If I understand your question correctly perl -i might also do the trick (see perlrun for details, or type perldoc perlrun on your machine if www.perldoc.com is down): 
perl -i -p -e '
BEGIN {
print "what length?\n";
$sequenceLength = getc;
}
s/$/$sequenceLength/' seq.txt

[download]

will print the same question you asked for and add it to all lines in the file seq.txt.

Note that your getccall will only fetch 1 character, so if I answer '33' to the first question, it will only add a '3'. You might want to replace the getc part with something like: 

$sequenceLength = <STDIN>; chomp($sequenceLength);

[download]

In reply to Re: how to read and write at the same time + a problem with seek by eXile
in thread how to read and write at the same time + a problem with seek by wolffm

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