Basic sed command on large one-line file: couldn’t re-allocate memory

#!/usr/bin/awk -f

BEGIN { FS = "\t"; OFS = "," }
{
    for(i = 1; i <= NF; i++) {
        if ($i + 0 == $i) { $i = "=" $i }
        else gsub(/"/, "\"\"", $i);
        $i = "\"" $i "\""
    }
    print
}

Assuming you name this convert.awk, you can either call with either

ec2-describe-snapshots -H --hide-tags | awk -f convert.awk > snapshots.csv

or (after adding execute permissions, chmod a+x convert.awk)

ec2-describe-snapshots -H --hide-tags | ./convert.awk > snapshots.csv

This will make a new column for each tab, which will keep the comment column together (unless it contains tabs), but add empty columns (though that is how your sample output looks, so maybe you actually do want that). If you want to split on all whitespace (this will collapse extra tabs within the table but put each word in the description as a new column), take out the FS="\t"; statement.

For future generations, if you don't need the "s or =s or embedded whitespace, you can make it a one-liner:

awk -v OFS=, '{$1=$1;print}'

An awk solution seen on #bash (Freenode):

awk '!seen[$0]++' filename