How can I use a file in a command and redirect output to the same file without truncating it?

Question

Basically I want to take as input text from a file, remove a line from that file, and send the output back to the same file. Something along these lines if that makes it any

Answer 1

You can use slurp with POSIX Awk:

!/seg[0-9]\{1,\}\.[0-9]\{1\}/ {
  q = q ? q RS $0 : $0
}
END {
  print q > ARGV[1]
}

Example