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Help with sed substitution


 
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# 15  
Old 12-12-2009
OK Guys....

Time out.

It is best to wait for the original poster to post back and get their feedback on what they believe works for them.

Good job.

Cheers.
# 16  
Old 12-12-2009
Haha, wow... I'm sorry I seemed to have sparked quite the discussion :-p I certainly appreciate all the help!

I am indeed an uber noob, and I have actually zero experience with awk. Although it seems to make sense to separate the data into "|" delimited fields, the sed command seems much simpler and easier for me to read (even though it is still a bit cryptic.... regex is seriously the bane of my existence). Plus, the case previously mentioned in which I might have more than one tcolor field, for example, would not be an issue.

That being said... unfortunately, doing the following:
Code:
grep "title=Boston" datafile | sed 's/tcolor=[^|]*/tcolor=blue/'

...does not seem to change the info in the datafile itself.


See, I have an html form that allows the user to enter in these fields into text boxes.. then they are added to the datafile via a .cgi script. If the user goes back and enters in new info for a data item who's title already exists in the datafile, then the new values are meant to replace the old.

So, if the datafile is as follows:
Code:
title=Boston|tcolor=green|desc=Large city in New England|url=http://www.boston.com
title=New York|tcolor=gold|desc=The largest city on the East Coast|url=http://www.newyork.com
title=Dallas|tcolor=silver|desc=A sprawling, cheery Texas city|url=http://www.dallas.com

and the .cgi script passes in the following parameters in the query string:
Code:
title=Boston&titlecolor=blue&descrip=&url=

Once the query string is parsed, what would be the best way to go about testing which parameter is not an empty string, and only updating that field (in this case the tcolor field in the $title line with $titlecolor) in the datafile?


...I hope that all makes sense, haha. Again, thank you for the help!
# 17  
Old 12-12-2009
Quote:
Originally Posted by stabby
Code:
title=Boston&titlecolor=blue&descrip=&url=

i assume you are going to parse the above string, so once again, the distinct field delimiter you can spot easily, its "&". split on "&" , then go through each item and check is there is anything after the "=" sign, if there is nothing, then its a null string. OR you can split on "=" , and make sure that the next field doesn't start with "&".


Code:
s="title=Boston&titlecolor=blue&descrip=&url="
echo $s|awk -F"&" '{
     for(i=1;i<=NF;i++){
        if ($i !~ /.*=$/){
            print $i
        }
     }
}'

# 18  
Old 12-12-2009
When a poster, who is not posting homework of course, asks for a reply in sed, it is generally best not to try to twist the arm of the original poster to a solution using awk or some other tool.

The poster asked for an sed solution and has commented they are not familiar with awk. So, let them have a solution that works for them.

I tend to agree that this has turned into a hypothetical discussion and does not address the needs or requirements of the original posters question.

Unless there is some reason the original poster wants to switch to awk, let's give him a solution in sed, which is what they asked for in the first place.
# 19  
Old 12-12-2009
Hi, you could combine a function and a sed statement to create a way of changing a field value based on a field name. E.g.

Code:
replace_field() 
{
  sed "/^$1/s/\(${2%=*}=\)[^|]*/\1${2#*=}/" infile;
}
replace_field "title=Boston" "desc=bla bla"

Code:
output:
title=Boston|tcolor=green|desc=bla bla|url=http://www.boston.com
title=New York|tcolor=gold|desc=The largest city on the East Coast|url=http://www.newyork.com
title=Dallas|tcolor=silver|desc=A sprawling, cheery Texas city|url=http://www.dallas.com

This probably looks quite cryptic but it is based on the previous examples with the values replaced by $2 (and parameter expansions). The function takes the first key to locate the line on which certain fields need to be replaced.

We can expand this to take more fields and replace the $2 expansions with field names so it becomes a bit more readable..
Code:
replace_fields() 
{
  key="$1"; shift
  sedstr="/^$key/{"
  while [ $# -gt 0 ]; do
       field="${1%=*}"
       val="${1#*=}"
       sedstr="${sedstr}s/\b\($field=\)[^|]*/\1$val/;"
       shift
  done
  sedstr="$sedstr}"
  sed "$sedstr" infile
}

replace_fields "title=New York" "tcolor=green" "desc=blabla"

Output:
Code:
title=Boston|tcolor=green|desc=Large city in New England|url=http://www.boston.com
title=New York|tcolor=green|desc=blabla|url=http://www.newyork.com
title=Dallas|tcolor=silver|desc=A sprawling, cheery Texas city|url=http://www.dallas.com

If you want to edit the file directly
you could use:
Code:
sed -i "$sedstr" infile

if your sed supports it or
Code:
sed "$sedstr" infile > /var/tmp/infile.tmp.$$ && mv /var/tmp/infile.tmp.$$ infile

It builds a sed statement based on the inout fields that are being provided. Unfortunately I haven't been able to make it simpler than this.

Also, it has to be said that this will probably only work in very low intensity situations. If more users will be creating updates simultaneously then problems may occur and you will have to create some sort of locking-and-wait mechanism. If it is getting more serious you will probably be better off using a simple database.

Last edited by Scrutinizer; 12-12-2009 at 05:49 PM..
# 20  
Old 12-13-2009
thanks a bunch, Scrutinizer.... I appreciate your explaining it to me. It works perfectly :-)
 

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