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The unix find(1) utility is very useful allowing me to perform an action on many files that match certain specifications, e.g.

find /dump -type f -name '*.xml' -exec java -jar ProcessFile.jar {} \;

The above might run a script or tool over every XML file in a particular directory.

Let's say my script/program takes a lot of CPU time and I have 8 processors. It would be nice to process up to 8 files at a time.

GNU make allows for parallel job processing with the -j flag but find does not appear to have such functionality. Is there an alternative generic job-scheduling method of approaching this?

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up vote 33 down vote accepted

xargs with the -P option (number of processes). Say I wanted to compress all the logfiles in a directory on a 4-cpu machine:

find . -name '*.log' -mtime +3 -print0 | xargs -0 -P 4 bzip2

You can also say -n <number> for the maximum number of work-units per process. So say I had 2500 files and I said:

find . -name '*.log' -mtime +3 -print0 | xargs -0 -n 500 -P 4 bzip2

This would start 4 bzip2 processes, each of which with 500 files, and then when the first one finished another would be started for the last 500 files.

Not sure why the previous answer uses xargs and make, you have two parallel engines there!

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With find/xargs, be careful: find defaults to newlines as output delimiters, but xargs defaults to any whitespace as input delimiters. Use -0 on both to be safe, or switch to GNU parallel which defaults to newlines as input delimiters (matching find's output). – ephemient Oct 23 '10 at 22:08
Wow, amazing! I just checked, and it's true, xargs has a -P option! – PP. Oct 25 '10 at 9:45

GNU parallel can help too.

find /dump -type f -name '*.xml' | parallel -j8 java -jar ProcessFile.jar {}

Note that without the -j8 argument, parallel defaults to the number of cores on your machine :-)

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No need to "fix" find - make use of make itself to handle the parallelism.

Have your process create a log file or some other output file, and then use a Makefile like this:

.SUFFIXES:  .xml .out

        java -jar ProcessFile.jar $< 1> $@

and invoked thus:

find /dump -type f -name '*.xml' | sed -e 's/\.xml$/.out/' | xargs make -j8

Better yet, if you ensure that the output file only gets created on successful completion of the Java process you can take advantage of make's dependency handling to ensure that next time around only unprocessed files get done.

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Brilliant! Thank you. – PP. Oct 21 '10 at 14:46
Hopefully there's no spaces or other "interesting" characters in those filenames; Make doesn't handle those very elegantly. – ephemient Oct 23 '10 at 22:10

Find has a parallel option you can use directly using the "+" symbol; no xargs required. Combining it with grep, it can rip through your tree quickly looking for matches. for example, if I'm looking for all files in my sources directory containing the string 'foo', I can invoke
find sources -type f -exec grep -H foo {} +

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Reading the find manual, you can see that the -exec command + syntax doesn't run it in parallel, but "group" many files together and run the command with multiple files as arguments at the same time. It happens that grep can look through its targets in parallel. – Gyscos Mar 21 at 23:27

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