.. highlight:: console

.. role:: bash(code)
   :language: bash
   :class: highlight

Pipes
-----
Earlier, we mentioned the possibility of redirecting output to a
file. Another very powerful redirection method are :dfn:`pipes`,
used like this::

	$ cmd1 | cmd2 | cmd3 | cmd4

Instead of redirecting the output of :command:`cmd1` to a file, it will be used directly as input to :command:`cmd2` and so on.

sort, grep, sed, awk, uniq, head, tail
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In this section, you'll see some commands that are useful in the
middle of pipes. Remember to use the man pages to find options!

#. Compare::

	$ echo -e "d\nb\nc\na"
	$ echo -e "d\nb\nc\na" | sort

#. Sort these lines by *numerical* value::

	$ echo -e "3\n1\n2\n12"

#. Copy the file :file:`/usr/share/dict/words` to your working directory,
   then use :command:`grep` to find out which words contain the string
   ``physics``::

		$ grep physics words

#. Alternatively, :command:`grep` can also be used as part of a pipe::

	$ cat words | grep physics

#. How many entries in :file:`words` do not contain ``physics``? Let
   :command:`grep` do the count for you.

#. Create a file called :file:`physicswords` that contains the *physics*
   words twice. Run :command:`sort` on it. Try inserting :command:`uniq` into the pipe before or after the sort.

#. Try::

	$ sort physicswords | uniq | head -3

   Do the same with :bash:`tail -3`.

#. :command:`sed` is a very powerful command, and worth spending more time on. It is most often used for text replacement using the ``s///`` construct::

	 	$ cat physicswords | sed s/physics/foo/

#. A similarly comprehensive command is :command:`awk`. Again, we can only
   show some common uses. Compare::

   	$ ls -l ~
   	$ ls -l ~ | awk '{print $1 " abc " $3 " def " $5}'
	
   What is happening? Also try::

    $ ls -l ~ | awk '{ sum+=$5 ; print sum } END { print "===\ntotal: "sum }'

   If you have any PDF files in :file:`~`, also try matching specific lines::

    $ ls -l ~ | awk '/pdf/ { sum+=$5 ; print sum } END { print "===\ntotal: "sum }'

#. List the three oldest files in your home directory.

#. Use :command:`ls`, :command:`grep`, :command:`sort`, :command:`head` and
   :command:`awk` to find the five largest files you have. Check all your
   directories, not just :file:`~`, do not include directory sizes!
   The output should list only the file names, one per line, largest
   first. If you feel up to a challenge, try :command:`find` instead
   of :command:`ls` and :command:`grep`.

date, cal
~~~~~~~~~

Two little useful tools to close this block (they fit equally badly
anywhere else)::

  $ date
  $ cal