Linux – How to grep multiple lines from a file in linux

The sed/wc complexity can be avoided in previous answers if awk is used. Using example provided from OP (showing complete lines before 10000 bytes):

awk '{i += (length() + 1); if (i <= 10000) print $ALL}' myfile.txt

Also showing the complete line containing 10000th byte if that byte is not at end of line:

awk '{i += (length() + 1); print $ALL; if (i >= 10000) exit}' myfile.txt

The answer above assumes:

1. Text file are of Unix line terminator (\n). For Dos/Windows text files (\r\n), change length() + 1 to length() + 2
2. Text file only contains single byte character. If there's multibyte character (such as under unicode environment), set environment LC_CTYPE=C to force interpretation on byte level.

some python: warning, I'm pretty new to it.

import io
import datetime

buffer = io.StringIO()
ago = datetime.date.today() - datetime.timedelta(days=30)
filename = "file"

with open(filename,"r") as f:
    line = f.readline()
    while line:
        if line.startswith("#"):
            date = (line.split())[1]
            if date >= ago.isoformat():
                buffer.write(line)
                line = f.readline()
                buffer.write(line)
        line = f.readline()

with open(filename, "w") as f:
    f.write(buffer.getvalue())