Question

I would like to change /etc/fstab inside a script. I want to add the aclattribute to the root partition.

One fstabline entry looks like this:

UUID=730aee20-52b7-4920-75cd-d0d995ef2445 /   ext3   errors=remount-ro 0   1

I want to change it to:

UUID=730aee20-52b7-4920-75cd-d0d995ef2445 /   ext3    acl,errors=remount-ro 0   1

I thought:
1. Search line with root partition /
2. insert acl after /

How can I do that with sed?

Answer 1

Who needs some 3rd party tool when we all have awk?

awk '$2~"^/$"{$4="acl,"$4}1' OFS="\t" /etc/fstab

Example Output

$ awk '$2~"^/$"{$4="acl,"$4}1' OFS="\t" /etc/fstab
/dev/sda2        swap             swap        defaults         0   0
/dev/sda5       /       ext4    acl,defaults    1       1
/dev/sda1        /boot            ext4        defaults         1   2
/dev/sda6        /home            ext4        defaults         1   2
/dev/sdb1        /backup          ext4        defaults         1   2
#/dev/cdrom      /mnt/cdrom       auto        noauto,owner,ro  0   0
/dev/fd0         /mnt/floppy      auto        noauto,owner     0   0
devpts           /dev/pts         devpts      gid=5,mode=620   0   0
proc             /proc            proc        defaults         0   0
tmpfs            /dev/shm         tmpfs       defaults         0   0

Explanation

1. $2~"^/$" Search the 2nd field $2 to see if it matches a forward slash by itself ^/$
2. {$4="acl,"$4} If we see a match, prepend acl to the beginning of the 4th field $4
3. }1 This is an awk shortcut which is equivalent to print $0, i.e. print the whole line (including any alterations we may have made)
4. OFS="\t" Set the Output Field Separator OFS to a tab \t. The default is space
5. /etc/fstab The file we want to use as input