Bash or Python to go backwards?

Question

I have a text file which a lot of random occurrences of the string @STRING_A, and I would be interested in writing a short script which removes only some of them. Particularly one that scans the file and once it finds a line which starts with this string like ``` @STRING_A ``` then checks if 3 lines backwards there is another occurrence of a line starting with the same string, like ``` @STRING_A @STRING_A ``` and if it happens, to delete the occurrence 3 lines backward. I was thinking about bash, but I do not know how to "go backwards" with it. So I am sure that this is not possible with bash. I also thought about python, but then I should store all information in memory in order to go backwards and then, for long files it would be unfeasible. What do you think? Is it possible to do it in bash or python? Thanks

Accepted Answer

Funny that after all these hours nobody's yet given a solution to the problem as actually phrased (as @John Machin points out in a comment) -- remove just the leading marker (if followed by another such marker 3 lines down), not the whole line containing it. It's not hard, of course -- here's a tiny mod as needed of @truppo's fun solution, for example:

```
from itertools import izip, chain
f = "foo.txt"
for third, line in izip(chain("   ", open(f)), open(f)):
    if third.startswith("@STRING_A") and line.startswith("@STRING_A"):
        line = line[len("@STRING_A"):]
    print line,
```

Of course, in real life, one would use an `iterator.tee` instead of reading the file twice, have this code in a function, not repeat the marker constant endlessly, &c;-).

Bash or Python to go backwards?

Solution

Problem