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host [SQL*Plus]

host shell-command
$ windows-cmd-command
! unix-shell-command
host executes the (shell) command (such as ls, dir, rm, del and so on) and prints the output.
On Unix, host can be abbreviated with an exlamation mark (!). On Windows, host can be abbreviated with an dollar sign ($).

Restriction level

The host command won't work if the restriction level is set to at least 1:
C:>sqlplus -r 1 rene/rene

SQL*Plus: Release - Production on Thu Aug 17 21:10:39 2006

Copyright (c) 1982, 2005, Oracle.  All rights reserved.

Connected to:
Oracle Database 10g Enterprise Edition Release - Production
With the Partitioning, OLAP and Data Mining options

RENE> host dir
SP2-0738: Restricted command "host" not available