.. _playing_sed_running:

How to run :command:`sed` ?
---------------------------
There are two ways of running :command:`sed`:

- you can give the commands on the command line::

    sed 's/yes/done/' file

  If you want to use several commands, you have to use the ``-e`` option::

     sed -e 's/yes/done/' -e 's/no/not-done/' file

- it is also possible to place the commands in a seperate file::

    sed -f sedsrc file

  where :file:`sedsrc` contains the necessary commands.

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.. _sed_exercises:

********

**Exercises**

********

#. On our parallel machines, the nodes where the command can run are listed in
   the file :file:`/usr/spool/machines`. However the first node is mentioned 2
   times. How can we remove the first line of this file, so that each node is
   mentioned one time ? How can we count how many nodes we have in this parallel
   environment ? (Use the command :command:`wc`)

   An example file is at ``/data/tccm/linux_bash/5/machines``

   .. Solution
    cat /usr/spool/machines | sed 1d
    cat /usr/spool/machines | sed 1d | wc -l

#. How can we use :file:`machines` from previous exercises to
   duplicate each node, so to make a machines file where each node is mentioned
   two times ?

   .. Solution
    cat /usr/spool/machines | sed -e 1d -e p

#. How should you make a general gaussian-inputfile, where the number of
   processors is not specified but filled in on run-time ? 

   - to get the number of cores available, see in the file
     :file:`/proc/cpuinfo` and use :command:`grep` and :command:`wc` [#store]_
   - to specify the number of procs in a gaussian-inputfile, you have to add a
     line like::

        %nprocshared=8
     
   How can you make this general so that the number is always correct for the
   node where the job is running ?

********

.. [#store] You can store the output of a command in a variable by including
            the command in $(...), (see :ref:`this section <shell_expansion_command>`)
            For example

            biggest_file=$(ls -lSr | tail -1)
..