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}}