1 More text processing

     2 ====================

     3 

     4 ``sort``

     5 --------

     6 Let's say we have a file which lists a few of the stalwarts of the open source community and a few details about them, like their "other" name, their homepage address, and what they are well known for or their claim to fame. 

     7 

     8 ::

     9 

    10   Richard Stallman%rms%GNU Project

    11   Eric Raymond%ESR%Jargon File

    12   Ian Murdock% %Debian

    13   Lawrence Lessig% %Creative Commons

    14   Linus Torvalds% %Linux Kernel

    15   Guido van Rossum%BDFL%Python

    16   Larry Wall% %Perl

    17 

    18 

    19 The sort command enables us to do this in a flash! Just running the sort command with the file name as a parameter sorts the lines of the file alphabetically and prints the output on the terminal. 

    20 ::

    21 

    22   $ sort stalwarts.txt 

    23   Eric Raymond%ESR%Jargon File

    24   Guido van Rossum%BDFL%Python

    25   Ian Murdock% %Debian

    26   Larry Wall% %Perl

    27   Lawrence Lessig% %Creative Commons

    28   Linus Torvalds% %Linux Kernel

    29   Richard Stallman%rms%GNU Project

    30 

    31 If you wish to sort them reverse alphabetically, you just need to pass the ``-r`` option. Now, you might want to sort the lines, based on each person's claim to fame or their "other" name. What do we do in that case? 

    32 

    33 Below is an example that sorts the file based on "other" names. 

    34 ::

    35 

    36   $ sort -t % -k 2,2  stalwarts.txt

    37 

    38   Ian Murdock% %Debian

    39   Larry Wall% %Perl

    40   Lawrence Lessig% %Creative Commons

    41   Linus Torvalds% %Linux Kernel

    42   Guido van Rossum%BDFL%Python

    43   Eric Raymond%ESR%Jargon File

    44   Richard Stallman%rms%GNU Project

    45 

    46 Sort command assumes white space to be the default delimiter for columns in each line. The ``-t`` option specifies the delimiting character, which is ``%`` in this case. 

    47 

    48 The ``-k`` option starts a key at position 2 and ends it at 2, essentially telling the sort command that it should sort based on the 2nd column, which is the other name. ``sort`` also supports conflict resolution using multiple columns for sorting. You can see that the first three lines have nothing in the "other" names column. We could resolve the conflict by sorting based on the project names (the 3rd column). 

    49 

    50 ::

    51 

    52   $ sort -t % -k 2,2 -k 3,3  stalwarts.txt

    53   

    54   Lawrence Lessig% %Creative Commons

    55   Ian Murdock% %Debian

    56   Linus Torvalds% %Linux Kernel

    57   Larry Wall% %Perl

    58   Guido van Rossum%BDFL%Python

    59   Eric Raymond%ESR%Jargon File

    60   Richard Stallman%rms%GNU Project

    61 

    62 ``sort`` also has a lot of other options like ignoring case differences, month sort(JAN<FEB<...), merging already sorted files. ``man sort`` would give you a lot of information. 

    63 

    64 

    65 ``uniq``

    66 --------

    67 

    68 Suppose we have a list of items, say books, and we wish to obtain a list which names of all the books only once, without any duplicates. We use the ``uniq`` command to achieve this. 

    69 

    70 ::

    71 

    72   Programming Pearls

    73   The C Programming Language

    74   The Mythical Man Month: Essays on Software Engineering 

    75   Programming Pearls

    76   The C Programming Language

    77   Structure and Interpretation of Computer Programs

    78   Programming Pearls

    79   Compilers: Principles, Techniques, and Tools

    80   The C Programming Language

    81   The Art of UNIX Programming

    82   Programming Pearls

    83   The Art of Computer Programming

    84   Introduction to Algorithms

    85   The Art of UNIX Programming

    86   The Pragmatic Programmer: From Journeyman to Master

    87   Programming Pearls

    88   Unix Power Tools

    89   The Art of UNIX Programming

    90 

    91 Let us try and get rid of the duplicate lines from this file using the ``uniq`` command. 

    92 

    93 ::

    94 

    95   $ uniq items.txt 

    96   Programming Pearls

    97   The C Programming Language

    98   The Mythical Man Month: Essays on Software Engineering 

    99   Programming Pearls

   100   The C Programming Language

   101   Structure and Interpretation of Computer Programs

   102   Programming Pearls

   103   Compilers: Principles, Techniques, and Tools

   104   The C Programming Language

   105   The Art of UNIX Programming

   106   Programming Pearls

   107   The Art of Computer Programming

   108   Introduction to Algorithms

   109   The Art of UNIX Programming

   110   The Pragmatic Programmer: From Journeyman to Master

   111   Programming Pearls

   112   Unix Power Tools

   113   The Art of UNIX Programming

   114 

   115 Nothing happens! Why? The ``uniq`` command removes duplicate lines only when they are next to each other. So, we get a sorted file from the original file and work with that file, henceforth. 

   116 

   117 ::

   118 

   119   $ sort items.txt > items-sorted.txt

   120   $ uniq items-sorted.txt

   121   Compilers: Principles, Techniques, and Tools

   122   Introduction to Algorithms

   123   Programming Pearls

   124   Structure and Interpretation of Computer Programs

   125   The Art of Computer Programming

   126   The Art of UNIX Programming

   127   The C Programming Language

   128   The Mythical Man Month: Essays on Software Engineering 

   129   The Pragmatic Programmer: From Journeyman to Master

   130   Unix Power Tools

   131 

   132 ``uniq -u`` command gives the lines which are unique and do not have any duplicates in the file. ``uniq -d`` outputs only those lines which have duplicates. The ``-c`` option displays the number of times each line occurs in the file. 

   133 ::

   134 

   135   $ uniq -u items-sorted.txt 

   136   Compilers: Principles, Techniques, and Tools

   137   Introduction to Algorithms

   138   Structure and Interpretation of Computer Programs

   139   The Art of Computer Programming

   140   The Mythical Man Month: Essays on Software Engineering 

   141   The Pragmatic Programmer: From Journeyman to Master

   142   Unix Power Tools

   143 

   144   $ uniq -dc items-sorted.txt      

   145   5 Programming Pearls

   146   3 The Art of UNIX Programming

   147   3 The C Programming Language

   148 

   149 

   150 ``join``

   151 --------

   152 

   153 Now suppose we had the file ``stalwarts1.txt``, which lists the home pages of all the people listed in ``stalwarts.txt``.

   154 ::

   155 

   156   Richard Stallman%http://www.stallman.org

   157   Eric Raymond%http://www.catb.org/~esr/

   158   Ian Murdock%http://ianmurdock.com/

   159   Lawrence Lessig%http://lessig.org

   160   Linus Torvalds%http://torvalds-family.blogspot.com/

   161   Guido van Rossum%http://www.python.org/~guido/

   162   Larry Wall%http://www.wall.org/~larry/

   163 

   164 It would be nice to have a single file with the information in both the files. To achieve this we use the ``join`` command. 

   165 ::

   166 

   167   $ join stalwarts.txt stalwarts1.txt -t %

   168   Richard Stallman%rms%GNU Project%http://www.stallman.org

   169   Eric Raymond%ESR%Jargon File%http://www.catb.org/~esr/

   170   Ian Murdock% %Debian%http://ianmurdock.com/

   171   Lawrence Lessig% %Creative Commons%http://lessig.org

   172   Linus Torvalds% %Linux Kernel%http://torvalds-family.blogspot.com/

   173   Guido van Rossum%BDFL%Python%http://www.python.org/~guido/

   174   Larry Wall% %Perl%http://www.wall.org/~larry/

   175 

   176 The ``join`` command joins the two files, based on the common field present in both the files, which is the name, in this case. 

   177 

   178 The ``-t`` option again specifies the delimiting character. Unless that is specified, join assumes that the fields are separated by spaces. 

   179 

   180 Note that, for ``join`` to work, the common field should be in the same order in both the files. If this is not so, you could use ``sort``, to sort the files on the common field and then join the files. In the above example, we have the common field to be the first column in both the files. If this is not the case we could use the ``-1`` and ``-2`` options to specify the field to be used for joining the files. 

   181 ::

   182 

   183   $ join -2 2 stalwarts.txt stalwarts2.txt -t %

   184   Richard Stallman%rms%GNU Project%http://www.stallman.org

   185   Eric Raymond%ESR%Jargon File%http://www.catb.org/~esr/

   186   Ian Murdock% %Debian%http://ianmurdock.com/

   187   Lawrence Lessig% %Creative Commons%http://lessig.org

   188   Linus Torvalds% %Linux Kernel%http://torvalds-family.blogspot.com/

   189   Guido van Rossum%BDFL%Python%http://www.python.org/~guido/

   190   Larry Wall% %Perl%http://www.wall.org/~larry/

   191 

   192 

   193 Generating a word frequency list

   194 ================================

   195 

   196 Now, let us use the tools we have learnt to use, to generate a word frequency list of a text file. We shall use the free text of Alice in Wonderland.

   197 

   198 The basic steps to achieve this task would be -

   199 

   200 1. Eliminate the punctuation and spaces from the document. 

   201 2. Generate a list of words.

   202 3. Count the words.

   203 

   204 We first use ``grep`` and some elementary ``regex`` to eliminate the non-alpha-characters. 

   205 ::

   206 

   207   $ grep "[A-Za-z]*" alice-in-wonderland.txt

   208 

   209 This outputs all the lines which has any alphabetic characters on it. This isn't of much use, since we haven't done anything with the code. We only require the alphabetic characters, without any of the other junk. ``man grep`` shows us the ``-o`` option for outputting only the text which matches the regular expression.

   210 ::

   211 

   212   $ grep "[A-Za-z]*" -o alice-in-wonderland.txt

   213 

   214 Not very surprisingly, we have all the words, spit out in the form of a list! Now that we have a list of words, it is quite simple to count the occurrences of the words. You would've realized that we can make use of ``sort`` and ``uniq`` commands. We pipe the output from the ``grep`` to the ``sort`` and then pipe it's output to ``uniq``.

   215 ::

   216   

   217   $ grep "[A-Za-z]*" -o alice-in-wonderland.txt | sort | uniq -c 

   218 

   219 Notice that you get the list of all words in the document in the alphabetical order, with it's frequency written next to it. But, you might have observed that Capitalized words and lower case words are being counted as different words. We therefore, replace all the Upper case characters with lower case ones, using the ``tr`` command. 

   220 ::

   221 

   222   $ grep  "[A-Za-z]*" -o alice-in-wonderland.txt | tr 'A-Z' 'a-z' | sort | uniq -c 

   223 

   224 Now, it would also be nice to have the list ordered in the decreasing order of the frequency of the appearance of the words. We sort the output of the ``uniq`` command with ``-n`` and ``-r`` options, to get the desired output. 

   225 ::

   226 

   227   $ grep  "[A-Za-z]*" -o alice-in-wonderland.txt | tr 'A-Z' 'a-z' | sort | uniq -c | sort -nr

   228 

   229 Basic editing and editors

   230 =========================

   231 

   232 vim

   233 ---

   234 Vim is a very powerful editor. It has a lot of commands, and all of them cannot be explained here. We shall try and look at a few, so that you can find your way around in vim. 

   235 

   236 To open a file in vim, we pass the filename as a parameter to the ``vim`` command. If a file with that filename does not exist, a new file is created. 

   237 ::

   238 

   239   $ vim first.txt

   240 

   241 To start inserting text into the new file that we have opened, we need to press the ``i`` key. This will take us into the *insert* mode from the *command* mode. Hitting the ``esc`` key, will bring us back to the *command* mode. There is also another mode of vim, called the *visual* mode which will be discussed later in the course. 

   242 

   243 In general, it is good to spend as little time as possible in the insert mode and extensively use the command mode to achieve various tasks. 

   244 

   245 To save the file, use ``:w`` in the command mode. From here on, it is understood that we are in the command mode, whenever we are issuing any command to vim. 

   246 

   247 To save a file and continue editing, use ``:w FILENAME``

   248 The file name is optional. If you do not specify a filename, it is saved in the same file that you opened. If a file name different from the one you opened is specified, the text is saved with the new name, but you continue editing the file that you opened. The next time you save it without specifying a name, it gets saved with the name of the file that you initially opened. 

   249 

   250 To save file with a new name and continue editing the new file, use ``:saveas FILENAME``

   251 

   252 To save and quit, use ``:wq``

   253 

   254 To quit, use ``:q``

   255 

   256 To quit without saving, use ``:q!``

   257 

   258 Moving around

   259 ~~~~~~~~~~~~~

   260 

   261 While you are typing in a file, it is in-convenient to keep moving your fingers from the standard position for typing to the arrow keys. Vim, therefore, provides alternate keys for moving in the document. Note again that, you should be in the command mode, when issuing any commands to vim. 

   262 

   263 The basic cursor movement can be achieved using the keys, ``h`` (left), ``l`` (right), ``k`` (up) and ``j`` (down). 

   264 ::

   265  

   266              ^

   267              k              

   268        < h       l >        

   269              j              

   270              v

   271 

   272 Note: Most commands can be prefixed with a number, to repeat the command. For instance, ``10j`` will move the cursor down 10 lines. 

   273 

   274 Moving within a line

   275 ++++++++++++++++++++

   276 

   277 +----------------------------------------+---------+

   278 | Cursor Movement                        | Command | 

   279 +========================================+=========+

   280 | Beginning of line                      | ``0``   |

   281 +----------------------------------------+---------+

   282 | First non-space character of line      | ``^``   |

   283 +----------------------------------------+---------+

   284 | End of line                            | ``$``   |

   285 +----------------------------------------+---------+

   286 | Last non-space character of line       | ``g_``  |

   287 +----------------------------------------+---------+

   288 

   289 Moving by words and sentences

   290 +++++++++++++++++++++++++++++

   291 +------------------------------+---------+

   292 | Cursor Movement              | Command |

   293 +==============================+=========+

   294 | Forward, word beginning      | ``w``   |

   295 +------------------------------+---------+

   296 | Backward, word beginning     | ``b``   |

   297 +------------------------------+---------+

   298 | Forward, word end            | ``e``   |

   299 +------------------------------+---------+

   300 | Backward, word end           | ``ge``  |

   301 +------------------------------+---------+

   302 | Forward, sentence beginning  | ``)``   |

   303 +------------------------------+---------+

   304 | Backward, sentence beginning | ``(``   |

   305 +------------------------------+---------+

   306 | Forward, paragraph beginning | ``}``   |

   307 +------------------------------+---------+

   308 | Backward, paragraph beginning| ``{``   |

   309 +------------------------------+---------+

   310 

   311 More movement commands

   312 ++++++++++++++++++++++

   313 +---------------------------------+------------+

   314 | Cursor Movement                 | Command    |

   315 +=================================+============+

   316 | Forward by a screenful of text  | ``C-f``    |

   317 +---------------------------------+------------+

   318 | Backward by a screenful of text | ``C-b``    |

   319 +---------------------------------+------------+

   320 | Beginning of the screen         | ``H``      |

   321 +---------------------------------+------------+

   322 | Middle of the screen            | ``M``      |

   323 +---------------------------------+------------+

   324 | End of the screen               | ``L``      |

   325 +---------------------------------+------------+

   326 | End of file                     | ``G``      |

   327 +---------------------------------+------------+

   328 | Line number ``num``             | ``[num]G`` |

   329 +---------------------------------+------------+

   330 | Beginning of file               | ``gg``     |

   331 +---------------------------------+------------+

   332 | Next occurrence of the text     | ``*``      |

   333 | under the cursor                |            |

   334 +---------------------------------+------------+

   335 | Previous occurrence of the text | ``#``      |

   336 | under the cursor                |            |

   337 +---------------------------------+------------+

   338 

   339 Note: ``C-x`` is ``Ctrl`` + ``x``

   340 

   341 The visual mode

   342 ~~~~~~~~~~~~~~~

   343 The visual mode is a special mode that is not present in the original vi editor. It allows us to highlight text and perform actions on it. All the movement commands that have been discussed till now work in the visual mode also. The editing commands that will be discussed in the future work on the visual blocks selected, too. 

   344 

   345 Editing commands

   346 ~~~~~~~~~~~~~~~~

   347 

   348 The editing commands usually take the movements as arguments. A movement is equivalent to a selection in the visual mode. The cursor is assumed to have moved over the text in between the initial and the final points of the movement. The motion or the visual block that's been highlighted can be passed as arguments to the editing commands. 

   349 

   350 +-------------------------+---------+

   351 | Editing effect          | Command |

   352 +=========================+=========+

   353 | Cutting text            | ``d``   |

   354 +-------------------------+---------+

   355 | Copying/Yanking text    | ``y``   |

   356 +-------------------------+---------+

   357 | Pasting copied/cut text | ``p``   |

   358 +-------------------------+---------+

   359 

   360 The cut and copy commands take the motions or visual blocks as arguments and act on them. For instance, if you wish to delete the text from the current text position to the beginning of the next word, type ``dw``. If you wish to copy the text from the current position to the end of this sentence, type ``y)``.

   361 

   362 Apart from the above commands, that take any motion or visual block as an argument, there are additional special commands. 

   363 

   364 +----------------------------------------+---------+

   365 | Editing effect                         | Command | 

   366 +========================================+=========+

   367 | Cut the character under the cursor     | ``x``   |

   368 +----------------------------------------+---------+

   369 | Replace the character under the        | ``ra``  |

   370 | cursor with ``a``                      |         |

   371 +----------------------------------------+---------+

   372 | Cut an entire line                     | ``dd``  |

   373 +----------------------------------------+---------+

   374 | Copy/yank an entire line               | ``yy``  |

   375 +----------------------------------------+---------+

   376 

   377 Note: You can prefix numbers to any of the commands, to repeat them.

   378 

   379 Undo and Redo

   380 ~~~~~~~~~~~~~

   381 You can undo almost anything using ``u``. 

   382 

   383 To undo the undo command type ``C-r``

   384 

   385 Searching and Replacing

   386 ~~~~~~~~~~~~~~~~~~~~~~~

   387 

   388 +-----------------------------------------+---------+

   389 | Finding                                 | Command |

   390 +=========================================+=========+

   391 | Next occurrence of ``text``, forward    |``\text``|

   392 +-----------------------------------------+---------+

   393 | Next occurrence of ``text``, backward   |``?text``|

   394 +-----------------------------------------+---------+

   395 | Search again in the same direction      | ``n``   |

   396 +-----------------------------------------+---------+

   397 | Search again in the opposite direction  | ``N``   |

   398 +-----------------------------------------+---------+

   399 | Next occurrence of ``x`` in the line    | ``fx``  |

   400 +-----------------------------------------+---------+

   401 | Previous occurrence of ``x`` in the line| ``Fx``  |

   402 +-----------------------------------------+---------+

   403 

   404 +---------------------------------------+------------------+

   405 | Finding and Replacing                 |  Command         |

   406 +=======================================+==================+

   407 | Replace the first instance of ``old`` |``:s/old/new``    |

   408 | with ``new`` in the current line.     |                  |

   409 +---------------------------------------+------------------+

   410 | Replace all instances of ``old``      |``:s/old/new/g``  |

   411 | with ``new`` in the current line.     |                  |

   412 +---------------------------------------+------------------+

   413 | Replace all instances of ``old``      |``:s/old/new/gc`` |

   414 | with ``new`` in the current line,     |                  |

   415 | but ask for confirmation each time.   |                  |

   416 +---------------------------------------+------------------+

   417 | Replace the first instance of ``old`` |``:%s/old/new``   |

   418 | with ``new`` in the entire file.      |                  |

   419 +---------------------------------------+------------------+

   420 | Replace all instances of ``old``      |``:%s/old/new/g`` |

   421 | with ``new`` in the entire file.      |                  |

   422 +---------------------------------------+------------------+

   423 | Replace all instances of ``old`` with |``:%s/old/new/gc``|

   424 | ``new`` in the entire file but ask    |                  |

   425 | for confirmation each time.           |                  |

   426 +---------------------------------------+------------------+

   427 

   428 SciTE

   429 -----

   430 

   431 SciTE is a *source code* editor, that has a feel similar to the commonly used GUI text editors. It has a wide range of features that are extremely useful for a programmer, editing code. Also it aims to keep configuration simple, and the user needs to edit a text file to configure SciTE to his/her liking. 

   432 

   433 Opening, Saving, Editing files with SciTE is extremely simple and trivial. Knowledge of using a text editor will suffice. 

   434 

   435 SciTE can syntax highlight code in various languages. It also has auto-indentation, code-folding and other such features which are useful when editing code. 

   436 

   437 SciTE also gives you the option to (compile and) run your code, from within the editor. 

   438 

   439 Personalizing your Environment

   440 ==============================

   441 

   442 .bashrc

   443 -------

   444 What would you do, if you want bash to execute a particular command each time you start it up? For instance, say you want the current directory to be your Desktop instead of your home folder, each time bash starts up. How would you achieve this? Bash reads and executes commands in a whole bunch of files called start-up files, when it starts up. 

   445 

   446 When bash starts up as an interactive login shell, it reads the files ``/etc/profile``, ``~/.bash_profile``, ``~/.bash_login``, and ``~/.profile`` in that order. 

   447 

   448 When it is a shell that is not a login shell, ``~/.bashrc`` is read and the commands in it are executed. This can be prevented using the ``--norc`` option. To force bash to use another file, instead of the ``~/.bashrc`` file on start-up, the ``--rcfile`` option may be used. 

   449 

   450 Now, you know what you should do, to change the current directory to you Desktop. Just put a ``cd ~/Desktop`` into your ``~/.bashrc`` and you are set!

   451 

   452 This example is quite a simple and lame one. The start-up files are used for a lot more complex things than this. You could set (or unset) aliases and a whole bunch of environment variables in the ``.bashrc``. We shall look at them, in the next section where we look at environment variables and ``set`` command.

   453 

   454 

   455 .vimrc

   456 ------

   457 ``.vimrc`` is a file similar to ``.bashrc`` for vim. It is a start-up file that vim reads and executes, each time it starts up. The options that you would like to be set every time you use vim, are placed in the ``.vimrc`` file, so that they are automatically set each time vim starts. The recommended place for having your ``.vimrc`` is also your home directory. 

   458 

   459 The file ``/etc/vimrc`` is the global config file and shouldn't usually be edited. You can instead edit the ``~/.vimrc`` file that is present in your home folder. 

   460 

   461 There are a whole bunch of variables that you could set in the ``.vimrc`` file. You can look at all the options available, using the ``:set all`` command in vim. You could use the ``:help option_name`` to get more information about the option that you want to set. Once you are comfortable with what you want to set a particular variable to, you could add it to ``.vimrc``. You should also look at ``:help vimrc`` for more info on the ``.vimrc`` file. If you already have a ``.vimrc`` file, you can edit it from within vim, using ``:e $MYVIMRC`` command. We shall look at some of the most commonly used options. 

   462 

   463 +----------------------------------+-----------------------------------------------------------------------------------+

   464 |Command                           | Vim action                                                                        |

   465 +==================================+===================================================================================+

   466 |``set nocompatible``              | Explicitly disable compatibility with vi                                          |

   467 +----------------------------------+-----------------------------------------------------------------------------------+

   468 |``set backspace=indent,eol,start``| In the insert mode, vim allows the backspace key to delete white spaces at the    |

   469 |                                  | start of line, line breaks and the character before which insert mode started.    |

   470 +----------------------------------+-----------------------------------------------------------------------------------+

   471 |set autoindent                    | Vim indents a new line with the same indentation of the previous line.            |

   472 +----------------------------------+-----------------------------------------------------------------------------------+

   473 |set backup                        | Vim keeps a backup copy of a file when overwriting it.                            |

   474 +----------------------------------+-----------------------------------------------------------------------------------+

   475 |set history=50                    | Vim keeps 50 commands and 50 search patterns in the history.                      |

   476 +----------------------------------+-----------------------------------------------------------------------------------+

   477 |set ruler                         | Displays the current cursor position in the lower right corner of the vim window. |

   478 +----------------------------------+-----------------------------------------------------------------------------------+

   479 |set showcmd                       | Displays the incomplete command in the lower right corner.                        |

   480 +----------------------------------+-----------------------------------------------------------------------------------+

   481 |set incsearch                     | Turns on incremental searching. Displays search results while you type.           |

   482 +----------------------------------+-----------------------------------------------------------------------------------+

   483 

   484 You can see the effect of the changes made to your ``.vimrc`` file by restarting vim. If you want to see the changes that you made to your ``.vimrc`` file immediately, you could source the file from within vim.

   485 

   486 If the ``.vimrc`` file has been sourced when this instance of vim was started, you could just resource the file again::

   487   :so $MYVIMRC

   488 

   489 If you just created the ``.vimrc`` file or it was not sourced when you stared this instance of vim, just replace the ``$MYVIMRC`` variable above, with the location of the ``.vimrc`` file that you created/edited.

   490 

   491 Subshells and ``source``

   492 ========================

   493 

   494 A subshell is just a separate instance of the shell which is a child process of the shell that launches it. Bash creates a subshell in various circumstances. Creation of subshells allows the execution of various processes simultaneously.   

   495 

   496   * When an external command is executed, a new subshell is created. Any built-in commands of bash are executed with int the same shell, and no new subshell is started. When an external command is run, the bash shell copies itself (along with it's environment) creating a subshell and the process is changed to the external command executed. The subshell is a child process of this shell. 

   497 

   498   * Any pipes being used, create a subshell. The commands on the input and output ends of the pipe are run in different subshells. 

   499 

   500   * You could also, explicitly tell bash to start a subshell by enclosing a list of commands between parentheses. Each of the commands in the list is executed within a single new subshell.   

   501 

   502 To avoid creating a subshell, when running a shell script, you could use the ``source`` command. 

   503 ::

   504 

   505   $ source script.sh

   506 

   507 This will run the ``script.sh`` within the present shell without creating a subshell. The ``.`` command is an alias for the source command. ``. script.sh`` is therefore equivalent to ``source script.sh``.