Grepping grep output fails

Posted by viraptor on Super User See other posts from Super User or by viraptor
Published on 2010-02-05T17:46:17Z Indexed on 2010/04/09 15:44 UTC
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Filed under:

linux

|

grep

|

pipe

I'm trying to grep the output of ngrep. Unfortunately when I add another grep to the pipeline, I get no output at all. It can be some other command too - cat / grep / tee - everything breaks the chain. Example:

# this works:
$ ngrep -l -q -T -Wbyline -d any udp and port 5060 |
    egrep -B1 '^SIP/2.0 180'
--
U +1.469535 xxx:5060 -> xxx:5060
SIP/2.0 180 Ringing.
--
U +0.001384 xxx:5060 -> xxx:2048
SIP/2.0 180 Ringing.

but

#these don't:
$ ngrep -l -q -T -Wbyline -d any udp and port 5060 |
    egrep -B1 '^SIP/2.0 180' | egrep '^U'
$ ngrep -l -q -T -Wbyline -d any udp and port 5060 |
    egrep -B1 '^SIP/2.0 180' | cat
$ ngrep -l -q -T -Wbyline -d any udp and port 5060 |
    egrep -B1 '^SIP/2.0 180' | tee test

If I use cat somefile instead of ngrep at the start, everything works as expected. Any ideas what could go wrong here?

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