Test

debian:/home/sander# ifdown wlan0 && ltup wlan0
There is already a pid file /var/run/dhclient.wlan0.pid with pid 4556
killed old client process, removed PID file
Internet Systems Consortium DHCP Client V3.1.1
Copyright 2004-2008 Internet Systems Consortium.
All rights reserved.
For info, please visit http://www.isc.org/sw/dhcp/

Listening on LPF/wlan0/00:24:2b:42:de:57
Sending on   LPF/wlan0/00:24:2b:42:de:57
Sending on   Socket/fallback
bash: ltup: command not found
debian:/home/sander# iwconfig
lo        no wireless extensions.

eth0      no wireless extensions.

wlan0     IEEE 802.11bg  Mode:Managed  Access Point: Not-Associated
          Tx-Power=off
          Retry  long limit:7   RTS thr:off   Fragment thr:off
          Encryption key:off
          Power Management:off

debian:/home/sander# ifconfig -a
eth0      Link encap:Ethernet  HWaddr 00:23:5a:25:26:c3
          BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)
          Interrupt:30 Base address:0xa000

lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:716 errors:0 dropped:0 overruns:0 frame:0
          TX packets:716 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:52272 (51.0 KiB)  TX bytes:52272 (51.0 KiB)

wlan0     Link encap:Ethernet  HWaddr 00:24:2b:42:de:57
          BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

debian:/home/sander#