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I have a CSV file like this:

hostname1 | role1 | environment | tag,list | |
hostname2 | role2 | environment | tag,list,longer | |
hostname3 | role3 | environment | | |

I need a sed expression that adds to the column with the tags (the 4th column) a new tag if there are no tags already, and inserts a new tag with a comma if there are existing tags. I have tried this:

sed "s/\(^$server |.*|.*|\) \(.*|.*|$\)/\1 new,\2/" testfile.csv

where server is defined externally like so:

server="hostname2"

but it doesn't work, and it doesn't deal with the comma if the tag list is empty.

How would you do this with sed?

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I don't know sed scripting, but found a good tutorial here: grymoire.com/Unix/Sed.html –  ThiagoPonte Jan 3 '13 at 17:15
    
This is probably better suited on stackoverflow.com –  user98085 Jan 3 '13 at 17:15

1 Answer 1

A common mistakes when discovering regular expression is to think that they can be used as parser (and so you see questions like "how can I parse an XML file using REGEXs?"). However you can't insert too much logic inside a regular expression: for complex problems, like yours, you either need a parser or more than one regular expression.

If you want to go with the way of regular expressions, you need two: one for the non-empty tags case and one for the empty tags field. The two regex could look like the following ones:

s/^(hostname123 \|.*?\|.*?\|\s*)(\S+\s*\|.*?\|)$/\1new,\2/
s/^(hostname123 ?\|.*?\|.*?\|)\s*(\|.*?\|)$/\1 new \2/

These two expressions can be used either with two sed piped together:

sed 'expression1' | sed 'expression2'

or, better, with one command in the following way:

sed -r -e '/^'"$server"' \|/ { s/^(.*?\|.*?\|.*?\|\s*)(\S+\s*\|.*?\|)$/\1new,\2/; s/^(.*?\|.*?\|.*?\|)\s*(\|.*?\|)$/\1 new \2/ }' testfile.csv

This is more efficient than using two seds because it does not parse the file twice.

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wow that was quick and awesome. But it doesn't work: $sed -r -e '/^$server \|/ { s/^(.*?\|.*?\|.*?\|\s*)(\S+\s*\|.*?\|)$/\1new,\2/; s/^(.*?\|.*?\|.*?\|)\s*(\|.*?\|)$/\1 new \2/ }' data2.csv hostname1 | role1 | environment | tag,list | | hostname2 | role2 | environment | tag,list,longer | | hostname3 | role3 | environment | | | –  Jason Jan 3 '13 at 18:15
    
Oops, sorry: the problem is that single quotes do not expand variables. The problem should be fixed now! –  Andrea Corbellini Jan 3 '13 at 19:12
    
brilliant! Can you explain one thing (otherwise I will ask a hundred questions). What is the purpose of the '?'s ? What do they do? –  Jason Jan 3 '13 at 19:30
    
@Jason: I'm not a good teacher. The concept is simple, but I don't know how to explain it in a few words. What I can do is to redirect you to the Python documentation (look for *?, +?, ??) which explains the concept very well, IMO. –  Andrea Corbellini Jan 3 '13 at 19:34
    
+1 that is the most polite and considerate RTFM I have ever received. Bravo, bravo! –  Jason Jan 3 '13 at 21:23

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