sponge - soak up standard input and write to a file

       Colin Watson and Tollef Fog Heen

moreutils                                          2006-02-19                                          SPONGE(1)

sponge reads standard input and writes it out to the specified file. Unlike a shell redirect, sponge
       soaks up all its input before writing the output file. This allows constructing pipelines that read from
       and write to the same file.

       sponge preserves the permissions of the output file if it already exists.

       When possible, sponge creates or updates the output file atomically by renaming a temp file into place.
       (This cannot be done if TMPDIR is not in the same filesystem.)

       If the output file is a special file or symlink, the data will be written to it, non-atomically.

       If no file is specified, sponge outputs to stdout.

       sponge - soak up standard input and write to a file

-a
           Replace the file with a new file that contains the file's original content, with the standard input
           appended to it. This is done atomically when possible.

sed'...'file|grep'...'|sponge[-a]file