introduction

Usage: SQLPLUS [ [<option>] [<logon>] [<start>] ]
where <option> ::= -H | -V | [ [-L] [-M <o>] [-R <n>] [-S] ]
      <logon>  ::= <username>[/<password>][@<connect_string>] | / | /NOLOG
      <start>  ::= @<URI>|<filename>[.<ext>] [<parameter> ...]
        "-H" displays the SQL*Plus version banner and usage syntax
        "-V" displays the SQL*Plus version banner
        "-L" attempts log on just once
        "-M <o>" uses HTML markup options <o>
        "-R <n>" uses restricted mode <n>
        "-S" uses silent mode

examples

1. [oracle@www oracle]$ sqlplus
   
   SQL*Plus: Release 9.2.0.1.0 - Production on Wed Feb 19 22:32:19 2003
   
   Copyright (c) 1982, 2002, Oracle Corporation.  All rights reserved.
   
   Enter user-name: scott
   Enter password:
   ERROR:
   ORA-01034: ORACLE not available
   ORA-27101: shared memory realm does not exist
   Linux Error: 2: No such file or directory
   
   

   the above error will occur whenever the wrong database is specified or the wrong password/user combination is given. Not a very helpful error description, it tends to lead one to assume there's a database problem.