Code:
oracle@simdes:~> cd $ORACLE_HOME
oracle@simdes:/oracle/simp> ls
Apache       arch        glibc-2.1.3-stubs.tar.gz  javavm  jsp      ocommon  owm      relnotes        slax       svrmgr
JRE          assistants  hs                        jdbc    ldap     odg      plsql    root.sh         sqlj       wwg
README.stub  bin         install                   jis     lib      oracore  precomp  root.sh.old     sqlplus    xdk
admin        dbs         install.platform          jlib    network  otrace   rdbms    setup_stubs.sh  starterdb
oracle@simdes:/oracle/simp> cd sqlplus/
oracle@simdes:/oracle/simp/sqlplus> ls
admin  demo  doc  lib  mesg
oracle@simdes:/oracle/simp/sqlplus> cdmkdir tmp
bash: cdmkdir: command not found
oracle@simdes:/oracle/simp/sqlplus> mkdir tmp
oracle@simdes:/oracle/simp/sqlplus> cd mesg/
oracle@simdes:/oracle/simp/sqlplus/mesg> ls
cpye.msb  cpyus.msb  cpyus.msg  sp1e.msb  sp1us.msb  sp1us.msg  sp2e.msb  sp2us.msb  sp2us.msg
oracle@simdes:/oracle/simp/sqlplus/mesg> ls -l
total 152
-rw-r--r--    1 oracle   oinstall     4096 mar 21 14:53 cpye.msb
-rw-r--r--    1 oracle   oinstall     4096 mar 21 14:53 cpyus.msb
-rw-r--r--    1 oracle   oinstall     1672 mar 21 14:53 cpyus.msg
-rw-r--r--    1 oracle   oinstall    11264 mar 21 14:53 sp1e.msb
-rw-r--r--    1 oracle   oinstall    10240 mar 21 14:53 sp1us.msb
-rw-r--r--    1 oracle   oinstall    15097 mar 21 14:53 sp1us.msg
-rw-r--r--    1 oracle   oinstall    29184 mar 21 14:53 sp2e.msb
-rw-r--r--    1 oracle   oinstall    25600 mar 21 14:53 sp2us.msb
-rw-r--r--    1 oracle   oinstall    40666 mar 21 14:53 sp2us.msg
oracle@simdes:/oracle/simp/sqlplus/mesg> mv sp*.* ../tmp
oracle@simdes:/oracle/simp/sqlplus/mesg> ls
cpye.msb  cpyus.msb  cpyus.msg
oracle@simdes:/oracle/simp/sqlplus/mesg> sqlplus internal
Message file sp1.msb not found
Error 6 initializing SQL*Plus

oracle@simdes:/oracle/simp/sqlplus/mesg> cp ../tmp/sp1us.msb .
oracle@simdes:/oracle/simp/sqlplus/mesg> sqlplus internal
Message file sp2.msb not found
Error 7 initializing SQL*Plus
oracle@simdes:/oracle/simp/sqlplus/mesg> cp ../tmp/sp2us.msb .
oracle@simdes:/oracle/simp/sqlplus/mesg> sqlplus internal

SQL*Plus: Release 8.1.7.0.0 - Production on Thu May 16 12:20:33 2002

(c) Copyright 2000 Oracle Corporation.  All rights reserved.


Connected to:
Oracle8i Enterprise Edition Release 8.1.7.0.1 - Production
With the Partitioning option
JServer Release 8.1.7.0.1 - Production

sys@DSIM>select sysdate from dual;

SYSDATE
---------
16-MAY-02

1 row selected.

sys@DSIM>quit
Disconnected from Oracle8i Enterprise Edition Release 8.1.7.0.1 - Production
With the Partitioning option
JServer Release 8.1.7.0.1 - Production
it works for me so I cant help, I didNOT copy msg because it is the ascii file of MSB files

Code:
oracle@simdes:/oracle/simp/sqlplus/mesg> cat sp1us.msg | more
/
/ Copyright (c) Oracle Corporation 1989, 1999. All Rights Reserved.
/
/ File          : SP1US.MSG
/ Created       : 05/22/90
/ By            : SYSTEM
/ Language      : American
/ Character set : US7ASCII
/
/ CONTENTS
/   Feedback messages for SQL*Plus
/
/ NOTES
/   The first colum is the SQL*Plus error number.
/   The second column is reserved and must (for now) be 0.
/   The remainder of the line is the error text.
/
/
00000,0, "Operation %ld succeeded."
// *Cause:
// *Action:
00001,0, "Table created."
// *Cause:
// *Action:
00002,0, "Table created."
// *Cause:
// *Action:
00003,0, "1 row created."
// *Cause:
// *Action:
00004,0, "%ld rows created."
// *Cause:
// *Action:
00005,0, "1 row selected."
// *Cause:
// *Action:
00006,0, "%ld rows selected."
// *Cause:
// *Action:
00007,0, "Cluster created."
// *Cause:
// *Action:
00008,0, "Cluster created."
// *Cause:
// *Action:
00009,0, "Cluster altered."

.............. etc etc