Read the log
Objective
Find the flag in a web server log, without reading every line.
The briefing
A web server log holds one line per request. Four fields, in order: the address that asked, the time, the request itself, and the status code.
address - - [time] "GET /path HTTP/1.1" status size
There are dozens of lines, and reading them by hand is a waste of your time. A search tool will find a pattern in a file and show you only the lines that match.
One warning. More than one line contains something that looks like a flag. Only one of them is real, and you can tell them apart by shape: your flag holds exactly sixteen characters, and every one of them is a digit or a letter from a to f.
Set it up
Run this in Termux. It teaches the concept above a second time, then writes the challenge files into your home directory.
ctf start 04 The tool tells you how many files it wrote, never their names.
Then find the flag and hand it in with ctf check 04 LEARMUX{...}.
Hints
Two of them, revealed one at a time. The tool works the same way and remembers how many you have seen, so nothing is spoiled by accident.