ls|xargs-t-I{} mv {} {}.old

U can make it clear to refer office instruction.Here I simplely to explain the meaning of the cmd.

1,pipe  |   Std  IO  , from left to right as args(just think so ) 

2 xargs  specify left cmd file name as tracking standard args to next command as args. Simplely U can regard filename as args to next cmd mv to complete rename all  files Name.