Parallel shell loops

Solution 1:

A makefile is a good solution to your problem. You could program this parallel execution in a shell, but it's hard, as you've noticed. A parallel implementation of make will not only take care of starting jobs and detecting their termination, but also handle load balancing, which is tricky.

The requirement for globbing is not an obstacle: there are make implementations that support it. GNU make, which has wildcard expansion such as $(wildcard *.c) and shell access such as $(shell mycommand) (look up functions in the GNU make manual for more information). It's the default make on Linux, and available on most other systems. Here's a Makefile skeleton that you may be able to adapt to your needs:

sources = $(wildcard *.src)

all: $(sources:.src=.tgt)

%.tgt: %.src
    do_something $< $$(derived_params $<) >$@

Run something like make -j4 to execute four jobs in parallel, or make -j -l3 to keep the load average around 3.

Solution 2:

I am not sure what your derived arguments are like. But with GNU Parallel http:// www.gnu.org/software/parallel/ you can do this to run one job per cpu core:

find . | parallel -j+0 'a={}; name=${a##*/}; upper=$(echo "$name" | tr "[:lower:]" "[:upper:]");
   echo "$name - $upper"'

If what you want to derive is simply changing the .extension the {.} may be handy:

parallel -j+0 lame {} -o {.}.mp3 ::: *.wav

Watch the intro video to GNU Parallel at http://www.youtube.com/watch?v=OpaiGYxkSuQ

Solution 3:

Wouldn't using the shell's wait command work for you?

for i in *
do
    do_something $i &
done
wait

Your loop executes a job then waits for it, then does the next job. If the above doesn't work for you, then yours might work better if you move pwait after done.

Solution 4:

Why has nobody mentioned xargs yet?

Assuming you have exactly three arguments,

for i in *.myfiles; do echo -n $i `derived_params $i` other_params; done | xargs -n 3 -P $PROCS do_something

Otherwise use a delimiter (null is handy for that):

for i in *.myfiles; do echo -n $i `derived_params $i` other_params; echo -ne "\0"; done | xargs -0 -n 1 -P $PROCS do_something

EDIT: for the above, each parameter should be separated by a null character, and then the number of parameters should be specified with the xargs -n.