I remember there was a trick to check the existence of a disk file (by using WebFOCUS Fex Code).
I am not able to recollect what was the exact syntax .... does anybody know ?

in a windows environment you can
CMD STATE D:\IBI\APPS\..\filename.ext
-RUN
-IF &RETCODE NE 0 THEN you got no file.
also there was an interesting post by jg
and he says

!IF EXIST C:\ibi\apps\baseapp\NEWXHT.XLS (DEL C:\ibi\apps\baseapp\NEWXHT.XLS /F)
 

here's the link to that code

or you can use the old DOS STATE

-* Note: zero is a good return code
DOS STATE \\IMPRS01\APPS\PMS\DATA\FTFLAG.DAT
-IF &RETCODE NE 0 GOTO NOFLAG;

then add appropriate labels and loop.

In the Unix/Linux environment, just use 'STATE' without the preceding CMS/DOS/etc. qualifier.
STATE filename.txt
-RUN
-IF &RETCODE EQ 0 GOTO FILEOK ;
-TYPE Error message
-GOTO ERREXIT

-FILEOK
... Rest of fex here

I am on Unix/Solaris. 'STATE' worked out for me. Thx.

Alternatively,

-UNIX ls /path/to/your/file 1>/dev/null 2>/dev/null
-IF &EXITRC NE 0 THEN GOTO WHATEVER;