LSNRCTL> set current_listener <listener name>
LSNRCTL> set log_file <sid name>.log

[Oracle@jumper admin]$ lsnrctl
LSNRCTL for linux: Version 9.2.0.4.0 - PRodUCtion on 10-NOV-2006 16:54:16
Copyright (c) 1991, 2002, Oracle Corporation.  All rights reserved.
Welcome to LSNRCTL, type "help" for information.
LSNRCTL> set current_listener LISTENER1
Current Listener is LISTENER1
LSNRCTL> set log_file
Parameter Value: a.log
Connecting to (DESCRipTION=(ADDRESS=(PROTOCOL=IPC)(KEY=EXTPROC)))
LISTENER1 parameter "log_file" set to a.log
The command completed successfully
LSNRCTL> status
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=IPC)(KEY=EXTPROC)))
STATUS of the LISTENER
------------------------
Alias                     LISTENER1
Version                   TNSLSNR for Linux: Version 9.2.0.4.0 - Production
Start Date                10-NOV-2006 16:54:12
Uptime                    0 days 0 hr. 2 min. 6 sec
Trace Level               off
Security                  OFF
SNMP                      OFF
Listener Parameter File   /opt/oracle/product/9.2.0/network/admin/listener.ora
Listener Log File         /opt/oracle/product/9.2.0/network/log/a.log
Listening Endpoints Summary...
  (DESCRIPTION=(ADDRESS=(PROTOCOL=ipc)(KEY=EXTPROC)))
  (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=172.16.33.11)(PORT=1521)))
Services Summary...
Service "PLSExtProc" has 1 instance(s).
  Instance "PLSExtProc", status UNKNOWN, has 1 handler(s) for this service...
Service "conner" has 1 instance(s).
  Instance "conner", status UNKNOWN, has 1 handler(s) for this service...
Service "eygle" has 1 instance(s).
  Instance "eygle", status UNKNOWN, has 1 handler(s) for this service...
The command completed successfully
LSNRCTL>

LSNRCTL> save_config
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=IPC)(KEY=EXTPROC)))
Saved LISTENER1 configuration parameters.
Listener Parameter File /opt/oracle/product/9.2.0/network/admin/listener.ora
Old Parameter File /opt/oracle/product/9.2.0/network/admin/listener.bak
The command completed successfully

[oracle@jumper oracle]$ tail -5 /opt/oracle/product/9.2.0/network/admin/listener.ora
#----ADDED BY TNSLSNR 14-NOV-2006 16:39:12---
LOG_FILE_LISTENER1 = a.log
#--------------------------------------------