extracting lines of text from a long file

Try:

awk -F, '/^#/{f=$NF".txt";cmt=$0; next} {printf "%s\n%s\n",cmt,$0 >f; close(f)}' file

Example

Applied to your sample input:

$ awk -F, '/^#/{f=$NF".txt";cmt=$0; next} {printf "%s\n%s\n",cmt,$0 >f; close(f)}' file

After the above is run, the following files are in the directory:

$ ls
file  importantname1.txt  importantname2.txt  importantname3.txt

The contents of the new files are:

$ cat importantname1.txt 
#info1 info2 info3 ,importantname1
importanttext1
$ cat importantname2.txt 
#info1 info2 info3 ,importantname2
importanttext2
$ cat importantname3.txt 
#info1 info2 info3 ,importantname3
importanttext3

How it works

Awk reads through the input file line by line. Our script classifies those lines as comments or not-comments. For comment lines, the file name and comment is saved. For non-comments, a new file is created and printed

• `-F,

  This tells awk to use a comma as the field separator on input. In this way, the file name will always be the last field.

• /^#/{f=$NF".txt";cmt=$0; next}

  If a line begins with #, we save the last field, $NF, plus .txt as a file name f. The whole of the comment line is saved as cmt. We then tell awk to skip the rest of the commands and jump to start over on the next line.

• printf "%s\n%s\n",cmt,$0 >f; close(f)

  For non-comment lines, we print the last seen comment, cmt, and the current line, $0, into last seen file name f. We then close the file handle for f.

Protecting against bad file names

If the fields that are to be used as file names contain /, the OS will interpret the file names as including directories. To avoid that, we can replace all / with - using gsub(/\//, "-", f) as follows:

awk -F, '/^#/{f=$NF".txt";gsub(/\//, "-", f); cmt=$0; next} {printf "%s\n%s\n",cmt,$0 >f; close(f)}' file

A combination of grep and csplit could do the job, by a) grepping all non-commeneted lines plus the preceeding info one and b) splitting the output based on the info comment line:

grep -v -B1 '^#' file | csplit -z - '/^#/' '{*}'

I.e. do not -v extract lines that have a # at the beginning ^# but one line preceeding such lines -B1. Then split the incoming piped input - at each # at the beginning of a line, ignore empty files -z and do so as often as possible {*}.

Renaming would have to be a separate step (csplit autonames the outpit as xx00, xx01 ... - change pre- and suffix with the -f and -b options, repectively)

#/bin/bash
for f in xx* ; do
   mv "$f" "$( sed -n '2p' "$f" )".txt
done