Sort a file based on key value pair at a different position in each line

Question 1

If sequence key is garanteed not only to be in each line, but to be unique and its value not to exceed lines count (as in example), you could do the following:

Allocate an array of lines size.
For every line:

2.1 Retrieve sequence number in text by slicing it with "sequence=" and a space.

2.2 Turn number in text into index.

2.3 Put a line into the corresponding cell of a new array.

In Python it would be like this:

lines = [
"key1=blah key2=something key4=else sequence=3",
"sequence=1 key2=xlde key7=eldl",
"blahkey=xxx sequence=2 keyx=adada"
]

new_lines = [""] * len(lines)

for line in lines:
    after_sequence = line.split("sequence=")[1]
    and_before_space = after_sequence.split(" ")[0]
    n = int(and_before_space) - 1
    new_lines[n] = line

print new_lines

Question 2

If I were doing this in perl, I would slurp the entire file in and munge it so that I could sort the original raw lines based on their sequence number. I'm not sure how consistent your file format is, but one perl approach might be:

#!/usr/bin/perl -w

my @data;

# slurp in each line, and tag it by its sequence number
foreach my $line ( <STDIN> )
{
    if ($line =~ /sequence=(\S+)/)
    {
        push @data, { sequence => $1, line => $line };
    } else
    {
        die "unhandled line: $line";  # only if needed
    }
}

# sort the lines by their sequence number into @sorted
my @sorted = sort { $a->{sequence} <=> $b->{sequence} } @data;

# generate the final result by extracting the original lines
# from the sorted array and concatenating them
my $result = join("", map { $_->{line} } @sorted);

# output the sorted result
print $result;

I tried this on your example above and it did the trick. You might massage the die line if you could have "garbage" lines in the input that the script can safely ignore.

Also, if you need to switch between ascending and descending sequence order, you can swap $a and $b in this line:

my @sorted = sort { $a->{sequence} <=> $b->{sequence} } @data;

If the sequence numbers aren't purely numeric, or you want to compare them as strings, change the <=> operator for cmp:

my @sorted = sort { $a->{sequence} cmp $b->{sequence} } @data;