Dfyjkt

I'm in the process of hardening some of our systems. As part of that hardening process, I need to update a few entries in the /etc/fstab to limit the capabilities of some of the various partitions.

With that said, I would like be able to use a sed in-line replace to update the rows. Below is a snippet from the current /etc/fstab:

# /etc/fstab
# Created by anaconda on Wed Feb 21 09:37:23 2018
/dev/mapper/vg1-lv_root / ext4 defaults 1 1
/dev/mapper/vg1-lv_home /home ext4 defaults 1 2
tmpfs /dev/shm tmpfs defaults 0 0

After the sed command is run I would like the file to look like the following:

# /etc/fstab
# Created by anaconda on Wed Feb 21 09:37:23 2018
/dev/mapper/vg1-lv_root / ext4 defaults 1 1
/dev/mapper/vg1-lv_home /home ext4 defaults,nodev 1 2
tmpfs /dev/shm tmpfs defaults 0 0

Basically, I need to add "nodev" to all the rows that are ext[2-4], that aren't the root partition.

The sed command that I put together comes close to doing this, but for whatever reason, I can't get the regex to not match the "/" partition, so it always updates that row also.

sed '/^[^#].*ext[2-4]/s/defaults/defaults,nodev/g' /etc/fstab

I would like to key off of the "/" surrounded by spaces, not the vg1-lv_root. The following works, but I don't like the solution because it's clunky:

sed '/^[^#].*ext[2-4]/s/defaults/defaults,nodev/g' /etc/fstab | sed '/^[^#].*root.*ext[2-4]/s/defaults,nodev/defaults/' > /etc/fstab

You could use awk to add the logic to add the string and column to reformat the final output file. Assuming you have write permissions to the /etc/ and /tmp/ folders

tempfile=$(mktemp /tmp/tmpfile.XXXXXXXX)

This would create the temporary file in the /tmp/ path in which you can write the awk output to and re-direct that back to the original file

awk '$3 ~ "ext[2-4]" $4=$4",nodev" 1 ' /etc/fstab | column -t > "$tempfile" && mv -- "$tempfile" /etc/fstab

The column -t part is just redundant and needed to look the output file more readable, rather to make it disordered and clunky.

Here's a simpler sed approach:

$ sed -E 's|(s/S+s+ext[2-4]s+defaults)|1,nodev|' fstab 
# /etc/fstab
# Created by anaconda on Wed Feb 21 09:37:23 2018
/dev/mapper/vg1-lv_root / ext4 defaults 1 1
/dev/mapper/vg1-lv_home /home ext4 defaults,nodev 1 2
tmpfs /dev/shm tmpfs defaults 0 0

The trick is to look for whitespace followed by a / and one or more non-whitespace characters (s/S+), then ext[2-4] but only if preceded by whitespace (s+ext[2-4]), more whitespace and defaults. That should only match the cases you are interested in. So if it does match, replace the entire match with itself plus nodev: 1,nodev.

I am not sure how portable this is, however. The -E for extended regular expressions is supported by many sed implementations, but it isn't POSIX. For a more portable approach, you can try the same idea in Perl:

$ perl -pe 's|(s/S+s+ext[2-4]s+defaults)|1,nodev|' fstab 
# /etc/fstab
# Created by anaconda on Wed Feb 21 09:37:23 2018
/dev/mapper/vg1-lv_root / ext4 defaults 1 1
/dev/mapper/vg1-lv_home /home ext4 defaults,nodev 1 2
tmpfs /dev/shm tmpfs defaults 0 0

In both cases, to edit the file in place, use -i:

perl -i -pe 's|(s/S+s+ext[2-4]s+defaults)|1,nodev|' fstab 
sed -E 's|(s/S+s+ext[2-4]s+defaults)|1,nodev|' fstab 

Or, for BSD or OSX sed:

sed -i '' -E 's|(s/S+s+ext[2-4]s+defaults)|1,nodev|' fstab 

Note that the above assume that the defaults option will either be the only one or, at least, the last one. They will fail if you have something like nodev,defaults for example.

Of course, ten minutes after posting the question, I was finally able to get it to do what I want with the following:

sed -r '/^[^#].*[ t]+/[^[:space:]].*ext[2-4]/s/defaults/defaults,nodev/g' /etc/fstab

If someone has a cleaner or more foolproof answer, I'd love to hear it. Thanks!

Excluding matches are difficult to maintain. It's safer to only match the partition you want. And, for only matching one line, the trailing 'g' isn't necessary.

sed -r 's#(/homes.*defaults)s#1,nodev #' /etc/fstab