file descriptor

The 3 most used file descriptors are stdin, stdout and stderr. A command's stdout can easily be redirected to another command's sdtin using pipe mechanism.

More about file descriptor can be found in this stackoverflow answer and this tiny tutorial. In short, file descriptors are located under /dev/fd/ (e.g. stdin is /dev/fd/0) and an unnamed pipe can be created using NN>&1 (redirecting stdout to /dev/fd/NN).

piping from multiple processes

The problem is the following: I have multiple command and I want to concatenate all the outputs. An obvious and easy way is to pipe all the outputs as input to the cat command. To do so, one must group all this commands together:

$ clear ; (ls /usr ; echo "foo bar" ; grep ntp /etc/passwd) | cat
bin
include
info
lib
lib64
libexec
local
lost+found
sbin
share
src
var
foo bar
ntp:x:87:87:Network Time Protocol:/var/lib/ntp:/bin/false

moreutils

`moreutils <http://joeyh.name/code/moreutils/>`__ (available in both archlinux and debian) is a tool collection. One (pee) is fitted to pipe an output to multiple command:

$ cd /usr && ls -1
bin
include
info
lib
lib64
libexec
local
lost+found
sbin
share
src
var
$ ls -1 | pee 'tail -1' 'head -1'
var
bin