UNIX tarpipe, gzip

tarpipe.md

```
$ (cd src && tar -c .) | (cd dst && tar -xp)
          1^           2^        3^
```

1^ this fork a new process that execute those commands and use the writing end of the pipe as its STDOUT
2^ the pipe has two ends (to write and read), and both of them have a fd
3^ this fork a new process that execute those commands using the reading end of the pipe as its STDIN

Also, using && instead of ; to separate commands means that if the first one fails, the second one won't be executed. 
On the other side, with ; both commands would be executed regardless of their result. So, in this case, we don't want
to execute tar -c inside another directory but src. (We don't want to compress or extract the wrong directory.)

$ tar -c foo.py
foo.py000644 000765 000024 00000000466 12753374164 013011 0ustar00rcanepastaff000000 000000 from os import fork, getpid, pipe, read, write

read_end, write_end = pipe()
child_pid = fork()

if child_pid:
  print "parent is about to read"
  data = read(read_end, 1000)
  print "parent read"
  print "data:", data
else:
  print "Child is about to write"
  write(write_end, "hi dad")
  print "Child wrote"

$ tar -c foo.py  | wc -l
      14

$ tar -c foo.py  | wc -c
   10240

$ tar -c foo.py  | gzip | wc -c
     295