One other item: You don't really need the cat statement, as grep can read from the file just as easily as it can read from the pipe:

grep eth0 /proc/net/dev | awk '{print $2}' | sed 's/eth0://g'
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then, of course, you really don't need the grep, either, as awk knows how to run a block only when it finds a regular expression:

awk '/eth0/ {print $2}' /proc/net/dev | sed 's/eth0://g'
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After that, we don't really need the sed statement either, as awk can also do a regular expression substitution:

awk '/eth0/ {print gensub("eth0","","g",$2); }' /proc/net/dev'
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Of course, since this is a perl site, you might just want to use perl...

...roboticus