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I have an MRTG configuration file containing lines starting with the word "Target". Now in such lines there is a pattern starting with the character "#", and ending with character ":".

A sample line could look like these (note two types but start/end markers are still the same):

   Target[]: #Gi1/1:public@

   Target[]: #Gi1/31:public@

What I need is for sed to find these lines and replace the pattern "#Gix/n:" with "ifInErrors#Gix/n&ifInErrors#Gix/n:", where x=1-9, n=1-48.

So the two sample lines shown above would be modified to these:

Target[]: ifInErrors#Gi1/1&ifInErrors#Gi1/1:public@

Target[]: ifInErrors#Gi1/31&ifInErrors#Gi1/31:public@
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2 Answers 2

What you need is a capturing group - you capture what you need in parentheses, and can reference it in the substitution. Like so:

 grep ^[[:space:]]*Target file | sed 's/\(#Gi[1-9]\/[1-9][0-9]*\):/ifInErrors\1\&ifInErrors\1:/'
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sed '/Target/s/\(#Gi[0-9]*_[0-9]*:\)/ifInErrors\1\&ifInErrors\1/' input.txt

sed '/Target/ is equivalent to grep Target | sed, except it involves one less process & one less pipe.

s/ means 'substitution', the most common sed command; s/foo/bar/ would replace instances of the string foo with bar, for example.

/\(#Gi[0-9]*\/[0-9]*:\) ... the brackets (which need to be escaped with a \) tell sed to mark everything between them as \1 (or \2 for the second marked pattern, \3 for the third etc). [0-9]* means 'any number of numbers', and \/ is an escaped / (it needs to be escaped because I'm using / as the separator for sed; if you use another separator, like |, then you wouldn't need to escape the /). So #Gi[0-9]*\/[0-9]*: is a pattern meaning 'start with #, followed by Gi, then any number of numbers, then /, then any number of numbers, ending with :'.

So \1 matches whatever string is detected by the pattern #Gi[0-9]*\/[0-9]*: In the first string given in the question,

Target[]: #Gi1/1:public@
##  The pattern #Gi[0-9]*\/[0-9]*: will match the substring

...therefore /ifInErrors\1\&ifInErrors\1/ tells sed 'replace #Gi1/1: with ifInErrors#Gi1/1:&ifInErrors#Gi1/1:'.

Some extra stuff: if you wanted to just print the lines that begin with 'Target', doing the sed substitution, you could use this line:

sed -n '/Target/s/\(#Gi[0-9]*\/[0-9]*:\)/ifInErrors\1\&ifInErrors\1/p'

-n tells sed 'don't print the output', and the p at the end tells it to print the lines sed is working on.

If you wanted to over-write your original file, you would use this:

sed -i '/Target/s/\(#Gi[0-9]*\/[0-9]*:\)/ifInErrors\1\&ifInErrors\1/'
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Can you elaborate a little? This isn't really an answer without text explaining it. –  Seth Jan 31 '13 at 4:44
@Seth Haha, fair enough, will edit in a explanation in a minute –  evilsoup Jan 31 '13 at 14:18

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