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Editorial Team
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Asked: 2026-05-24T02:04:43+00:00

This line works: sed -r -e ‘s/^([^#a-z]+)localhost/\1hostname.domain hostname localhost/’ /etc/hosts But adding the itty

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This line works:

sed -r -e 's/^([^#a-z]+)localhost/\1hostname.domain hostname localhost/' /etc/hosts

But adding the itty option "i":

sed -ir -e 's/^([^#a-z]+)localhost/\1hostname.domain hostname localhost/' /etc/hosts

Results in:

sed: -e expression #1, char 60: invalid reference \1 on `s' command's RHS

Can someone tell me what’s going on??

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1 Answer

  1. Editorial Team

    You’ve turned off the -r (extended syntax) option, because what you append to -i isn’t more options, but an optional backup suffix. From the manpage:

    -i[SUFFIX], --in-place[=SUFFIX]

   edit files in place (makes backup if extension supplied)

    So just separate them:

    sed -i -r -e 's/^([^#a-z]+)localhost/\1hostname.domain hostname localhost/' /etc/hosts
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