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How can I modify the following content of a file:

cat:persian/young-1

cat:winter/young-2

cat:summer/wild-3

dog:persian/young-1

dog:winter/young-2

dog:summer/wild-3

To :

cat:persian/young-1

cat:winter/young-2

cat:summer/wild-3



dog:persian/young-1

dog:winter/young-2

dog:summer/wild-3

It's not specific to dog or cat, it's more of symbolic representation of whatever the first word/term is

You could do something like:

awk -F: 'NR>1 && $1 "" != last {print ""}; {print; last = $1}'

The "" is to force string comparison. Without it, it wouldn't work properly in input like:

100:foo

100:bar

1e2:baz

1e2:biz

Where 100 and 1e2 would be compared as numbers.

Here's one way. If the first field isn't the same as the one in the previous line, print a break....

$ awk -F: '$1!=a&&a{print ""}{a=$1}1' myfile

cat:persian/young-1

cat:winter/young-2

cat:summer/wild-3



dog:persian/young-1

dog:winter/young-2

dog:summer/wild-3

$

Explanation:

• 
  -F: = set the field delimiter to :
  


• 
  $1!=a&&a = if first field is not equal to variable "a" (the previous first field), and variable "a" is set to something (i.e. we're not dealing with the very first line in the file)


• 
  {print ""} = print a blank line


• 
  {a=$1} = for every line read, set variable "a" to the first field


• 
  1 = print the line

I tried with this way

en ~]# awk '/cat/{print $0}' filename| sed '$s/.*/&n/g';awk '/dog/{print $0}' filename



cat:persian/young-1

cat:winter/young-2

cat:summer/wild-3



dog:persian/young-1

dog:winter/young-2

dog:summer/wild-3

[root@praveen ~]#

You could employ the sed editor for this job too:

$ sed -e '

   $q;N

   P

   /^([^:]*):.*n1:/!{x;p;x;}

   D

' input



cat:persian/young-1

cat:winter/young-2

cat:summer/wild-3



dog:persian/young-1

dog:winter/young-2

dog:summer/wild-3

Basic idea is to have 2 lines at any time in the pattern space. Compare the strings before the leading colons ":" in the pattern space. Should they not match, means a
changeover of an animal is detected, and we promptly display an empty line.

It could be done with awk:

awk -F: ' $0!="" && last!="" && $1!=last"" {print""} {last=$1} 1'

• -F : used to split the fields on a : character.




• $0 != "" is required to avoid converting one empty line into three.
  
  This allows re-processing an already processed file without adding empty lines.




• last != "" is required to avoid the file first line (where last is empty).



• 
  

  $1 != last"" compare the present first field to the previous one.
  
  The trailing "" ensure that the comparison is done in text mode. Otherwise, equivalent (to awk) numeric values will be seen as equal, like:

  
  
  

  7:first line
  
  7e0:second line
  
  7.0000:third line
  
  7.000000000000000000008:fourth line
  
  

  
  



• {print""} print an empty line (if all the tests above match).




• {last=$1} store the line first field in the variable last.




• 1 print the line contents (always).