Linux redirection: read from and write to the same file. Use a sponge!

Installation on Debian/Ubuntu:

sudo apt-get install moreutils

From the man page:

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

It also creates the output file atomically by renaming a temp file into place, and preserves the permissions of the output file if it already exists. If the output file is a special file or symlink, the data will be written to it.

If no output file is specified, sponge outputs to stdout.

For example:

sed '...' file | grep '...' | sponge file

That’s it!

Remark: The moreutils package is in conflict with GNU parallel (parallel package) because the moreutils package comes with its own parallel command. However, the parallel command provided by the moreutils package is much less powerful than the GNU parallel. If you want to keep the GNU parallel command and use the sponge command too, you can install sponge locally in $HOME/bin for example. To do so:

git clone git://git.joeyh.name/moreutils
cd moreutils
gcc sponge.c -o ~/bin/sponge