed files spawn off a process to
                decompress and use its output as the FITS stream. If the
                keyword is not 1, then use its value as a string giving the
                command needed for decompression.
    /FPACK - Signal that the file is compressed with the FPACK software.
              http://heasarc.gsfc.nasa.gov/fitsio/fpack/ ) By default,
              (FXPOSIT will assume that if the file name extension ends in
              .fz that it is fpack compressed.) The FPACK software must
              be installed on the system
    /NO_FPACK - The unit will only be used to read the FITS header. In
                that case FPACK compressed files need not be uncompressed.
      LUNIT - Integer giving the file unit number. Use this keyword if
                you want to override the default use of GET_LUN to obtain
                a unit number.
    /READONLY - If this keyword is set and non-zero, then OPENR rather
                than OPENU will be used to open the FITS file. Note that
                compressed files are always set to /READONLY
    /SILENT If set, then suppress any messages about invalid characters
                in the FITS file.

Optional Output Keywords

      EXTNUM - Nonnegative integer give the extension number actually read
              Useful only if the extension was specified by name.
      ERRMSG = If this keyword is present, then any error messages will be
                returned to the user in this parameter rather than
                depending on the MESSAGE routine in IDL. If no errors are
                encountered, then a null string is returned.
      UNIXPIPE - If set to 1, then the FITS file was opened with a UNIX pipe
                rather than with the OPENR command. This is only required
                when reading a FPACK, bzip or Unix compressed file. Note
                that automatic byteswapping cannnot be set for a Unix pipe,
                since the SWAP_IF_LITTLE_ENDIAN keyword is only available for the
                OPEN command, and it is the responsibility of the calling
                routine to perform the byteswapping.

Side Effects

      Opens and returns a file unit.


      Open the appropriate file, or spawn a command and intercept
      the output.
      Call FXMOVE to get to the appropriate extension.

Procedure Calls


Modification History

      Derived from William Thompson's FXFINDEND routine.
      Modified by T.McGlynn, 5-October-1994.
      Modified by T.McGlynn, 25-Feb-1995 to handle compressed
          files. Pipes cannot be accessed using FXHREAD so
          MRD_HREAD was written.
      W. Landsman 23-Apr-1997 Force the /bin/sh shell when uncompressing
      T. McGlynn 03-June-1999 Use /noshell option to get rid of processes left by spawn.
                                  Use findfile to retain ability to use wildcards
      W. Landsman 03-Aug-1999 Use EXPAND_TILDE under Unix to find file
      T. McGlynn 04-Apr-2000 Put reading code into FXMOVE,
                                  additional support for compression from D.Palmer.
      W. Landsman/D.Zarro 04-Jul-2000 Added test for !VERSION.OS EQ 'Win32' (WinNT)
      W. Landsman 12-Dec-2000 Added /SILENT keyword
      W. Landsman April 2002 Use FILE_SEARCH for V5.5 or later
      W. Landsman Feb 2004 Assume since V5.3 (OPENR,/COMPRESS available)
      W. Landsman,W. Thompson, 2-Mar-2004, Add support for BZIP2
      W. Landsman Don't leave open file if an error occurs
      W. Landsman Sep 2004 Treat FTZ extension as gzip compressed
      W. Landsman Feb 2006 Removed leading spaces (prior to V5.5)
      W. Landsman Nov 2006 Allow specification of extension name
                                  Added EXTNUM, ERRMSG keywords
      W. Landsman/N.Piskunov Dec 2007 Added LUNIT keyword
      W. Landsman Mar 2009 OPEN with /SWAP_IF_LITTLE_ENDIAN
                                  Added UNIXPIPE output keyword
      N. Rich May 2009 Check if filename is an empty string
      W. Landsman May 2009 Support FPACK compressed files
                                  Added /FPACK, /HEADERONLY keywords
      W.Landsman July 2009 Deprecated /HEADERONLY add /NO_FPACK
      W.Landsman July 2011 Check for SIMPLE in first 8 chars
              Use gunzip to decompress Unix. Z file since compress utility
              often not installed anymore)
      W. Landsman October 2012 Add .fz extension if /FPACK set
      W. Landsman July 2013 More diagnostics if file not found

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