diff options
Diffstat (limited to 'ult/ult_4/script.rst')
| -rw-r--r-- | ult/ult_4/script.rst | 447 |
1 files changed, 236 insertions, 211 deletions
diff --git a/ult/ult_4/script.rst b/ult/ult_4/script.rst index 09de00e..4b1bef0 100644 --- a/ult/ult_4/script.rst +++ b/ult/ult_4/script.rst @@ -3,15 +3,17 @@ .. At the end of this tutorial, you will be able to: - .. 1. Understand what is Redirection and Piping. - .. 2. Learn various features of shell. + .. 1. Display the contents of files. + .. 2. Read only parts of a file. + .. 3. Look at the statistical information of a file. .. Prerequisites .. ------------- -.. 1. Using Linux tools - Part 1 -.. 2. Using Linux tools - Part 2 -.. 3. Using Linux tools - Part 3 +.. 1. Getting started with Linux +.. 2. Basic File Handling +.. 3. File permissions and ownership + Script ------ @@ -24,7 +26,7 @@ team along with the logo of MHRD }}} .. R1 Hello friends and Welcome to the tutorial on -'Using linux tools - Part 4'. +'Advanced File Handling'. .. L2 @@ -34,8 +36,9 @@ Hello friends and Welcome to the tutorial on At the end of this tutorial, you will be able to, - 1. Understand what is Redirection and Piping. - #. Learn various features of the shell. + 1. Display the contents of files. + #. Read only parts of a file. + #. Look at the statistical information of a file. .. L3 @@ -43,367 +46,389 @@ At the end of this tutorial, you will be able to, .. R3 -Before beginning this tutorial, we would suggest you to complete the -tutorial on "Using Linux tools from Part 1 to Part 3". +Before beginning this tutorial,we would suggest you to complete the +former tutorials as being displayed currently. .. R4 -Let us begin with the concept of 'Redirection and Piping' which -performs the same operations as the ``cut`` and ``paste`` commands. - -Consider the files ``marks.txt`` and ``students.txt``.The contents of -the files are as following: +Let us begin with how to read a file as a whole. +The ``cat`` command is the most commonly used command for this purpose. +To view the contents of a file, say, ``foo.txt``, we +simply say, .. L4 -{{{ Open the terminal }}} +{{{ Switch to terminal }}} :: - cat marks1.txt - cat students.txt + cat foo.txt .. R5 -Now, let us view the contents of both these files side-by-side. +You can see the contents of the file on the terminal. + +The cat command could also be used to concatenate the text of multiple +files. Say, we have two files,``foo.txt`` and ``bar.txt``, .. L5 :: - cut -d " " -f 2- marks1.txt | paste -d " " students.txt - + cat foo.txt bar.txt .. R6 -Now, in order to view the same output in a new file at an other -location, we say, +It shows the output of both the files concatenated on the standard output. +But if we had a long file,the output of ``cat`` command is not convenient +to read. +Let's look at the ``less`` command which turns out to be more useful in +such a case. + +``less `` allows you to view the contents of a text file one screen at a +time. .. L6 :: - cut -d " " -f 2- marks1.txt > /tmp/m_tmp.txt - paste -d " " students.txt m_tmp.txt + less wonderland.txt + +<Press enter few times and show the contents of the file, then press Ctrl-z> .. R7 -First, let us try to understand the second solution,which is a two -step approach. -Later, we shall look at the first solution. +This shows us the file, one screen at a time. .. L7 .. L8 -{{{ Show slide, with Redirection }}} +{{{ Show slide with, less }}} .. R8 -The standard output, in general, goes to the display. -Hence, the output of the commands that we type, come out to the display. -This may not be what we always require. +``less`` has a list of commands that it allows you to use, once you have +started viewing a file. A few of the common ones have been listed below. -For instance, in the solution above, we use the cut command and get only -the required columns of the file and write the output to a new temporary -file. The ``>`` character is used to state that we wish to redirect the -output, and it is followed by the location to which we wish to redirect. -For example, + * q: to Quit. - command > file1 + * [Arrows]/[Page Up]/[Page Down]/[Home]/[End]:to Navigate through + the content. -.. L9 + * ng: to jump to a given line number (n).Default, is the first line of + a file. -{{{ Show slide, with Redirection... }}} + * /pattern: Search for pattern. Regular expressions can be used. -.. R9 - -Similarly, the standard input (stdin) can be redirected as, - - command < file1 + * h: for Help. -The input and the output redirection could be combined in a single command, -as, - - command < infile > outfile +.. R9 -There is actually a third kind of standard stream, called the Standard -error (stderr). Any error messages that you get, are coming through this -stream. Like ``stdout``, ``stderr`` also streams to the display by default, -but it could be redirected to a file, as well. +Let us move ahead with the topic. Often, we would like to get only some +statistical information about a file, rather than it's full contents. +The ``wc`` command prints these details for a file. -.. R10 +.. L9 +:: -For instance, let's reproduce an error using the ``cut`` command used -before. We shall change the ``-f`` option to ``-c`` + wc wonderland.txt .. L10 -{{{ Switch to terminal }}} -:: +{{{ Highlight the required portions accordingly while narrating }}} - cut -d " " -c 2- marks1.txt > /tmp/m_tmp.txt +.. R10 + +As you can see, we get some information about the file. +The first number is the number of lines, the second is the number of words +and the third is the number of characters in the file. .. R11 -This displays an error saying that the delimiter option should be used -with the fields option only. You may verify this by looking at the -``m_tmp.txt`` file, which is now empty.We can now, redirect the -``stderr`` also to a file, instead of showing it on the display. +Let us now look at a couple of commands that let's you see parts of files, +instead of the whole file. The ``head`` and ``tail`` commands allow you to see +parts of files. As their names suggest, they display the start and end of a +file, respectively. .. L11 :: - cut -d " " -f 2- marks1.txt 1> /tmp/m_tmp.txt 2> /tmp/m_err.txt + head wonderland.txt .. R12 -The above command redirects all the errors to the ``m_err.txt`` file -and the output to the ``m_tmp.txt`` file. When redirecting, 1 stands -for ``stdout`` and 2 stands for ``stderr``. - -Let us complete the solution by using the ``paste`` command. +It prints only the first 10 lines of the file. Similarly tail will print the +last 10 lines of the file. If we wish to change the number of lines that we +wish to view, we use the option ``-n``. .. L12 :: - paste -d " " students.txt m_tmp.txt + head -n 1 wonderland.txt .. R13 -So, in two steps we solved the problem of getting rid of the roll numbers -from the marks file and displaying the marks along with the names of the -students. Now, that we know how to redirect output, we could choose to -write the output to a file, instead of showing on the display. +It prints only the first line of the file. Similarly, we could print only +the last line of the file. -Let us now look at the first solution. +The most common use of the tail command is to monitor a continuously +changing file, for instance, a log file. Say, you have a process running, +which is continuously logging it's information to a file, for instance, the +logs of the system messages. .. L13 :: - cut -d " " -f 2- marks1.txt | paste -d " " students.txt - - -.. L14 - -{{{ Show slide, with Piping }}} + tail -f /var/log/dmesg .. R14 -First of all, the hyphen at the end is to ask the paste command to -read the standard input, instead of looking for a FILE. The ``man`` -page of ``paste`` command gives us this information. - -The character ``|`` is called a pipe. -Now, let us observe the ``cut`` command. If we look at the command only -upto the ``|`` character, it appears as a normal ``cut`` command . -So, the ``|`` character here, seems -to be joining the two commands in some way. -Essentially, what we are doing is, to redirect the output of the first -command to ``stdin`` and the second command takes the input from the ``stdin``. - -More generally, - - command1 | command2 +This will show the last 10 lines of the file as expected, but along with +that, it starts monitoring the file. Any new lines added at the end of the +file, will be shown. To interrupt tail, while it is monitoring, hit +``Ctrl-C``. -executes ``command1`` and sends it's output to the ``stdin``, which is then -used as the input for ``command2``. This activity is commonly called piping. +We looked at a couple of functions that allowed us to view a part of a file, +line-wise. We shall now look at a couple of commands that will allow us to look +at, only certain sections of each line of a file and merge those parts. +Let's take the ``/etc/passwd`` file as our example file. It contains +information about each user of the system. -.. L15 +.. L14 +:: -{{{ Show slide, with Piping... }}} + cat /etc/passwd .. R15 -This is roughly equivalent to using two redirects and a temporary file. - - command1 > tempfile - command2 < tempfile - rm tempfile +In the output, let us look at only the first, fifth, sixth and the last +columns.The first column is the ``user name``, the fifth column is the +``user info``, the sixth column is the ``home folder`` and the last column +is the ``path of the shell program`` that the user uses. +Let's say, we wish to look at only the user names in the file, how do we do it? -Also, given that a pipe is just a way to send the output of a command to -the ``stdin``, it should be obvious to you that we can use a chain of -pipes. Any number of commands can be piped together and therefore it should - be noted that it is not restricted to only two commands. +.. L15 +:: + + cut -d : -f 1 /etc/passwd -The Bash shell has some nice features, that make our job of using the shell -easier and much more pleasant. Let us have a look at few of them here. +.. R16 -Bash provides the feature of 'tab completion'. What does tab completion mean? -When you are typing a word, bash helps you to complete the word. -This can be done by entering some portion of the word and thereafter, -pressing the tab key. +It gives us the required output. Let us understand this operation in detail. +The first option ``-d`` specifies the delimiter between the various fields in +the file, in this case it is the ``semicolon``. If no delimiter is specified, +the TAB character is assumed to be the delimiter. The ``-f`` option specifies, +the field number that we wish to choose. +You can print multiple fields, by separating the field numbers with a +comma. -If you do not get the desired word on pressing the tab key, it implies that -either the word doesn't exist or the word cannot be decided unambiguously. -In the latter case, pressing the tab key for a second time,will list out -all the possibilities. +Pause the video here, try out the following exercise and resume the video. .. L16 -{{{ Show slide, with Tab-completion }}} - -.. R16 - -Bash provides tab completion for the following. +.. L17 - 1. File Names - 2. Directory Names - 3. Executable Names - 4. User Names (when they are prefixed with a ~) - 5. Host Names (when they are prefixed with a @) - 6. Variable Names (when they are prefixed with a $) +{{{ Show slide with exercise }}} .. R17 -For example, - -.. L17 - -{{{ Switch to terminal }}} -:: - - pas<TAB><TAB> - ~/<TAB><TAB> +Print only the first, fifth and the seventh fields of the file ``/etc/passwd``. .. R18 -Bash also saves the history of the commands you have typed earlier. -This feature enables you to goto the previously typed commands and -use them as and when necessary. The up and down arrow keys will help -you to navigate -through these commands in the bash history. +Switch to the terminal for solution .. L18 -:: - <UP-ARROW> +{{{ continue from paused state }}} +{{{ Switch to the terminal }}} + +:: + + cut -d : -f 1,5,7 /etc/passwd .. R19 -You may also search incrementally, for commands in your bash history. -``Ctrl-r`` searches for the commands that you have typed earlier. However, -it should be noted that the number of commands saved in the history is -limited, generally upto a 1000 commands. +We get the correct output. +Instead of choosing by fields, ``cut`` also allows us to choose on the +basis of characters or bytes. For instance, we could get the first 4 +characters of all the entries of the file, ``/etc/passwd`` by saying, .. L19 :: - <Ctrl-r> pas + cut -c 1-4 /etc/passwd .. R20 -Unix recognizes certain special characters, called "meta characters", as -command directives. The shell meta characters are recognized anywhere they -appear in the command line, even if they are not surrounded by a blank space. -For this reason, it is always recommended to use only the characters A-Z, -a-z, 0-9, period, dash and underscore, when naming files and -directories on Unix. If your file or directory has a shell meta character -in the name, you may find it difficult to use this name in a shell command. +The end limits of the ranges can take sensible default values, if they are +left out. For example, .. L20 +:: -.. L21 - -{{{ Show slide, with Shell Meta Characters }}} + cut -c -4 /etc/passwd .. R21 -The characters that you see on the slide are the shell meta characters +It gives the same output as before. If the start position has not been +specified, it is assumed to be the start of the line. Similarly if the end +position is not specified, it is assumed to be the end of the line. + +.. L21 +:: + + cut -c 10- /etc/passwd .. R22 -Lets take an example, +It prints all the characters from the 10th character up to the end of the +line. +Let us now solve an inverse problem. Let's say we have two columns of data +in two different files, and we wish to view them side by side. .. L22 -{{{ Switch to terminal }}} -:: +.. L23 - ls file.? +{{{ Show slide with, paste }}} .. R23 -It means, run on a directory containing the files file, file.c, file.lst, -and myfile would list the files file.c and file.lst. However, - -.. L23 -:: - - ls file.? +For instance, consider a file 'students.txt' containing the names of students +and another file 'marks.txt' containing their respective marks. .. R24 -Run on the same directory would only list file.c because the ? only matches -one character, no more, no less. This helps you save time, while typing. - -For example, if there is a file called -california_cornish_hens_with_wild_rice and no other files whose names begin -with 'c', you could view the file without typing the whole name by typing -this +we wish to view the contents, side by side. The ``paste`` command allows +us to do that. .. L24 :: - more c* + paste students.txt marks.txt + paste -s students.txt marks.txt .. R25 -Here, the c* matches that long file name. -File-names containing meta characters can pose many problems and should -never be intentionally created. +The first command gives us the output of the two files, next to each other +and the second command gives us the output one below the other. + +Now, this problem is a bit unrealistic because, we wouldn't have the marks +of students in a file, without any information about the student to which +they belong. Let's say our marks file had the first column as the roll +number of the student, followed by the marks of the students. What would we +do then, to get the same output that we got before? + +Essentially we need to use both the ``cut`` and ``paste`` commands, but +how do we do that? That brings us to the concept of Redirection and Piping +which is covered in the next spoken tutorial. .. L25 .. L26 -{{{ Switch to Summary slide }}} +{{{ Switch to summary slide }}} .. R26 -This brings us to the end of the end of this tutorial. -In this tutorial, we have learnt to, +This brings us to the end of this tutorial. +In this tutorial, we have learnt to, - 1. Use the ``cut`` and ``paste`` commands in redirection. - 2. Use the pipe ( | ) character. - 3. Implement features of shell, like tab-completion and history. + 1. Display the contents of files using the ``cat`` command. + #. View the contents of a file one screen at a time using the + ``less`` command. .. L27 - -{{{ Show self assessment questions slide }}} - -.. R27 -Here are some self assessment questions for you to solve: +{{{ Switch to slide Summary... }}} - 1. Bash does not provide tab completion for Host Names. - True of False? +.. R27 - 2. In a file /home/test.txt ,first line is "data:myscripts:20:30". How do we - view only the minutes (last field, 30). - - - cut -d : -f 4 /home/test.txt - - cut -f 3 /home/test.txt - - cut -d : -f 3 /home/test.txt - - None of these + 1. Display only specific contents of file using the ``head`` and + ``tail`` commands. + #. Use the ``cut``, ``paste`` and ``wc`` commands. + .. L28 -{{{ Solutions for the self assessment questions on slide }}} +{{{ Show self assessment questions slide }}} .. R28 -And the answers: +Here are some self assessment questions for you to solve - 1. False. Bash provides tab completion for Host Names when they are prefixed - with a @ sign. +1. How to view lines from 1 to 15 in ``wonderland.txt``? - 2. The correct option would be -:: - - cut -d : -f 4 /home/test.txt +2. In ``cut`` command, how to specify space as the delimiter? .. L29 -{{{ Show the Thank you slide }}} +{{{ Solution of self assessment questions on slide }}} .. R29 +And the answers, + +1. We can use the head command as, +:: + + head -15 wonderland.txt + +2. We use the -d option with the command as, +:: + + cut -d " " <filename> + +.. L30 + +{{{ Show the SDES & FOSSEE slide }}} + +.. R30 + +Software Development techniques for Engineers and Scientists - SDES, is an +initiative by FOSSEE. For more information, please visit the given link. + +Free and Open-source Software for Science and Engineering Education - FOSSEE, is +based at IIT Bombay which is funded by MHRD as part of National Mission on +Education through ICT. + +.. L31 + +{{{ Show the ``About the Spoken Tutorial Project'' slide }}} + +.. R31 + +Watch the video available at the following link. It summarises the Spoken +Tutorial project.If you do not have good bandwidth, you can download and +watch it. + +.. L32 + +{{{ Show the `` Spoken Tutorial Workshops'' slide }}} + +.. R32 + +The Spoken Tutorial Project Team conducts workshops using spoken tutorials, +gives certificates to those who pass an online test. + +For more details, contact contact@spoken-tutorial.org + +.. L33 + +{{{ Show the ``Acknowledgements'' slide }}} + +.. R33 + +Spoken Tutorial Project is a part of the "Talk to a Teacher" project. +It is supported by the National Mission on Education through ICT, MHRD, +Government of India. More information on this mission is available at the +given link. + +.. L34 + +{{{ Show the Thank you slide }}} + +.. R34 + Hope you have enjoyed this tutorial and found it useful. Thank you! |