[SOLVED] Unable to edit fstab with sed to edit mount options

Honest Abe · 02-28-2019, 02:12 AM

Hi LQ,

This is my target file & it's contents..

Code:

vim /tmp/fstab_bk
# Commented Line followed by empty line
/dev/mapper/vg_root-lv_root     /       ext4    defaults 1 2
UUID=STRING1       /boot   ext4    defaults 1 2
/dev/mapper/vg_root-lv_home     /home   ext4    defaults 1 2
/dev/mapper/vg_root-lv_opt      /opt    ext4    defaults 1 2
# Commented Line followed by empty line

/dev/mapper/vg_root-lv_tmp      /tmp    ext4    defaults,noexec         1       2
/dev/mapper/vg_root-lv_usr      /usr    ext4    defaults 1 2
/dev/mapper/vg_root-lv_var      /var    ext4    defaults 1 2
/dev/mapper/vg_root-lv_swap     swap    swap    defaults 1 2
tmpfs   /dev/shm        tmpfs   nodev,nosuid,noexec     0       0
devpts  /dev/pts        devpts  STRING2  0       0
sysfs   /sys    sysfs   defaults 1 2
proc    /proc   proc    defaults 1 2
/tmp    /var/tmp        none    bind,nodev,nosuid,noexec    0      0
/dest1  /MP1    ext4    defaults,nodev 1 2
/dest2         /MP2      ext3    defaults 1 2
# Commented Line followed by empty line

/dest3           /MP3                  ext4    defaults        1       2

I am trying to write a script that would add 'nosuid' option for all non-root filesystems. So, my output file should be something like this -

Code:

# Commented Line followed by empty line
/dev/mapper/vg_root-lv_root     /       ext4    defaults 1 2
UUID=STRING1       /boot   ext4    defaults 1 2
/dev/mapper/vg_root-lv_home     /home   ext4    defaults 1 2
/dev/mapper/vg_root-lv_opt      /opt    ext4    defaults 1 2
# Commented Line followed by empty line

/dev/mapper/vg_root-lv_tmp      /tmp    ext4    defaults,noexec         1       2
/dev/mapper/vg_root-lv_usr      /usr    ext4    defaults 1 2
/dev/mapper/vg_root-lv_var      /var    ext4    defaults 1 2
/dev/mapper/vg_root-lv_swap     swap    swap    defaults 1 2
tmpfs   /dev/shm        tmpfs   nodev,nosuid,noexec     0       0
devpts  /dev/pts        devpts  STRING2  0       0
sysfs   /sys    sysfs   defaults 1 2
proc    /proc   proc    defaults 1 2
/tmp    /var/tmp        none    bind,nodev,nosuid,noexec    0      0
/dest1  /MP1    ext4    defaults,nosuid,nodev 1 2
/dest2         /MP2      ext3    defaults,nosuid 1 2
# Commented Line followed by empty line

/dest3           /MP3                  ext4    defaults,nosuid        1       2

I understand that I first need to discard all root and runtime directories (/proc, /dev/shm and such), so I can not do a blind sed string replacement on the file itself. Right now I can replace the string on the line output-

Code:

for i in $(cat /tmp/fstab_bk | grep -Ev "^$|^#|vg_root|UUID|nosuid|proc|sysfs|devpts"| awk '{print $1, $4}');do  echo "string is $i";echo $i | sed  's/defaults/defaults,nosuid/'  ; done
string is /dest1
/dest1
string is defaults,nodev
defaults,nosuid,nodev
string is /dest2
/dest2
string is defaults
defaults,nosuid
string is /dest3
/dest3
string is defaults
defaults,nosuid

This looks hopeful and my approach this problem is to match the $1 value with the first column of the file (so, matching /destN) and check if $4 contains the string "default" and replace it with "defaults,nosuid" for those lines only. For the life of me, I can't achieve this.

Here's what I am trying, but I am guessing there are syntax errors here -

Code:

]#  for i in $(cat /tmp/fstab_bk | grep -Ev "^$|^#|vg_root|UUID|nosuid|proc|sysfs|devpts"| awk '{print $1}';
do
sed -i -e '/^$i / s/defaults/defaults,nosuid/' /tmp/fstab_bk
done
>

I am specifically looking for a one liner and since I am pretty mediocre in sed, a bit of explanation would be awesome too. Can I please get some pointers ? Also, it'd be interesting to show other approaches as well.

berndbausch · 02-28-2019, 03:11 AM

This is too complex for my poor brain, but this line:

Code:

sed -i -e '/^$i / s/defaults/defaults,nosuid/' /tmp/fstab_bk

will perform the substitution in lines that start with dollar-i. Since there are no such lines, it does nothing. If you want $i to be interpreted as a shell variable, use double quotes around the sed expression.

Since you use awk anyway, why not use it for the substitution as well? I don't know if awk has an option that is equivalent to sed's -i, but this is easily worked around.

You can discard unwanted lines by checking the second column (for root) or column 3:

Code:

$2=="/" || $3=="tmpfs" || $3=="devpts" || (...etc...) { next }

Then, in the remaining lines, add nosuid to the fourth columm:

Code:

{ $4 = $4 ",nosuid"; print }

That should be it, more or less. Not a one-liner, but perhaps easier to read than your construct.

Honest Abe · 02-28-2019, 04:06 AM

Thanks for your attention @berndbausch

Code:

sed -i -e '/^$i / s/defaults/defaults,nosuid/' /tmp/fstab_bk

Quote:

will perform the substitution in lines that start with dollar-i. Since there are no such lines, it does nothing.

Sorry, could you please explain that ? Because $i is actually holding the value for first column of the fstab file.

Code:

# for i in $(cat /tmp/fstab_bk | grep -Ev "^$|^#|vg_root|UUID|nosuid|proc|sysfs|devpts"| awk '{print $1}');do  echo "string is $i"; done
string is /dest1
string is /dest2
string is /dest3

I am trying to construct the awk script with your method and will post results.

berndbausch · 02-28-2019, 04:19 AM

Quote:

Sorry, could you please explain that ? Because $i is actually holding the value for first column of the fstab file.

Your problem is that shell variables are not resolved inside single quotes. When quoted like this, a dollar is a dollar. No replacement.

Use double quotes instead. In my mother tongue, Gänsefüßchen (much cuter, I think). Try this:

Code:

i=123
echo ‘$i’
echo “$i”

Honest Abe · 02-28-2019, 04:40 AM

Ah correct, overlooked that.

Honest Abe · 02-28-2019, 11:25 AM

@berndbausch, tried awk too. Maybe you could help in streamlining this ?

Code:

 # awk '{if ($1!="*vg_root*" || $1!="UUID*" || $1!="^$" || $1!="^#" || $3!="proc" || $3!="sysfs" || $3!="tmpfs" || $3!="devpts" || $4!="*nosuid*"){ $4=$4 ",nosuid"; print}}' /tmp/fstab_bk
# Commented Line followed,nosuid by empty line
/dev/mapper/vg_root-lv_root / ext4 defaults,nosuid 1 2
UUID=STRING1 /boot ext4 defaults,nosuid 1 2
/dev/mapper/vg_root-lv_home /home ext4 defaults,nosuid 1 2
/dev/mapper/vg_root-lv_opt /opt ext4 defaults,nosuid 1 2
# Commented Line followed,nosuid by empty line
   ,nosuid
/dev/mapper/vg_root-lv_tmp /tmp ext4 defaults,noexec,nosuid 1 2
/dev/mapper/vg_root-lv_usr /usr ext4 defaults,nosuid 1 2
/dev/mapper/vg_root-lv_var /var ext4 defaults,nosuid 1 2
/dev/mapper/vg_root-lv_swap swap swap defaults,nosuid 1 2
tmpfs /dev/shm tmpfs nodev,nosuid,noexec,nosuid 0 0
devpts /dev/pts devpts STRING2,nosuid 0 0
sysfs /sys sysfs defaults,nosuid 1 2
proc /proc proc defaults,nosuid 1 2
/tmp /var/tmp none bind,nodev,nosuid,noexec,nosuid 0 0
/dest1 /MP1 ext4 defaults,nodev,nosuid 1 2
/dest2 /MP2 ext3 defaults,nosuid 1 2
# Commented Line followed,nosuid by empty line
   ,nosuid
/dest3 /MP3 ext4 defaults,nosuid 1 2

at present , I can see that the conditionals are completely ignored.

pan64 · 02-28-2019, 12:22 PM

I would rather try the following:

Code:

awk ' /vg_root/ { continue }
      /UUID/ { continue }
# all similar conditions comes here
     { $4=$4 ",nosuid"; print}'

but it is not tested, and probably I misunderstood something. (this is only a tip to see a different approach, because I did not really catch what do you need)

Honest Abe · 02-28-2019, 12:41 PM

@pan64, Thanks for your attention.

Gave a quick try and got an error.

Code:

 # awk ' /vg_root/ { continue }
/UUID/ { continue }
> /nosuid/ { continue }
> /"^$"/ { continue } 
> { $4=$4 ",nosuid"; print}' /tmp/fstab_bk
awk: cmd. line:1: error: `continue' is not allowed outside a loop
awk: cmd. line:1: error: `continue' is not allowed outside a loop

If you can help with specifics, I have posted my original/target file and the intended result I am expecting. I am looking for a short one liner kind of thing and the fact that I came close at my first attempt with sed makes me hopeful that it's possible. I shall dig more on awk, but if I can get more pointers that'd be great.

Honest Abe · 02-28-2019, 11:37 PM

So before I dig deeper into awk for a possible solution, I wanted to know whether a brutish approach works..
This is working -

Code:

#cat /tmp/fstab_bk | while read line; do echo $line; if [[ $line =~ .*vg_root.*  ||  $line =~ .*UUID.* ||  $line =~ .*^#.*  || $line =~ .*^$.*  ||  $line =~ .*nosuid.* || $line =~ .*proc.*  || $line =~ .*sysfs.*  ||  $line =~ .*devpts.* ]]; then echo $line >> /tmp/fstab_bk_bk; else echo $line | sed 's/defaults/defaults,nosuid/' >> /tmp/fstab_bk_bk;fi; done

comparison of input & output -

Code:

# sdiff /tmp/fstab_bk /tmp/fstab_bk_bk
# Commented Line followed by empty line                         # Commented Line followed by empty line
/dev/mapper/vg_root-lv_root     /       ext4    defaults 1 2  | /dev/mapper/vg_root-lv_root / ext4 defaults 1 2
UUID=STRING1       /boot   ext4    defaults 1 2               | UUID=STRING1 /boot ext4 defaults 1 2
/dev/mapper/vg_root-lv_home     /home   ext4    defaults 1 2  | /dev/mapper/vg_root-lv_home /home ext4 defaults 1 2
/dev/mapper/vg_root-lv_opt      /opt    ext4    defaults 1 2  | /dev/mapper/vg_root-lv_opt /opt ext4 defaults 1 2
# Commented Line followed by empty line                         # Commented Line followed by empty line

/dev/mapper/vg_root-lv_tmp      /tmp    ext4    defaults,noex | /dev/mapper/vg_root-lv_tmp /tmp ext4 defaults,noexec 1 2
/dev/mapper/vg_root-lv_usr      /usr    ext4    defaults 1 2  | /dev/mapper/vg_root-lv_usr /usr ext4 defaults 1 2
/dev/mapper/vg_root-lv_var      /var    ext4    defaults 1 2  | /dev/mapper/vg_root-lv_var /var ext4 defaults 1 2
/dev/mapper/vg_root-lv_swap     swap    swap    defaults 1 2  | /dev/mapper/vg_root-lv_swap swap swap defaults 1 2
tmpfs   /dev/shm        tmpfs   nodev,nosuid,noexec     0     | tmpfs /dev/shm tmpfs nodev,nosuid,noexec 0 0
devpts  /dev/pts        devpts  STRING2  0       0            | devpts /dev/pts devpts STRING2 0 0
sysfs   /sys    sysfs   defaults 1 2                          | sysfs /sys sysfs defaults 1 2
proc    /proc   proc    defaults 1 2                          | proc /proc proc defaults 1 2
/tmp    /var/tmp        none    bind,nodev,nosuid,noexec    0 | /tmp /var/tmp none bind,nodev,nosuid,noexec 0 0
/dest1  /MP1    ext4    defaults,nodev 1 2                    | /dest1 /MP1 ext4 defaults,nosuid,nodev 1 2
/dest2         /MP2      ext3    defaults 1 2                 | /dest2 /MP2 ext3 defaults,nosuid 1 2
# Commented Line followed by empty line                         # Commented Line followed by empty line

/dest3           /MP3                  ext4    defaults       | /dest3 /MP3 ext4 defaults,nosuid 1 2

Right now, the spacing between words are not being conserved. However, syntactically the output looks okay.

pan64 · 03-01-2019, 12:31 AM

Quote:

Originally Posted by Honest Abe

@pan64, Thanks for your attention.

Gave a quick try and got an error.

Code:

 # awk ' /vg_root/ { continue }
/UUID/ { continue }
> /nosuid/ { continue }
> /"^$"/ { continue } 
> { $4=$4 ",nosuid"; print}' /tmp/fstab_bk
awk: cmd. line:1: error: `continue' is not allowed outside a loop
awk: cmd. line:1: error: `continue' is not allowed outside a loop

If you can help with specifics, I have posted my original/target file and the intended result I am expecting. I am looking for a short one liner kind of thing and the fact that I came close at my first attempt with sed makes me hopeful that it's possible. I shall dig more on awk, but if I can get more pointers that'd be great.

Yes, that was my mistake. It is next instead of continue. Sorry.
Also if you want to check empty lines you can write:

Code:

/^$/ { next } # << no " required
# or
length == 0 { next }

In sed you can do the same:

Code:

sed '/nosuid/n; /vg_root/n; /^$/n  ..... s/defaults/defaults,nosuid/'

ondoho · 03-01-2019, 01:08 AM

ok, i didn't read all of it, but allow me this one comment:
whenever i see a thread title "use sed to accomplish this-n-that"
my first thought is: "But is sed the right tool for the job?"
________________________

Quote:

Originally Posted by berndbausch

Use double quotes instead. In my mother tongue, Gänsefüßchen (much cuter, I think). Try this:

Code:

i=123
echo ‘$i’
echo “$i”

Gänsefüßchen!
but something's off with your formatting.
consider this:

Code:

i=123
echo ‘$i’
echo '$i'
echo “$i”
echo "$i"

berndbausch · 03-01-2019, 02:52 AM

Quote:

Originally Posted by ondoho

ok, i didn't read all of it, but allow me this one comment:
whenever i see a thread title "use sed to accomplish this-n-that"
my first thought is: "But is sed the right tool for the job?"
________________________

Gänsefüßchen!
but something's off with your formatting.
consider this:

Code:

i=123
echo ‘$i’
echo '$i'
echo “$i”
echo "$i"

That's probably Windows and/or Firefox mangling my Gänsefüßchen.

And sed is not the right tool here, true.

Honest Abe · 03-01-2019, 12:09 PM

Quote:

Originally Posted by pan64

Yes, that was my mistake. It is next instead of continue. Sorry.
Also if you want to check empty lines you can write:

Code:

/^$/ { next } # << no " required
# or
length == 0 { next }

This works..

Code:

# awk ' /vg_root/ { next } 
> /UUID/ { next }
> /^#/ { next }
> /^$/ { next }
> /sysfs/ { next }
> /proc/ { next }
> /tmpfs/ { next }
> /devpts/ { next }
> { $4=$4 ",nosuid"; print}' /tmp/fstab_bk
/tmp /var/tmp none bind,nodev,nosuid,noexec,nosuid 0 0
/dest1 /MP1 ext4 defaults,nodev,nosuid 1 2
/dest2 /MP2 ext3 defaults,nosuid 1 2
/dest3 /MP3 ext4 defaults,nosuid 1 2

Quote:

In sed you can do the same:

Code:

sed '/nosuid/n; /vg_root/n; /^$/n  ..... s/defaults/defaults,nosuid/'

Actually, if you see my approach I was trying to get this done within a loop. Thanks for direction.

Honest Abe · 03-02-2019, 02:19 AM

Quote:

Originally Posted by pan64

In sed you can do the same:

Code:

sed '/nosuid/n; /vg_root/n; /^$/n  ..... s/defaults/defaults,nosuid/'

I think this needs some work with respect to pattern matching -

Code:

# sed '/*nosuid*/n; /\/dev\/mapper\/vg_root*/n; /^$/n; /^#/n; /UUID*/n; /*sysfs*/n; /*proc*/n; /*tmp*/n; /*devpts*/n; s/defaults/defaults.nosuid/' /tmp/fstab_bk
# Commented Line followed by empty line
/dev/mapper/vg_root-lv_root     /       ext4    defaults.nosuid 1 2
UUID=STRING1       /boot   ext4    defaults 1 2
/dev/mapper/vg_root-lv_home     /home   ext4    defaults.nosuid 1 2
dev/mapper/vg_root-lv_opt      /opt    ext4    defaults 1 2
# Commented Line followed by empty line[/B]

[B]/dev/mapper/vg_root-lv_tmp      /tmp    ext4    defaults,noexec         1       2
/dev/mapper/vg_root-lv_usr      /usr    ext4    defaults.nosuid 1 2
/dev/mapper/vg_root-lv_var      /var    ext4    defaults 1 2
/dev/mapper/vg_root-lv_swap     swap    swap    defaults.nosuid 1 2
tmpfs   /dev/shm        tmpfs   nodev,nosuid,noexec     0       0
devpts  /dev/pts        devpts  STRING2  0       0
sysfs   /sys    sysfs   defaults.nosuid 1 2
proc    /proc   proc    defaults.nosuid 1 2
/tmp    /var/tmp        none    bind,nodev,nosuid,noexec    0      0
/dest1  /MP1    ext4    defaults.nosuid,nodev 1 2
/dest2         /MP2      ext3    defaults.nosuid 1 2
# Commented Line followed by empty line

/dest3           /MP3                  ext4    defaults.nosuid        1       2

Bold lines are somehow not matching my expected results, whereas the blue lines are expected.
[The dot (.) in the replacement string was a typo, but I left it as it is, since it's all being done on a test file ]

Here is one more attempt, different regex ..

Code:

# sed '/nosuid/n; /vg_root/n; /^$/n; /^#/n; /UUID/n; /sysfs/n; /proc/n; /tmp/n; /devpts/n; s/defaults/defaults.nosuid/' /tmp/fstab_bk
# Commented Line followed by empty line
/dev/mapper/vg_root-lv_root     /       ext4    defaults.nosuid 1 2
UUID=STRING1       /boot   ext4    defaults 1 2
/dev/mapper/vg_root-lv_home     /home   ext4    defaults.nosuid 1 2
/dev/mapper/vg_root-lv_opt      /opt    ext4    defaults 1 2
# Commented Line followed by empty line

/dev/mapper/vg_root-lv_tmp      /tmp    ext4    defaults,noexec         1       2
/dev/mapper/vg_root-lv_usr      /usr    ext4    defaults.nosuid 1 2
/dev/mapper/vg_root-lv_var      /var    ext4    defaults 1 2
/dev/mapper/vg_root-lv_swap     swap    swap    defaults.nosuid 1 2
tmpfs   /dev/shm        tmpfs   nodev,nosuid,noexec     0       0
devpts  /dev/pts        devpts  STRING2  0       0
sysfs   /sys    sysfs   defaults.nosuid 1 2
proc    /proc   proc    defaults 1 2
/tmp    /var/tmp        none    bind,nodev,nosuid,noexec    0      0
/dest1  /MP1    ext4    defaults.nosuid,nodev 1 2
/dest2         /MP2      ext3    defaults.nosuid 1 2
# Commented Line followed by empty line

/dest3           /MP3                  ext4    defaults.nosuid        1       2

pan64 · 03-02-2019, 12:38 PM

sorry, I'm trying to understand. * in those regexps are not ok. Also the dot in defaults.nosuid looks strange.
For me it looks like that sed works properly, just the expression is not the one you really need.
The logic is to "skip" all the lines which should not be changed:

Code:

/sysfs/n;

and finally substitute defaults. I was thinking about replacing n to {p;d}, that will lead to something like this:

Code:

sed '/UUID/{p;d};/^#/{p;d};/^$/{p;d};/sysfs/{p;d};/proc/{p;d};/tmpfs/{p;d};/devpts/{p;d};s/defaults/defaults,nosuid/'

but I'm unsure at this moment