awk and sed help for script

technocp · 04-24-2010, 01:14 PM

hello friends,
I am creating a script for making a public folder in linux. The idea behind is to allow any user to write files in this folder and further each of those files can be used by any user.

for this purpose I am using acl. This first requires to set acl as mount option in /etc/fstab which I need to do with help of (awk and sed) command available in linux.

the line below is the fstab entry which I need to make change in.

UUID=e9a3db4b-d8c0-40b4-a661-131e13afea1e / ext3 relatime,errors=remount-ro 0 1

I want my script to identify this line (most probably with / which denotes the mountpoint) and then add (acl,) without brackets in options

currently I use the command as below
sed 's/realtime/acl,relatime/' /etc/fstab

but the above command is in reference with (relatime) what I am planning is that my script should be able to find the line which has / and then go to options and add acl there.

can anyone please help me for this purpose

colucix · 04-24-2010, 02:53 PM

I suggest a solution without sed or awk:

Code:

while read line
do
  mp=$(echo $line | cut -d' ' -f2)
  if [[ "$mp" == "/" ]]
  then
    op=$(echo $line | cut -d' ' -f4)
    line=${line/$op/acl,$op}
  fi
  echo "$line"
done < /etc/fstab

Indeed awk would have the advantage to use a more compact and elegant code, but there is not a simple way to preserve the original (multiple) spacing of /etc/fstab (the code above preserve it). On the other hand sed has the ability to edit the file in place, without going through temporary files. It is a matter of choice at this point..

grail · 04-24-2010, 10:40 PM

I would caution using / as your identifier as remember you may have /usr or /home lines as well.
For your sed you can add a search to the fron of your change:

Code:

sed '/<something on line your looking for>/s/realtime/acl,relatime/' /etc/fstab

This will tell sed to only make a change to the line you find.

awk on the other hand would be a little nicer as the whole field will equal just / and if you make
your change to the line and not the field you can maintain the formatting:

Code:

awk '$2=="/"{sub(/realtime/,"acl,&")}1' /etc/fstab

grail · 05-16-2010, 12:14 AM

Please mark as SOLVED, unless you still have questions.