shell script to read from multiple files in parallel

If the files are on separate disks, run one grep command on each disk.

For files on the same disk, the bottleneck is reading from the disk. Reading from multiple files in parallel will only make the speed worse.

If the files are on a RAID-0 array, you might get a speed increase by running two grep commands at the same time. Benchmark to see if you really gain time. The low-tech way:

grep file1 file2 file3 &
grep file4 file5 file6

With GNU parallel:

parallel -j 2 grep ::: file1 file2 file3 file4 file5 file6

If you're getting files from find:

find … -print0 | parallel -0 -j 2

Remember: if the files are on the same disk, a single grep command is the fastest.

I'm guessing that your files are quite large (otherwise you probably wouldn't care about parallelising the job).

The GNU parallel suggestions are good (and GNU's xargs also has a -P option for parallel execution) BUT given that grepping a file (or files) is an I/O-bound operation, not CPU-bound, you may find that running multiple greps in parallel actually slows things down because you now have multiple processes competing for disk access.

I/O speed is the limiting factor here, not CPU power. Even a single grep process is probably spending most of its time waiting for data from disk (i.e. CPU is mostly idle).

If the files are not physically close to each other on the disk, it could be MANY times slower as the disk heads have to move around a lot more (of course, this would not be a problem on an SSD or a ramdisk or if the files are already cached)

You might try GNU parallel:

find . -type f | parallel -k -j150% -n 1000 -m grep -H -n STRING {}

( from http://www.gnu.org/software/parallel/man.html#example__parallel_grep )

Edit: Note that other comments that state that grep will run faster sequentially, if the bottleneck is IO, are correct.