[SOLVED] Trying to understand the behavior of grep - line seperator

Issue

I'm trying to mimic grep

One thing I don't understand is: when should I print a line separator --? For example, running the command:

cat test.txt | grep -b -v -A 1 -i on (I'll paste text.txt at the bottom)

I get the following output:

87:aaa
91:
92:from the start  Her daughters were identical twins  but the resemblance was purely physical 
185-Madeline had been   difficult baby from day one  She was one who cried nonstop  who
269:refused to nurse  When Paige would finally get her settled and carefully place her down 
358:she rarely got as far as the nursery door before Madelines blood curdling cries began
444-again  often waking Erica as well  Paige would want to sob on the floor  she was so 
529:absolutely depleted 
550:Madeline was an infant who made Paige understand why Shaken Baby Syndrome was   thing 

It makes perfect sense in terms of matching lines and trailing context (because -A) but I thought a line separator should be printed between continous blocks of printed lines:

For example, for the same input, my program prints:

87:aaa
91:
92:from the start  Her daughters were identical twins  but the resemblance was purely physical 
185-Madeline had been   difficult baby from day one  She was one who cried nonstop  who
--
269:refused to nurse  When Paige would finally get her settled and carefully place her down 
358:she rarely got as far as the nursery door before Madelines blood curdling cries began
444-again  often waking Erica as well  Paige would want to sob on the floor  she was so 
--
529:absolutely depleted 
550:Madeline was an infant who made Paige understand why Shaken Baby Syndrome was   thing

My reasoning - on the output itself:

87:aaa -- no match, so print (because -v)
91:    -- no match so print (because -v)
92:from the start  Her daughters were identical twins  but the resemblance was purely physical -- no match, so print (because -v) 
185-Madeline had been   difficult baby from day one  She was one who cried nonstop  who -- match, print because -A 1 and -v -> extra line - last extra line , so must print a line seperator if found a new match
-- -> print a line separator - found a new line that has no match (because -v )
269:refused to nurse  When Paige would finally get her settled and carefully place her do
..
.
.

The text.txt:

Their mother  Paige  would have told you there was something wrong with Madeline right
aaa

from the start  Her daughters were identical twins  but the resemblance was purely physical 
Madeline had been   difficult baby from day one  She was one who cried nonstop  who
refused to nurse  When Paige would finally get her settled and carefully place her down 
she rarely got as far as the nursery door before Madelines blood curdling cries began
again  often waking Erica as well  Paige would want to sob on the floor  she was so 
absolutely depleted 
Madeline was an infant who made Paige understand why Shaken Baby Syndrome was   thing 

So my question is: when exactly does grep prints a line separator ?

Solution

Note: I'm not answering about -v and context matching, since I don't think they are well defined or even supposed to work. For example, this output doesn't make sense:

$ seq 5 | grep -v -A1 '3'
1
2
3
4
5

The separator -- won't be added if two or more groups of matching lines have overlapping lines or are next to each other in input file. Consider this sample input:

$ cat context.txt
wheat
    roti
    bread

blue
    toy
    flower
    sand stone
light blue
    flower
    sky
    water
dark red
    ruby
    blood
    evening sky

Case 1: groups are next to each other

$ grep -C1 'flower' context.txt
    toy
    flower
    sand stone
light blue
    flower
    sky

Case 2: overlapping groups

$ grep -A4 'flower' context.txt
    flower
    sand stone
light blue
    flower
    sky
    water
dark red
    ruby

Answered By - Sundeep

Answer Checked By - Cary Denson (WPSolving Admin)