sponge

Soak up the input before writing the output file. More information: https://manned.org/sponge.

• Append file content to the source file:
  cat {{path/to/file}} | sponge -a {{path/to/file}}
• Remove all lines starting with # in a file:
  grep -v '^{{#}}' {{path/to/file}} | sponge {{path/to/file}}