#!/bin/bash
for file in folder/*.txt do
  grep 'sometext:' $file | sed '/^.*:s*//' >> list.txt
  #doing simliar stuff with other lines in the current file
done

randomtext: ...
sometext: Hello
randomtext: ...
someothertext: World
somedifferenttext: !
randomtext:

grep 'sometext:' folder/*.txt

$ awk '{
        if($1~/sometext|someothertext|somedifferenttext/){
            printf "%s,",$2
        }
        if(FNR==1 && NR>1){
            print ""
        }
    }
    END{ print "" }' folder/*txt
Hello,World,!,
Hello,World,!,
Hello,World,!,
Hello,World,!,
Hello,World,!,
Hello,World,!,
Hello,World,!,
Hello,World,!,
Hello,World,!,
Hello,World,!,
Hello,World,!,

if($1=="sometext:" || $1=="someothertext:" || $1=="somedifferenttext:"){

awk '{
    if($1=="sometext:" ||
       $1=="someothertext:" ||
       $1=="somedifferenttext:"){
        $1="";
        printf "%s,",$0
    }
    if(FNR==1 && NR>1){print ""}
    }END{print ""}' folder/*txt | sed 's/^ //'

$ perl -lane '
    if($F[0]=~/(sometext|someothertext|somedifferenttext):/){
        push @k,@F[1..$#F]
    }
    if(eof){
        print join ",", @k; @k=();
    }' folder/file*
Hello,World,!
Hello,World,!
Hello,World,!
Hello,World,!
Hello,World,!
Hello,World,!
Hello,World,!
Hello,World,!
Hello,World,!
Hello,World,!
Hello,World,!

$ perl -lane '
    if($F[0]=~/^(sometext|someothertext|somedifferenttext):$/){
        push @k,@F[1..$#F]
    }
    if(eof){
        print join ",", @k , ""; @k=();
    }' folder/file*
Hello,World,!,
Hello,World,!,
Hello,World,!,
Hello,World,!,
Hello,World,!,
Hello,World,!,
Hello,World,!,
Hello,World,!,
Hello,World,!,
Hello,World,!,
Hello,World,!,

$ for f in folder/*; do
    grep -hoP '^(sometext|someothertext|somedifferenttext): K.*' "$f" |
        perl -pe 's/n/,/; END{print "n"}';
  done
Hello,World,!,
Hello,World,!,
Hello,World,!,
Hello,World,!,
Hello,World,!,
Hello,World,!,
Hello,World,!,
Hello,World,!,
Hello,World,!,
Hello,World,!,
Hello,World,!,