~$ sudo strace -etrace=open,close "lpr -P HP_Deskjet_6500_USB_1 -o OptionDuplex=False -o PageSize=Letter -o PageRegion=Letter -o Duplex=None -o InputSlot=Auto -o ColorModel=RGB -o MediaType=Automatic -o OutputMode=Auto -o hpInkSet=Standard"
strace: lpr -P HP_Deskjet_6500_USB_1 -o OptionDuplex=False -o PageSize=Letter -o PageRegion=Letter -o Duplex=None -o InputSlot=Auto -o ColorModel=RGB -o MediaType=Automatic -o OutputMode=Auto -o hpInkSet=Standard: 
command not found

~$ which strace
/usr/bin/strace

~$ which lpr   
/usr/bin/lpr