sponge

Overwrite file with pipeline output

$ cat [file.txt] | sort | sponge [file.txt]

$ grep "[pattern]" [file.txt] | sponge [file.txt]

$ sed 's/old/new/g' [file.txt] | sponge [file.txt]

$ echo "new line" | sponge -a [file.txt]

sponge [options] file

sponge reads standard input and writes to a file. Unlike shell redirection, it reads all input before opening the output file, allowing safe in-place modifications.
The tool is part of moreutils and solves the problem of using a file as both input and output in a pipeline.

-a

Append to file instead of overwriting.

Without sponge (fails):

$ sort file.txt > file.txt  # Results in empty file!

$ sort file.txt | sponge file.txt

Loads entire input into memory. Very large files may exhaust memory. Part of moreutils package. Creates temp file internally.

sponge is part of moreutils, a collection of Unix tools created by Joey Hess. It addresses a common shell scripting pitfall where redirection to the input file truncates it before reading.

tee(1), cat(1), moreutils(7)