Writing Shell Scripts

As Miriad commands can be invoked directly from the command line, scripts and command procedures to run a sequence of Miriad commands can be developed using the normal host's facilities. This is a somewhat advanced topic - you will probably want to be familiar with the shell scripts and Miriad before you attempt to develop your own script.

There are numerous books written of shell programming or the like - a manual like this cannot be expected to cover the subject in the depth that these books go into. Instead a simple annotated example will be given using the C-Shell commonly used on UNIX systems. To aid description, line numbers are given on the left side of each line (these line numbers are not part of the shell script).

  1:  #!/bin/csh
  2:
  3:  # Delete any datasets called "multi.uv".
  4:
  5:  rm -rf multi.uv
  6:
  7:  fits in=MULTI.UV op=uvin out=multi.uv
  8:
  9:  foreach srcnam ( 1934-638 0823-500 vela )
 10:    uvaver vis=multi.uv "select=source(${srcnam})" out=${srcnam}.uv
 11:  end
 12:
 13:  mfcal vis=1934-638.uv interval=10 refant=3
 14:  gpcal vis=1934-638.uv interval=10 options=xyvary refant=3
 15:  gpcopy vis=1934-638.uv out=0823-500.uv
 16:
 17:  gpcal vis=0823-500.uv interval=10 \
 18:                 options=nopol,xyvary,qusolve refant=3
 19:
 20:  gpboot vis=0834-500.uv cal=1934-638.uv
 21:  uvplt vis=0823-500 stokes=i,q,u,v axis=real,imag device=0823.ps/ps
 22:
 23:  lpr 0823.ps

• Line 1: C-shell scripts must always start with the rather cryptic #!/bin/csh. This allows the system to determine the appropriate interpreter for the script (i.e. the C-shell).
• Line 3: Comments are introduced by a hash character.
• Line 5: Delete a Miriad dataset called multi.uv. The r flag (recursive) is needed to delete a directory (which is how Miriad stores a dataset). The f (force) flags causes rm to work in non-interactive mode.
• Line 7: Execute the Miriad task fits. This reads a visibility FITS disk file (MULTI.UV), and saves it as a Miriad dataset (multi.uv).
• Lines 9 and 11: This is the C-shell's equivalent of a DO-loop. The values listed inside the brackets (source names in this instance) are progressively assigned to the control variable of the loop (srcnam here). The net result is that this loop will be executed three times, with srcnam being successively set to 1934-638, 0823-500 and vela. The ``things'' inside the brackets are separate by spaces - the C-shell generally likes spaces to separate different components of a command.
• Line 10: The body of the loop. The uvaver command will be executed three times. The shell substitutes the string ${srcnam} with the current value of the srcnam variable. Thus the select parameter being successively set with source(1934-638), source(0823-500) and source(vela) on the three times through the loop. The output dataset of the command also changes. Note that the select parameter is quoted ("select=... "). This is as brackets (()) are special to the shell. Quoting them prevents this interpretation. Note however that the ${srcnam} is still treated as special, even though it is in quotes (software does not have to be consistent!).
• Lines 13-15: Execute various Miriad commands on data-sets produced by line 10.
• Lines 17-18: Yet another command. However this one was too long to fit on one line. In the C-shell, a line can be continued onto the next by ending it with a backslash.
• Lines 20 and 21: More of the same. Generate a postscript output file, 0823.ps, using task uvplt.
• Line 23: Spool this postscript file to the laser printer.

On UNIX systems, after having developed a script, you will need to change the ``file mode'' of the script to indicate that the script is executable. For example, to mark the shell script calibrate as executable, use the UNIX command

     % chmod +x calibrate