If I put (most of) the error message into a google search, the first hit I get (at an ibm.com site) tells me:

This issue is caused by the Sybase Open Client being unable to find the database server name. This is typically provided in the Sybase client interfaces file. ... Sybase documentation states that the default location of the interfaces file is the directory pointed to by the $SYBASE environment variable. Please ensure that the interfaces file exists in this directory.

... which makes your situation look like a good candidate for shell environment problem. Modify your script slightly so that it prints (or stores in a temp file) the contents of %ENV; look at that output, and confirm that (a) the $SYBASE variable is set to some path, and (b) that path actually points to the expected data file.