Append line to a block of text incrementing the value from previous line

Question

I have a text file name class.txt with follow contents.

[serverClass:MAIL]
whitelist.0=LATE
whitelist.1=ONTIME

[serverClass:LETTER]
whitelist.0=FIRST
whitelist.1=SECOND
whitelist.2=THIRD
whitelist.3=FOURTH

[serverClass:NOTES]
whitelist.0=TEST
whitelist.1=CAR
whitelist.2=SPOON
whitelist.3=GAME

Let's say I want to add a new line to one of these blocks e.g. a new entry SAMPLE in block LETTER so whitelist number should auto increment when the new entry is added. Desired output

[serverClass:MAIL]
whitelist.0=LATE
whitelist.1=ONTIME

[serverClass:LETTER]
whitelist.0=FIRST
whitelist.1=OLD
whitelist.2=NEW
whitelist.3=FOURTH
whitelist.4=SAMPLE

[serverClass:NOTES]
whitelist.0=TEST
whitelist.1=CAR
whitelist.2=SPOON
whitelist.3=GAME

is there a way to do this with sed ?

Answer 1

As I said in my comment on Ralph's answer, there are better tools for this job e.g. with awk you could use paragraph mode, append whitelist.0=SAMPLE if the block is empty otherwise extract the no. from the last field (in this case the field is a line) and append whitelist.NR+1=SAMPLE to that block:

awk -vRS= -vORS='\n\n' 'BEGIN{z="whitelist.0=SAMPLE";FS="\n"}
/LETTER/{if (/[0-9]=/){split($NF, a, /[.=]/);sub(/0/, a[2]+1, z)}
sub (/$/,"\n"z ,$0)};1' infile

Answer 2

Great! Now, I've learnt something new. I've used sed for all sorts of (minor) substitutions, and not realized you can actually program in it. Obviously a quite weak "machine", since it has only two registers, and a rather obscure language.

Since sed doesn't take input variables, I've mad it an sh script that invokes sed with a dynamic program, which is prepared by replacing BLOCK and ENTRY with the actual tags from the command line. That replacement is done with a separate simple sed substitution to prepare the specific program to achieve the entry insertion.

The following script seems to do the work. If called addentry, it would be invoked as

$ addentry LETTER SAMPLE < data

to reproduce the input data with the entry inserted as its output. I suppose there's an -i option to sed for "in-place editing", if needs be.

#!/bin/sh

/bin/sed -n "$(cat << EOF | sed -e "s/BLOCK/$1/g;s/ENTRY/$2/g"
# Initialize hold space with 0
1 { x ; /^$/ s/^$/0/; x }

# Lines outside the interesting block are just printed
/\s*serverClass:BLOCK/,/^$/! { p }

# Lines of the interesting block are considered more in detail
/\s*serverClass:BLOCK/,/^$/ {

  # The final empty line is replaced by the new entry, using the line
  # counter from the "hold buffer"
  /^$/ { g; s/\(.*\)/whitelist.\1=ENTRY/p; s/.*//p; b xx }

  # print the line
  p

  # Jump forward for the block leader (note xx is a label)
  /serverClass:/ { b xx }

  # Increment hold space counter
  # (Only handles 0-9 here; room for improvement)
  x; y/0123456789/1234567890/; h

  # If the block ends the file without blank line, then add the
  # new entry at end.
  $ { g; s/\(.*\)/whitelist.\1=ENTRY/p; b xx }

  # Label xx is here
  :xx
}
EOF
)"

Many thanks for raising this (for me) interesting challenge.