# Example 1: capitalisation
tr a-z A-Z < xy.txt | reservoir -o xy.txt
tr a-z A-Z < xy.txt | sponge xy.txt

# Example 2: evaluate #ifdefs of all C files in directory
# unifdef is from http://dotat.at/prog/unifdef/
# MAKE A BACKUP BEFORE RUNNING THIS LOOP, YOUR FILES WILL BE OVERWRITTEN!
find . -iname \*.c | while read file ; do unifdef $file | reservoir -o $file ; done

URLs:  
<http://www.chiark.greenend.org.uk/~sgtatham/utils/>    <http://kitenet.net/~joey/code/moreutils/>

Use one of these nifty utilities to automatically process files in-place.  Both store the results of programs which write to stdout until they are done, and then write the result.  See also `sed`'s `-i` option.  Be aware that this may clobber your files if an error occurs during processing, so it is best to back them up (though `reservoir` refuses to truncate files to nothing by default).

Filter files in place

filter

Bash