scripting insertion of variable to config file

Question

As part of a post-preseeding script, I want to disable ipv6.

In /etc/default/grub I want to change this line:

GRUB_CMDLINE_LINX=""

so that it reads:

GRUB_CMDLINE_LINUX="ipv6.disable=1"

Which can be done with:

sed -i "s/GRUB_CMDLINE_LINUX=\"\"/GRUB_CMDLINE_LINUX=\"ipv6.disable=1\"/" /etc/default/grub

But sed won't match if GRUB_CMDLINE_LINUX already has arguments in it.

How can I add the parameter while preserving any existing arguments (if any) ?

So if empty, fill with "ipv6...". Otherwise, keep the value? — fedorqui
– fedorqui, Commented Jul 15, 2013 at 15:14
Maybe you should try GRUB_CMDLINE_LINUX instead of GRUB_CMD_LINE_LINUX (delete the underscore between CMD and LINE). — uzsolt
– uzsolt, Commented Jul 15, 2013 at 15:15
thanks uzolt, just a typo. @fedorqui, no, if there's a value already there, keep it and add ipv6.disable=1 inside the quotes, if there's no value, just add it. — jon
– jon, Commented Jul 15, 2013 at 15:17

jon · Accepted Answer · 2013-07-15 15:37:58Z

3

The problem is that you are nuking the entire line. Try

s/GRUB_CMD_LINE_LINUX=\"/GRUB_CMD_LINE_LINUX=\"ipv6.disable=1XXX/

to insert your assignment to the front and replace XXX with the character used as a separator. Perhaps ;. Alternatively

s/(GRUB_CMD_LINE_LINUX=[^\"]*)\"$/\1XXXipv6.disable=1"/

to insert at the end of the line. ie after all existing params. You may have to escape the parenthesis and/or " in the second version.

Solved with:

sed "s/GRUB_CMDLINE_LINUX=\"\(.*\)\"/GRUB_CMDLINE_LINUX=\"\1 ipv6.disable=1\"/" /etc/default/grub

jon

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answered Jul 15, 2013 at 15:17

user1666959

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