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.. link:
.. description: is a unix tool aiding the Telefonbuch project for export and search queries.
.. tags: project
.. date: 2015/02/01 19:10:08
.. title: el
.. subtitle: extract lines from text files
.. slug: ../arts/software/el/index
.. prio: 100
.. contents::
:depth: 1
:class: ezjail-toc
----
Overview
========
*el* (from extract lines) is a unix tool to efficiently extract lines from a text file using a dynamic line end index. Its command line syntax is tuned to work with other posix tools – especially grep – to easily operate on poor man's databases where each of the database's columns is in a separate newline separated file.
Since *el* mmaps the whole file, it does not work on streams.
----
Build instructions
==================
Get the source from my git repository ``git clone git://erdgeist.org/el`` and type ``make`` in the directory ``el``. *el* has no further dependencies and can just be copied to ``/usr/local/bin`` or wherever your binaries reside.
----
Invocation
==========
In its simplest form just run::
echo 1 2 3 4 5 6 7 8 9 10 | el file.txt
to extract lines 1 to 10 from the file ``file.txt`` and print them to stdout. *el* also accepts white space separated line numbers with the ``-i`` parameter and can default to hexadecimal numbers with the ``-x`` switch::
el -i "10 11 12 13" -x file1.txt file2.txt
extracts line 16 to 19 from the files ``file.txt`` and ``file2.txt``, each line separated by the tab character. If your line numbers start at ``0``, *el* will add 1, if it is run with the ``-0`` switch.
The reason reading line numbers from stdin and not from command line by default becomes apparent, when you use the combining features of *el*, as it can parse and generate output as the ``grep`` posix tool does.
If you use the ``-n`` switch, each extracted line is prefixed by it's line number and a colon, just as ``grep`` does. If you use the ``-g`` switch, *el* will also **expect** the line numbers on stdin to be terminated by a colon followed by garbage until next newline. So you can feed results from ``grep -n`` directly to it. With the ``-G`` option, not only will *el* parse the line numbers from ``grep``'s output, but also append the extracted lines from each file to the input line. In order to chain several ``grep`` and *el* commands, best combine the ``-Gn`` switches as in this example::
grep -n Manager positions.txt | el -Gn firstname.txt lastname.txt | grep Jon | el -Gn salary.txt
This will produce, an output like this::
17:Manager\tJon\tMiller\t$1200
23:Manager\tJon\tDoe\t$1600
given that all the text files contain corresponding rows of information about the staff.
----
Author
======
*el* was written by `Dirk Engling <mailto:erdgeist@erdgeist.org>`_, who likes to hear from happy customers.
----
License
=======
*el* is considered `beer ware </beerware.html>`_.
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