Retrieving lines from a file depending on other lines

Question

Imagine the following file structure:

foo.bar.1
blabla
moreblabla
relevant=yes
foo.bar.2
relevant=no
foo.bar.3
blablabla
foo.bar.4
relevant=yes

I want to retrieve all foo.bar lines where within the block following themselves and before the next foo.bar there is a line stating relevant=yes.

So the output should be:

foo.bar.1
foo.bar.4

I could of course write a program/script iterating through the lines, remembering the foo.bars and print them when there is a line saying relevant=yes following them an before the next foo.bar. But I thought there might be an out-of-the box way using standard Unix utilities (grep/sed/awk)?

Thanx for any hints!

Answer 1

If the input is processed line by line, then processing needs to go like this:

• if the current line is foo.bar, store it, forgetting any previous foo.bar line that wasn't enabled for output;
• if the current line is relevant=yes, this enables the latest foo.bar for output.

This kind of reasoning is a job for awk. (It can also be done in sed if you like pain.)

awk '
    /^foo\.bar/ { foobar = $0 }
    /^relevant=yes$/ {if (foobar != "") {print foobar; foobar = ""}}
'

Answer 2

Here's one way with sed:

sed '/foo\.bar/h;/relevant=yes/!d;x;/foo\.bar/!d' infile

Lines matching foobar are copied to hold space. All lines, except those matching relevant=yes are deleted. Exchange hold space with pattern space (this only happens when lines match relevant=yes) and delete if it doesn't match foobar.

Answer 3

Pythonic way:

>>> with open("/home/xieerqi/textfile.txt") as file:
...   for line in file:
...       if line.__contains__("foo"):
...          VAR = line
...       if line.__contains__("relevant=yes"):
...          print VAR
... 
foo.bar.1

foo.bar.4

Put together in a script:

DIR:/xieerqi
skolodya@ubuntu:$ chmod +x  relevance.py                                       

DIR:/xieerqi
skolodya@ubuntu:$ ./relevance.py textfile.txt                                  
foo.bar.1
foo.bar.4

DIR:/xieerqi
skolodya@ubuntu:$ cat relevance.py                                             
#!/usr/bin/env python
import sys

with open(sys.argv[1]) as file:
   for line in file:
       if line.__contains__("foo"):
          VAR = line.strip("\n")
       if line.__contains__("relevant=yes"):
          print VAR