[SOLVED] sed trick to do conditional append

the_gripmaster · 01-23-2012, 09:18 PM

Existing file

Code:

root:x:0:
wheel:x:10:user1,user2
mail:x:12:mail

Desired file

Code:

root:x:0:
wheel:x:10:user1,user2,user3
mail:x:12:mail

What I want to do is use sed to append user3 at the end of the wheel group.

jhwilliams · 01-23-2012, 10:14 PM

Code:

sed 's/^\(wheel:.*\)/\1,user3/' /etc/group

the_gripmaster · 01-23-2012, 10:38 PM

Quote:

Originally Posted by jhwilliams

Code:

sed 's/^\(wheel:.*\)/\1,user3/' /etc/group

Thanks.

jschiwal · 01-24-2012, 12:47 AM

I'd recommend changing it slightly, using the pattern /wheel:/ to select the line to edit.

Code:

/^wheel:/s/.*/&,user3/

It only edits lines starting with "wheel".

Dark_Helmet · 01-24-2012, 12:58 AM

And a slight modification to jschiwal's pattern

Code:

/^wheel:/s/\(.*\)/\1,user3/

EDIT:
Just to clarify, this form and jhwilliams's are the same thing--just formatted differently (as far as I can tell)

jschiwal · 01-24-2012, 06:14 AM

Thanks Dark Helmet. I modified my post. Didn't type the ampersand. I get so caught up avoiding auto-discorrection, I end up making dumb omissions.

the_gripmaster · 01-24-2012, 06:17 AM

Why is sed so cryptic?

jhwilliams · 01-24-2012, 08:53 AM

Quote:

Originally Posted by the_gripmaster

Why is sed so cryptic?

I remember feeling that way, too. Here, this is a /little/ clearer, with extended regex:

Code:

sed -r 's/(wheel:.*)/\1,user3/' /etc/group

Regex is a powerful tool - it's a trade off between ease of use and capability of expression. I think if we tried to create such a regular expression language of our own it might end up being just as ugly, if only different.

Incidentally, if I am correctly guessing what you're trying to do, you might find it easier to just add the group membership to the user's list with this command:

Code:

sudo usermod -a -G wheel user3

trey85stang · 01-24-2012, 10:30 AM

Quote:

Originally Posted by the_gripmaster

Why is sed so cryptic?

It's not really cryptic, its just a language you don't understand (yet). Basically as said above the language you do not understand is regular expressions... (also the shell escapes get confusing but come second nature when you realize they are shell escapes) Here is a good getting started tutorial: http://gnosis.cx/publish/programming...pressions.html

You don't have to get too advanced to get a good grasp and be able to start using them, also regular expressions are supported in many languages and commands so knowing a little bit comes in handy.

Quote:

Originally Posted by Dark_Helmet

And a slight modification to jschiwal's pattern

Code:

/^wheel:/s/\(.*\)/\1,user3/

EDIT:
Just to clarify, this form and jhwilliams's are the same thing--just formatted differently (as far as I can tell)

the above sed line will not work correctly if there is no user in the wheel group... (the sed line will work. the group entry will not.)

This should work though...

Code:

sed "/^wheel:/s/\(.*\)/\1,user3/;s/:,/:/" /etc/group

the_gripmaster · 01-24-2012, 06:39 PM

Quote:

Originally Posted by jhwilliams

Incidentally, if I am correctly guessing what you're trying to do, you might find it easier to just add the group membership to the user's list with this command:

Code:

sudo usermod -a -G wheel user3

Excellent! I wonder why I never thought about adding users to the wheel group this way.