Sponge from moreutils – Differences and Useful Examples

command lineio-redirectionshellUtilities

> brew install moreutils                                                          
==> Downloading https://homebrew.bintray.com/bottles/moreutils-0.55.yosemite.bottle.tar.gz    
######################################################################## 100.0%               
==> Pouring moreutils0.55.yosemite.bottle.tar.gz       
?  /usr/local/Cellar/moreutils/0.55: 67 files, 740K

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.

I don't understand. Please give me some useful examples.

What does soaks up mean?

Best Answer

Assume that you have a file named input, you want to remove all line start with # in input. You can get all lines don't start with # using:

grep -v '^#' input

But how do you make changes to input? With standard POSIX toolchest, you need to use a temporary file, some thing like:

grep -v '^#' input >/tmp/input.tmp
mv /tmp/input.tmp ./input

With shell redirection:

grep -v '^#' input >input

will truncate input before you reading from it.

With sponge, you can:

grep -v '^#' input | sponge input
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