http://en.wikipedia.org/wiki/True_and_false

$ ls /bin
$ echo $?

$ ls /noexistfile
$ echo $?

test "$1" = "" && \
{                                   
  echo usage:                       
  echo /root/ex1.sh url           
  exit 1                            
}

• //val.bmstu.ru/unix/virus.zip

$ f=/etc/passwd
$ ls $f >/dev/null 2>&1 && echo file $f exist || echo  file $f not exist

$ f=virus.zip
$ clamdscan $f || rm $f

test -e $f && (clamdscan $f >/dev/null 2>&1  || rm -v $f)