NCL: system interactions

• Setting NCL variables via command line arguments.
• Writing shell scripts to set NCL variables via command line arguments and execute the NCL script.
• Writing shell scripts to set NCL variables via environment variables and execute the NCL script.
• Using NCL to execute UNIX system commands.

NCL command line arguments allow you to set NCL variables and execute simple NCL code snipplets from the UNIX command line or a shell script.

As a simple example, assume you have a 3-line ncl script called myscript.ncl:

  print("year: " + year) print("month: " + month) print("title: " +
  title)

However, using command line arguments, you can set these variables when you run NCL from the UNIX command line:

   ncl year=2014 month=5 'title="This is a title"' myscript.ncl

This is similar to the above example, but now you are writing a shell script to set the NCL variables and execute the NCL script:

test_sh_script.sh (bash/ksh):

#!/bin/sh

ncl year=2014 month=10 'title="This is a title"' myscript.ncl

test_csh_script.csh (bash/ksh):

#!/bin/csh -f

ncl year=2014 month=10 'title="This is a title"' myscript.ncl

To run either shell script, make it executable first:

chmod gou+x test_sh_script.sh
./test_sh_script.sh

chmod gou+x test_csh_script.csh
./test_csh_script.csh

For full details, see the Command Line Options section in the NCL Reference Manual.

You can use UNIX environment variables to set values inside a shell script, that you can then retrieve from within an NCL script using NCL's getenv function.

Note: command line arguments (see above section) are the preferred method for doing this sort of thing.

Starting with the same 3-line script from above, You would need to add three lines to it to retrieve these environment variable settings:

myscript_env.ncl:

  year  = toint(getenv("YEAR"))
  month = toint(getenv("MONTH"))
  title = getenv("TITLE")
  print("year:  " + year) 
  print("month: " + month) 
  print("title: " + title)

You would then set these environment variables from within a csh/tcsh script or a bash/ksh script, before executing the script:

test_sh_script_env.sh (bash/ksh):

#!/bin/sh

export YEAR=2014
export MONTH=10
export TITLE="This is a title"

ncl myscript_env.ncl

test_csh_script_env.csh (csh/tcsh):

#!/bin/csh -f

setenv YEAR 2014
setenv MONTH 10
setenv TITLE "This is a title"

ncl myscript_env.ncl

chmod gou+x test_sh_script_env.sh
./test_sh_script_env.sh

chmod gou+x test_csh_script_env.csh
./test_csh_script_env.csh

Executing UNIX commands from NCL can be done via NCL's systemfunc function, and/or the system procedure. These provide users with the ability to execute shell commands or other software tools, like netCDF operators (NCO) , Climate Data Operators (CDO), convert (ImageMagick), wgrib, wget. See below for some examples.