Find and print two blocks of lines from file with sed in one pass - sed

Question

I am trying to come up with an sed command to find and print two blocks of variable number of lines from a text file that look like this:
...
INFO first block to match
id: "value"
...
last line of the first block
INFO next irrelevant block
id: "different value"
...
INFO second block to match
id: "value"
...
last line of the second block
...
I only have prior knowledge of the id value and the fact that each block starts with a line that has "INFO". I want to match each block from that first line and not include the first line of the next block in the output:
INFO first block to match
id: "value"
...
last line of the first block
INFO second block to match
id: "value"
...
last line of the second block
Ideally I would prefer to do it in a single pass, not have the file scanned multiple times from top to bottom. Currently I have this (it only matches the first block, and I need both):
sed -n -e "/INFO/{"'$!'"{N;/INFO.*id: \"value\"/{:l;p;n;/^[^\\[]/bl;}}}" file.log
EDIT
Linebreak between blocks is certainly nice, but entirely optional.
EDIT 2
Please note that INFO and id: "value" do not have to be in the beginning of the line, and all other words in my example are arbitrary and not known in advance. There can be any number of blocks (including 0) between and around the ones I need to match.

Answer 1

sed is powerful, terse, and dumb. awk is smarter!
awk '/^INFO/{f = /match/? 1: 0} f'
edit: I see you want a linebreak between each "block"; will update if I find a tighter way:
awk '/^INFO/{f = /match/? 1: 0; if(i++) $0 = RS $0} f'
/^INFO/{action}: Execute {action} only on lines beginning with "INFO"
variable = if ? then : else: Conditional Expression (ternary operator)
if(i++): The first time this is evaluated, i will be zero, thus the expression will be false. This prevents an extra line break at the first block.
$0 = RS $0: Prepend a Record Separator (newline) to $0 (entire record)
f If f is greater than zero, {print $0} is implied.

Answer 2

This might work for you (GNU sed):
sed -nE ':a;/^INFO/{N;/^id: "value"/M!D;:b;H;$!{n;/^INFO/!bb};x;s/^/x/;/^x{2}/{s/^x*.//p;q};x;ba}' file
This solution stores the required blocks in the hold space, prefixed by a counter. Once the required number of blocks are stored the counters are removed, the blocks printed and the process quit.
The solution (based only on the input provided) supposes that an id (if it exists) always follows the the INFO line.

Answer 3

Here is an alternative solution using a combination of sed and awk. It allows you to parse the input blockwise or recordwise. This approach relies on setting awk record separator (RS) to the empty string which makes awk read a full block in at a time.
So there are 2 steps:
Make the input record-parsable.
Process each record.
For your example this could something like this:
sed '1!s/^INFO/\n&/' infile | awk '/id: "value"/' RS= ORS='\n\n'
Output:
INFO first block to match
id: "value"
...
last line of the first block
INFO second block to match
id: "value"
...
last line of the second block

Answer 4

awk is good for this, and if you could set RS to a multi-character expression it would be ideal. (gnu awk allows this, but why bother with gnu awk when there is perl?)
perl -wnle 'BEGIN{$/="INFO"; undef $\} print "$/$_" if m/id: \"value\"/' input
Basically, this sets the record separator ($/) to the string "INFO" (so now each of your "records" is a "line" to perl). If the record matches the pattern id: "value", it is printed with "INFO" prepended to the start. (without -n, perl would retain the record separator the end of each record, which is not quite what you want). By omitting the "undef $\", you can get an extra newline between records. Some code golf could probably cut the length of this in half, but my perl is a bit rusty. Waiting for the shorter version in comments.

Answer 5

This may or may not be what you want depending on what your real data looks like:
$ awk '/INFO/{info=$0; f=0} /id: "value"/{print info; f=1} f' file
INFO first block to match
id: "value"
...
last line of the first block
INFO second block to match
id: "value"
...
last line of the second block
or if you want to do more with each block than just print it as you go then some variation of this is better:
$ awk '
/INFO/ { prt() }
{ block = block $0 ORS }
END { prt() }
function prt() {
if (block ~ /id: "value"/) {
printf "%s", block
}
block=""
}
' file
INFO first block to match
id: "value"
...
last line of the first block
INFO second block to match
id: "value"
...
last line of the second block
The above will behave the same using any awk in any shell on any UNIX box.