LinuxQuestions.org - [SOLVED] shellscripting: pipes and output redirection

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shellscripting: pipes and output redirection

I'm puzzled.

Please see these simple commands:

Code:

# dbus-monitor --system >> /data/eject.txt

This one works as expected ... dbus-monitor never terminates and whenever it outputs new lines, they are appended to "/data/eject.txt".

Code:

# dbus-monitor --system | grep "Eject"

This one does work too. Any line containing the string "Eject" appears immediately in my shell window.

Code:

dbus-monitor --system | grep "Eject" >> /data/eject.txt

This one does not work as expected. To be more precisely, nothing happens. Nothing gets written to "/data/eject.txt", nothing gets written on the screen.

So I tried

Code:

dbus-monitor --system | grep "Eject" | tee -a /data/eject.txt

Same result, nothing happens ... neither in "/data/eject.txt" nor on my screen.

As I said ... I am puzzled.

I suspect, I missed something elementary here about pipes and redirected output ...

hmm,
looks OK.

first of all
check permissions, check disk usage.

Try to add the option --line-buffered to grep: it forces the output to flush one line at a time. Otherwise the streamed output is flushed after a long time/size and you can't see it in real time.

Quote:

Originally Posted by colucix (Post 4073636)

add the option --line-buffered to grep

Yeah, this did do the trick!

Thanks for the exceptional fast help, especially as I would never have suspected good old grep to be the culprit.

And my trust in pipes and redirection is restored :-)