xrdls something similar to ls

Xrootd does not provide the directory listing (ls) semantics as a file system does.
The xrdls is a command line tool that looks a little bit like an ls. By defaults it
loops over all xrootd data servers and obtains a directory listing for each of them.

• -l prints a long listing
• specify dataserver if the listing from only this server is requested. Default is to
  loop over all servers

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