ult/session4.rst
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     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 donot 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                                          |
       
   390 +========================================+=========+
       
   391 | Next occurence of ``text``, forward    |``\text``|
       
   392 +----------------------------------------+---------+
       
   393 | Next occurence 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 occurence of ``x`` in the line    | ``fx``  |
       
   400 +----------------------------------------+---------+
       
   401 | Previous occurence of ``x`` in the line| ``Fx``  |
       
   402 +----------------------------------------+---------+
       
   403 
       
   404 +---------------------------------------+------------------+
       
   405 | Finding and Replacing                                    |
       
   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 
       
   429 scite
       
   430 -----
       
   431 
       
   432 
       
   433 Personalizing your Environment
       
   434 ==============================
       
   435 
       
   436 .bashrc
       
   437 -------
       
   438 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 startup files, when it starts up. 
       
   439 
       
   440 When bash starts up as an interactive login shell, it reads the files ``/etc/profile``, ``~/.bash_profile``, ``~/.bash_login``, and ``~/.profile`` in that order. 
       
   441 
       
   442 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 startup, the ``--rcfile`` option may be used. 
       
   443 
       
   444 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!
       
   445 
       
   446 This example is quite a simple and lame one. The startup 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.
       
   447 
       
   448 
       
   449 .vimrc
       
   450 ------
       
   451 ``.vimrc`` is a file similar to ``.bashrc`` for vim. It is a startup 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. 
       
   452 
       
   453 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. 
       
   454 
       
   455 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. 
       
   456 
       
   457 +----------------------------------+-----------------------------------------------------------------------------------+
       
   458 |Command                           | Vim action                                                                        |
       
   459 +==================================+===================================================================================+
       
   460 |``set nocompatible``              | Explicitly disable compatibility with vi                                          |
       
   461 +----------------------------------+-----------------------------------------------------------------------------------+
       
   462 |``set backspace=indent,eol,start``| In the insert mode, vim allows the backspace key to delete white spaces at the    |
       
   463 |                                  | start of line, line breaks and the character before which insert mode started.    |
       
   464 +----------------------------------+-----------------------------------------------------------------------------------+
       
   465 |set autoindent                    | Vim indents a new line with the same indentation of the previous line.            |
       
   466 +----------------------------------+-----------------------------------------------------------------------------------+
       
   467 |set backup                        | Vim keeps a backup copy of a file when overwriting it.                            |
       
   468 +----------------------------------+-----------------------------------------------------------------------------------+
       
   469 |set history=50                    | Vim keeps 50 commands and 50 search patterns in the history.                      |
       
   470 +----------------------------------+-----------------------------------------------------------------------------------+
       
   471 |set ruler                         | Displays the current cursor position in the lower right corner of the vim window. |
       
   472 +----------------------------------+-----------------------------------------------------------------------------------+
       
   473 |set showcmd                       | Displays the incomplete command in the lower right corner.                        |
       
   474 +----------------------------------+-----------------------------------------------------------------------------------+
       
   475 |set incsearch                     | Turns on incremental searching. Displays search results while you type.           |
       
   476 +----------------------------------+-----------------------------------------------------------------------------------+
       
   477 
       
   478 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.
       
   479 
       
   480 If the ``.vimrc`` file has been sourced when this instance of vim was started, you could just resource the file again::
       
   481   :so $MYVIMRC
       
   482 
       
   483 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.
       
   484 
       
   485 Subshells and ``source``
       
   486 ========================
       
   487 
       
   488 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.   
       
   489 
       
   490   * 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. 
       
   491 
       
   492   * Any pipes being used, create a subshell. The commands on the input and output ends of the pipe are run in different subshells. 
       
   493 
       
   494   * 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.   
       
   495 
       
   496 To avoid creating a subshell, when running a shell script, you could use the ``source`` command. 
       
   497 ::
       
   498 
       
   499   $ source script.sh
       
   500 
       
   501 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``.