change only some entries in /etc/fstab file in bash - bash

Question

Below is a sample /etc/fstab with 9 entries. How can I change column 4 (from-defaults to-noexec,nodev) for nfs shares only? How can I use an exceptions file to do this? or is there any other way if i want to do this system wide?
/dev/appvg/me /me ext4 defaults 0 0
/dev/appvg/you /you ext4 defaults 0 0
/dev/appvg/we /we ext4 defaults 0 0
/dev/appvg/us /us ext4 defaults 0 0
nas-prd-unix.com:/vol/unix/he /he nfs defaults 0 0
nas-prd-unix.com:/vol/unix/she /she nfs defaults 0 0
nas-prd-unix.com:/vol/unix/it /it nfs defaults 0 0
nas-prd-unix.com:/vol/unix/one /one nfs defaults 0 0
nas-prd-unix.com:/vol/unix/two /two nfs defaults 0 0

Answer 1

In awk:
$ awk '$1 ~ /test[3469]/{sub(/defaults {6}/,"defaluts,nodev",$0)} 1' /etc/fstab
/dev/appvg/test1 /test1 nfs defaults 0 0
/dev/appvg/test2 /test2 nfs defaults 0 0
/dev/appvg/test3 /test3 nfs defaluts,nodev 0 0
/dev/appvg/test4 /test4 nfs defaluts,nodev 0 0
/dev/appvg/test5 /test5 nfs defaults 0 0
/dev/appvg/test6 /test6 nfs defaluts,nodev 0 0
/dev/appvg/test7 /test7 nfs defaults 0 0
/dev/appvg/test8 /test8 nfs defaults 0 0
/dev/appvg/test9 /test9 nfs defaluts,nodev 0 0
If you insist on exclusion file:
$ cat exclusions.txt
/dev/appvg/test1
/dev/appvg/test2
/dev/appvg/test5
/dev/appvg/test7
/dev/appvg/test8
Script:
$ cat script.awk
NR==FNR { a[$1];next } # remember exclusions
!($1 in a) { sub(/defaults {6}/,"defaluts,nodev",$0) } # don't replace on them
1 # print
Run it:
$ awk -f script.awk exclusions.txt /etc/fstab
# for output, see above.

Answer 2

You can use a regular expression with a look-behind assertion. These are supported by Perl, for example:
perl -pe 's/test[3469].*\Kdefaults/defaults,nodev/' /etc/fstab
To change the file in-place with a backup you can do:
perl -p -i.bak -e 's/test[3469].*\Kdefaults/defaults,nodev/' /etc/fstab

Answer 3

Somewhat similar to Eugene's answer...
# Your input data or file "data.txt"
bash$> cat data.txt
/dev/appvg/test1 /test1 nfs defaults 0 0
/dev/appvg/test2 /test2 nfs defaults 0 0
/dev/appvg/test3 /test3 nfs defaults 0 0
/dev/appvg/test4 /test4 nfs defaults 0 0
/dev/appvg/test5 /test5 nfs defaults 0 0
/dev/appvg/test6 /test6 nfs defaults 0 0
/dev/appvg/test7 /test7 nfs defaults 0 0
/dev/appvg/test8 /test8 nfs defaults 0 0
/dev/appvg/test9 /test9 nfs defaults 0 0
bash$>
# Apply a perl command, which performs in place editing.
** Caution: If you don't formulate the command properly, it may damage the file **
bash$> perl -pi -e 'if (m/test[3469]/) {$_ =~ s/defaults/defaults,nodev/g}' data.txt ; cat data.txt
/dev/appvg/test1 /test1 nfs defaults 0 0
/dev/appvg/test2 /test2 nfs defaults 0 0
/dev/appvg/test3 /test3 nfs defaults,nodev 0 0
/dev/appvg/test4 /test4 nfs defaults,nodev 0 0
/dev/appvg/test5 /test5 nfs defaults 0 0
/dev/appvg/test6 /test6 nfs defaults,nodev 0 0
/dev/appvg/test7 /test7 nfs defaults 0 0
/dev/appvg/test8 /test8 nfs defaults 0 0
/dev/appvg/test9 /test9 nfs defaults,nodev 0 0
bash$>
If you do this often enough, then use this script, and just feed parameters to it as shown and it will formulate a command and execute it for you.
# Script output
bash$> ./run.sh "test[3469]" "defaults" "defaults,nodev"
Formed Command is : perl -pi -e 'if(m/test[3469]/) {s/defaults/defaults,nodev/g}' data.txt
/dev/appvg/test1 /test1 nfs defaults 0 0
/dev/appvg/test2 /test2 nfs defaults 0 0
/dev/appvg/test3 /test3 nfs defaults,nodev 0 0
/dev/appvg/test4 /test4 nfs defaults,nodev 0 0
/dev/appvg/test5 /test5 nfs defaults 0 0
/dev/appvg/test6 /test6 nfs defaults,nodev 0 0
/dev/appvg/test7 /test7 nfs defaults 0 0
/dev/appvg/test8 /test8 nfs defaults 0 0
/dev/appvg/test9 /test9 nfs defaults,nodev 0 0
bash$>
Here is the script, which can be made general purpose if one needs to do this frequently, although this is not the only way. One can devise tonnes of other ways to arrive at the same result. This is just one approach.
bash$> cat run.sh
#!/bin/bash
# Taking Input parameters in a,b,c
a="$1"
b="$2"
c="$3"
# Old command for reference
#perl -pi -e 'if (m/test[3469]/) {$_ =~ s/defaults/defaults,nodev/g}' data.txt ; cat data.txt
# Now formulate a command and use substitution
cmd="perl -pi -e 'if(m/$a/) {s/$b/$c/g}' data.txt"
echo "Formed Command is : $cmd"
# Execute the formulated command
eval "$cmd"
# Check the output after execution.
echo ; cat data.txt
bash$>