Question

2

Upstart: pipe the output of multiple commands

rated 0 times [ 2] [ 0] / answers: 1 / hits: 774 / 2 Years ago, tue, november 15, 2022, 11:11:59

I try to detect if there is a network link (cable plugged/unplugged) in an upstart script (Ubuntu 12.04). Here the relevant part of my script (which didndt work):

script

if  [ /sbin/ethtool eth0 | /bin/grep "Link detected: yes" > /dev/null ] ; then

    exec prog1

else

    exec prog2

fi



end script

(try to start prog1 if there is a link otherwise prog2.) How to fix that?

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kroapathe

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answered 2 Years ago velelf · Accepted Answer

The correct way to do this is to use

if /sbin/ethtool eth0 | /bin/grep -q "Link detected: yes" ; then

    exec prog1

else

    exec prog2

fi

The -q argument to grep will discard stdout, and the if statement checks the status of the command it runs. [ /sbin/ethtool eth0 | /bin/grep -q "Link detected: yes" ] is not a valid command, because [ is actually a program that accepts arguments like grep. So [ cannot understand /sbin/ethtool eth0 | /bin/grep -q "Link detected: yes" and it fails.