Shell parallel processing

This is the description of the tool from the gnu site - Parallel
GNU parallel is a shell tool for executing jobs in parallel using one
or more computers. A job it can be a single command or a small script
that has to be run for each of the lines in the input. The typical
input is a list of files, a list of hosts, a list of users, a list of
URLs, or a list of tables. A job can also be a command that reads from
a pipe. GNU parallel can then split the input into blocks and pipe a
lock into each command in parallel.

The tool can do many things and has some very useful tools that come with it
see sql and niceload.
Bellow you can see some example on how to use it.

#!/bin/sh

# tail log files on different computers
# create a hosts file with all the computers you want to connect

echo '10.100.218.79' >> host.file
echo '107.22.24.219' >> host.file
cat host.file | parallel ssh {} "tail /var/log/php-fpm/error.log | awk '{print \$1,\$2,\$3,\$4,\$5,\$6}'"

[20-Feb-2012 15:05:05.323178] DEBUG: pid 7812, fpm_pctl_perform_idle_server_maintenance(),
[20-Feb-2012 15:05:06.324028] DEBUG: pid 7812, fpm_pctl_perform_idle_server_maintenance(),
[20-Feb-2012 15:05:07.324877] DEBUG: pid 7812, fpm_pctl_perform_idle_server_maintenance(),
[20-Feb-2012 15:05:08.325727] DEBUG: pid 7812, fpm_pctl_perform_idle_server_maintenance(),
[20-Feb-2012 15:05:09.326568] DEBUG: pid 7812, fpm_pctl_perform_idle_server_maintenance(),
[20-Feb-2012 15:05:10.327418] DEBUG: pid 7812, fpm_pctl_perform_idle_server_maintenance(),
[20-Feb-2012 15:05:11.328265] DEBUG: pid 7812, fpm_pctl_perform_idle_server_maintenance(),
[20-Feb-2012 15:05:12.329118] DEBUG: pid 7812, fpm_pctl_perform_idle_server_maintenance(),
[20-Feb-2012 15:05:13.329960] DEBUG: pid 7812, fpm_pctl_perform_idle_server_maintenance(),
[20-Feb-2012 15:05:14.330806] DEBUG: pid 7812, fpm_pctl_perform_idle_server_maintenance(),

GNU's site has lots more example see Examples

Recursive grep with filters - command line

Problem:
you have a list o directories that you want to search a pattern BUT you want to exclude a few directories

Solution:
use ls and grep recursive and apply a filter for the directory that you want to exlude

shell$ pwd
/home/
shell$ ls
work documents logs bin
# you want to search on all except logs for pattern 'invoice'

shell$ for i in `ls` ;do  if [ $i != 'logs' ]; then grep -r 'invoice' $i; fi; done
... output

what it happens is this:

the for loop executes based on the command `ls`
all the output is taken and feed it to the do
inside the do we are filtering off 'logs' (with the if) and then search for invoice with grep -r