sed to change character in text file only once.

pobrika · 09-07-2009, 07:50 PM

Ok I have been all over the net and read books and still can't figure this one out, please someone show me the light.

I have a script that looks like:

Code:

cat servers.txt
trivia:P:N
trivia:D:N
tucana:P:Y
vesta:D:Y
victor:D:Y
vulcan:D:Y
wn-nzp-pin-1:P:Y
wnsf2sc-pvg:P:N
wnsf2sc0-pvg:P:N
wnsf2sc1-pvg:P:N
wnt3pp1:P:N

I want to be able to find the lines that matches my input and change the N to a Y, but only for the lines that matches the name and not any other N's
My problem is the line does not always contain a P as it can be a D as well so my matching did not work.

If my script issues the name $1=triva the lines will change to:

Code:

trivia:P:Y
trivia:D:Y

I have the following code so far but as you can see it does not change the D's

Code:

sed -i 's/trivia:P:Y/trivia:P:N/g' servers.txt

Thanks

*** UPDATE ***

should I be using a method as follows? I am still stuck on the changing all instances though.

Code:

$1=server

sed -i 's/$server1:P:Y/$server:P:N/g' server.txt
sed -i 's/$server1:D:Y/$server:D:N/g' server.txt

kbp · 09-07-2009, 10:46 PM

Hi pobrika,

Here's an example you can work from:

Code:

#!/bin/bash

SERVER=$1

if [ -z $SERVER ]
then
        echo "Usage: $0 servername"
        exit
fi

sed -i "s/\(${SERVER}\:[PD]\:\)N/\1Y/" servers.txt

Notes:
Sed requires double quotes to interpret variables inside the regex
Colons need to be escaped
'$' will not be interpreted on the RHS so use backreference - '( )' / '\1'
Use '[]' to define a set or range to match

cheers,

kbp

ghostdog74 · 09-07-2009, 11:04 PM

Code:

name="trivia"
awk -v name=$name -F":" '$1~name&&$NF=="N"{$NF="Y"}1' file > temp
mv temp file

ta0kira · 09-08-2009, 12:13 AM

Quote:

Originally Posted by kbp

Code:

#!/bin/bash

SERVER=$1

if [ -z $SERVER ]
then
        echo "Usage: $0 servername"
        exit
fi

sed -i "s/\(${SERVER}\:[PD]\:\)N/\1Y/" servers.txt

Another way to do it, i.e. without having to concern yourself with the rest of the line is to do this:

Code:

sed -i "/$SERVER/s/:N$/:Y/" servers.txt

Two things to watch out for here. The first is the name of the server. If for some reason you're going to use a path, change all / to something else, e.g. @. The second thing is the -i option. This is very handy, however, you should skip it initially and preview using less to make sure you don't permanently ruin your file (or use something like -i.bak.)
Kevin Barry

pobrika · 09-08-2009, 06:40 AM

Cheers everyone that is exactly what I was looking for I knew there must be a one liner for the sed command, annoyed with myself for not being able to find the answer on my own

I'll do some playing around and see which method works best in my script.

ta0kira · 09-08-2009, 10:30 AM

Here is a good place to learn more complex things:
http://www.grymoire.com/Unix/Sed.html

Kevin Barry

pobrika · 09-08-2009, 11:49 PM

Just a follow up to say I managed to get my script to work, the only problems I had was the testing was done on my trusty rhel4 server and the script would not run on a solaris 8 platform.
Turned out the -i option did not work.
I ended up using:

Code:

sed "s/\(${SERVER}\:[PD]\:\)N/\1Y/" /usr/local/bin/servers.txt > /tmp/servers.txt
mv /tmp/servers.txt /usr/local/bin/servers.txt

I hate writing files out and back in as it is "messy" but it works.

ta0kira · 09-09-2009, 07:43 AM

Quote:

Originally Posted by pobrika

Just a follow up to say I managed to get my script to work, the only problems I had was the testing was done on my trusty rhel4 server and the script would not run on a solaris 8 platform.
Turned out the -i option did not work.
I ended up using:

Code:

sed "s/\(${SERVER}\:[PD]\:\)N/\1Y/" /usr/local/bin/servers.txt > /tmp/servers.txt
mv /tmp/servers.txt /usr/local/bin/servers.txt

I hate writing files out and back in as it is "messy" but it works.

Not only that, if the file has multiple links (i.e. hard linked) they might not keep pointing to the same file unless you do this instead of mv:

Code:

cat /tmp/servers.txt > /usr/local/bin/servers.txt
rm /tmp/servers.txt

Also, if this is in a script you might want to use mktemp in case it happens to get called twice concurrently. That way you won't overwrite something in progress.
Kevin Barry