$ ls /bin
$ echo $?

$ ls /noexistfile
$ echo $?

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

$ f=/etc/rc.conf
$ ls $f >/dev/null 2>&1 && echo Yes || echo No

$ f=/tmp/virus.zip
$ clamscan $f || rm $f

$ test -e $f && (clamdscan $f || rm $f)