# do it once
seq 1 | parallel -n0 "curl -H 'Content-Type: application/json' http://httpbin.org/post -X POST -d '{\"url\":\"http://google.com/\"}'"

# do it twice
seq 2 | parallel -n0 "curl -H 'Content-Type: application/json' http://httpbin.org/post -X POST -d '{\"url\":\"http://google.com/\"}'"

# do it 4 times, but at 2 a time
seq 4 | parallel -n0 -j2 "curl -H 'Content-Type: application/json' http://httpbin.org/post -X POST -d '{\"url\":\"http://google.com/\"}'"

# you can also put all your commands into a file
echo "ls\nls" > somefile
cat somefile | parallel

# finally, just straight command line parameters
parallel echo ::: a b c

# by default, it runs as many jobs in parallel as there are cores in your computer

# if you try and set more jobs than there are cores, they will be concurrent, but 
# they will only ever be parallel up to your core count

# for httpie use the --ignore-stdin flag, or else it gets mixed up
seq 1 | parallel -n0 "http --ignore-stdin POST http://httpbin.org/post url=http://google.com/"