Re: [ferret_users] Check file exist before saving it

["Yu, Hao-Cheng" <danish.url@xxxxxxxxx>]

Hi,

thanks for all reply!

Actually I try to write a ferret script to get file from OPeNDAP and saving it.

And hope this script can check file existence, 

also can use "do while loop" to make sure all downloads can be completed!

(Because download sometimes stop due to ferret happened "killed" signal!)

Maybe try to use "spawn" to call shell-script will be good idea!

Anyway, thanks for the idea and scripts.

Regards, dan 

2011/10/26 Ansley Manke <ansley.b.manke@xxxxxxxx>

Hi all
Thanks for the replies.  This sounds like a good topic for an FAQ.  There's one more way that occurs to me, the TEST_OPENDAP function.  It returns the flag that's sent back from the netCDF library on a request to open the dataset.

If the flag is zero, the file exists and can be correctly opened.

yes? list test_opendap("z_wide_variation.nc")
             VARIABLE : TEST_OPENDAP("z_wide_variation.nc")
             X        : 1
          0.0000

If the file exists but is not a valid netCDF file or opendap url, you get various other codes depending on the error (invalid remote  url, not-a-nc file, etc.)

yes? list test_opendap("ferret.jnl")
             VARIABLE : TEST_OPENDAP("ferret.jnl")
             X        : 1
         -51.00

If the file does not exist, you 'll get -2.

yes? list test_opendap("this_file_doesnt_exist.nc")
             VARIABLE : TEST_OPENDAP("this_file_doesnt_exist.nc")
             X        : 1
          2.000

So you could test for the result of the function being zero.

Ansley

On 10/25/2011 2:19 AM, Ryo Furue wrote:

Hi Dan,

| I would like to check file existence before saving it.
|
You can use shell commands for that:

  let ret = {spawn:"[ -e CTL.nc ]; echo $?"}
  if `ret[i=1] eq "0"` then
    ! . . . file exists . . .
  else
    ! . . . file does not exist . . .
  endif

The shell command "[ -e file ]" returns 0 if the file exists
or a different value if it does not.

The command "echo $?" just prints the return value of the
preceding command to the standard output.

The SPAWN _expression_ of Ferret catches the standard output of the
shell command(s).

There may be a Ferret-proper way.

Regards,
Ryo