grep -v not working when piping from iwconfig but works when piping from cat on a file

Question

I am running iwconfig | grep -v "no wireless extensions" but can't get the -v option to work as expected. I want to exclude lines including "no wireless extensions", i.e., I want to display only the active/working wireless interface or whatever this should be called.

In the beginning I thought perhaps the command outputs to a stream different than the one piped to grep, so I tried cat myFile | grep -v myExclusionPattern. This works as expected, so I concluded -v does what I expect it does. I then tried iwconfig | grep "no wireless extensions" and again the result is the expected - meaning the output of iwconfig is what is piped to grep. So I am left with the question why specifically -v is not working when piping the results of iwconfig to grep.

Here is my output to iwconfig:

enp4s0    no wireless extensions.

docker0   no wireless extensions.

lo        no wireless extensions.

wlp5s0    IEEE 802.11  ESSID:"myEssid"  
          Mode:Managed  Frequency:5.24 GHz  Access Point: 74:83:C2:75:86:2A   
          Bit Rate=6 Mb/s   Tx-Power=30 dBm   
          Retry short limit:7   RTS thr:off   Fragment thr:off
          Power Management:on
          Link Quality=59/70  Signal level=-51 dBm  
          Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:0
          Tx excessive retries:0  Invalid misc:747   Missed beacon:0

Summarizing:

1. grep -v works for me when I pipe to grep the output of a file with cat.
2. I can't reproduce the same behavior as in (1) when piping the output of iwconfig to grep.

I read the following questions on grep -v, but am not able to find the answer to the above in any of them:

• Why "grep -q -v" only works with single line input?
• Pipe find into grep -v
• grep -v without output?
• grep -v -f alternative

Answer 1

iwconfig outputs to both standard output and to standard error, depending on whether it found or did not find any wireless extensions for an interface. Piping only affects standard output.

Example removing the output sent to the standard error stream (only shows interfaces that have wireless extensions):

$ /usr/sbin/iwconfig 2>/dev/null
wlp4s0    IEEE 802.11  ESSID:off/any
          Mode:Managed  Access Point: Not-Associated   Tx-Power=22 dBm
          Retry short limit:7   RTS thr:off   Fragment thr:off
          Power Management:on

Example removing the output sent to the standard output stream (only shows interfaces where iwconfig failed to find wireless extensions):

$ /usr/sbin/iwconfig >/dev/null
lo        no wireless extensions.

enp0s31f6  no wireless extensions.

wwan0     no wireless extensions.

docker0   no wireless extensions.

br-ca679f9ee354  no wireless extensions.

veth232fd86  no wireless extensions.

vboxnet0  no wireless extensions.

Answer 2

I see no one mentioned this alternative, but I would consider using

iwconfig 2>&1 | grep -v "no wireless extensions" a solution as well (perhaps even more elegant ?)

Now both errors and regular output are sent to the same file descriptor (standard output), which is what is piped to grep, which can finally filter the whole data.