mjhjmtu

I can print the braceted lines in the file a

first
[
third
fourth
]
sixth
[
eighth 
]
tenth

by doing

% <a sed -n '/[/,/]/p' 

which prints

[
third
fourth
]
[
eighth
]

But what if I want only the second match, ie. the last three lines?

Easier to do with awk, assuming the blocks defined by [ and ] do not themselves contain [ or ] inside the block

$ awk -v b=2 '/[/c++ c==b; /]/ && c==bexit' ip.txt
[
eighth 
]

• 
  -v b=2 variable to specify which block is required


• 
  /[/c++ increment counter if line matches starting condition


• 
  c==b; print input record if counter is equal to block required


• 
  /]/ && c==bexit exit on matching ending condition

another way to write this:

awk -v b=2 '/[/c++ c==bprint $0; if(/]/) exit' ip.txt

$ sed -n '/^[/h; /^[/,/^]/H; $x;s/^[n//;p;' file
[
eighth
]

Annotated sed script (assumes -n):

/^[/h; # replace hold space with this line
/^[/,/^]/H; # append these lines to hold space with embedded newlines
$ # at last line of input
 x; # swap in the hold space
 s/^[n//; # delete the first "[" and the newline following it
 p; # print

That is, whenever we find a line that starts with [, clear the hold space by copying the line there. Then keep appending lines to the hold space until we find the corresponding line that starts with ].

At the end, we will have a hold space with one [ too many, so delete that (and the embedded newline after it) before printing the data.

Using sed editor, we can perform it as follows:

sed -ne ' ;# "-n" suppresses autoprint of pattern space
 /^[/!d ;# skip lines that donot begin a fresh block
 :a;$!N;/n]/!ba ;# keep appending lines to pattern space until end of block
 G ;# append our makeshift counter (hold space) to pattern spc
 s/n2$//p ;# only when counter had 2 chars in it, we print block
 /n$/!q ;# target block, 2nd block has been printed so quit 
 x;s/$/n/;x ;# target block not reached, so increment
' input.file

with Perl, we can use the ... operator in tandem with the boolean $k == 2, indicating that we have reached the intended target block and need to print it.

perl -lne 'print if /^[/ && ++$k == 2 ... /^]/' input.file