That would work but what would tail do different than sed here? Both would still need to read and write the full file (minus 4 lines for tail) which without testing indicates the time would be the same.
I deleted the line with `sed -i "4d" sqldump`. The duplicated disk space comes from sed writing a temporary file out to the current directory, it does this in chunks as the command runs.
It makes sense for sed to write a temp file even with `-i` because what happens if your power goes out mid-way through the command without the temp file? You'd have data loss. To combat that sed will write a temp file out and when the operation is completed the last step is to move the temp file to the original file to overwrite it. You might not notice this unless you monitor the directory for changes while sed runs. You can spam `ls -la` as sed runs on a decently sized file to see the temp file.
