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diff --git a/ult/ult_3/script.rst b/ult/ult_3/script.rst index adc24b8..089adb9 100644 --- a/ult/ult_3/script.rst +++ b/ult/ult_3/script.rst @@ -3,15 +3,14 @@ .. At the end of this tutorial, you will be able to: - .. 1. Display the contents of files. - .. 2. Read only parts of a file. - .. 3. Look at the statistical information of a file. - + .. 1. Change file permissions + .. 2. Change ownership of files + .. Prerequisites .. ------------- -.. 1. Using Linux tools - Part 1 -.. 2. Using Linux tools - Part 2 +.. 1. Getting started with Linux +.. 2. Basic File Handling Script ------ @@ -23,8 +22,7 @@ team along with the logo of MHRD }}} .. R1 -Hello friends and Welcome to the tutorial on -'Using linux tools - Part 3'. +Hello friends and Welcome to the tutorial on "File permissions and ownership". .. L2 @@ -34,9 +32,8 @@ Hello friends and Welcome to the tutorial on At the end of this tutorial, you will be able to, - 1. Display the contents of files. - #. Read only parts of a file. - #. Look at the statistical information of a file. + 1. Change file permissions + #. Change ownership of files .. L3 @@ -45,174 +42,174 @@ 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 - Part 1" and "Using Linux tools - Part 2". - -.. R4 - -Let us begin with how to read a while as a whole. -The ``cat`` command is the most commonly used command to display the -contents of files. To view the contents of a file, say, ``foo.txt``, we -simply say, +former tutorials as being displayed currently. .. L4 -:: - cat foo.txt +.. R4 -.. R5 +Let us now look at file permissions. Linux is a multi-user environment and +allows users to set permissions to their files to allow only a set of +people to read or write it. Similarly, it is not "safe" to allow system +files to be edited by any user. All this access control is possible in +Linux. -You can see the contents of the file on the terminal. +.. R5 -The cat command could also be used to concatenate the text of multiple -files. Say, we have two files,``foo.txt`` and ``bar.txt``, +To start, in the root directory, say, .. L5 + +{{{ Open the terminal }}} :: - cat foo.txt bar.txt + cd / + ls -l .. R6 -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. +You get a list of all the sub-directories, with a lot of additional information. +Let us try and understand the output. .. L6 -:: - - less wonderland.txt - -.. R7 - -This shows us the file, one screen at a time. .. L7 -.. L8 +{{{ Highlight the required portions accordingly while narrating }}} -{{{ Show slide with, less }}} +.. R7 -.. R8 +The first column denotes the type and the access permissions of the file. +The second is the number of links. The third and fourth are the owner and +group of the file. The next field is the size of the file in bytes. The +next field is the date and time of modification and the last column is the +file name. +We shall look at the permissions of the file now, ie., the first column of +the output. + +The first character in the first column specifies, whether the item is a +file or a directory. Files have a ``-`` as the first character and +directories have a ``d``. + +The rest of the 9 characters are actually sets of 3 characters each. The +first set of 3 characters defines the permissions of the user, the next 3 +is for the group and the last three is for others. +Based on the values of these characters, access to files is provided or denied, +to each of the users. + +So, what does each of the three characters stand for? Let's suppose, we are +looking at the set, corresponding to the permissions of the user. In the +three characters, the first character can either be an ``r`` or a ``-``. +Which means, the user can either have the permission to read the file or +not. If the character is ``r``, then the user has the permission to read +the file, else not. Similarly, ``w`` stands for write permissions and +decides whether the user is allowed to write to the file or not. ``x`` stands +for execute permissions. You cannot execute a file, if you do not have the +permission to execute it. + +Similarly, the next set of characters decides the same permissions for the +members of the group, that the file is associated with. The last set of +characters defines these permissions for the users, who are neither owners +of the file nor in the group, with which the file is associated. + +Now, it's not as if these permissions cannot be changed. If you are the +owner of a file, you can change the permissions of a file, using the +``chmod`` command. -``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. - * q: Quit. +.. R8 - * [Arrows]/[Page Up]/[Page Down]/[Home]/[End]: Navigation. +Let's say, we wish to give the execute permissions for a file, to both the +user and the group, how do we go about doing it? To be more explicit, given +a file ``foo.sh``, with the permissions flags as ``-rw-r--r--``, change it +to ``-rwxr-xr--``. - * ng: Jump to line number n. Default is the start of the file. +The following command does it for us, - * /pattern: Search for pattern. Regular expressions can be used. +.. L8 +:: - * h: Help. + chmod ug+x foo.sh + ls -l foo.sh .. R9 -Let us move ahead with the topic. Often we just would like to get some -statistical information about the file, rather than viewing the contents -of the file. The ``wc`` command prints these details for a file. +As you can see, the permissions have been set to the required value. But +what did we exactly do? .. L9 -:: - - wc wonderland.txt .. L10 -{{{ Highlight the required portions accordingly while narrating }}} +{{{ Switch to slide,Symbolic modes }}} .. 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 - -Let us now look at a couple of commands that let you see parts of files, -instead of the whole file. The ``head`` and ``tail`` commands let you see -parts of files, as their names suggest, the start and the end of a file, -respectively. +Let us understand these parameters one by one. +The ``u`` is the user who is the owner of the file. +``g`` stands for group which consists of users who are members of the +file’s group. The reference ``o``, which we shall use later in the tutorial, +stands for others who are users of the file but not the owners or members of +a group. .. L11 -:: - head wonderland.txt +{{{ Switch to slide,Symbolic modes... }}} -.. R12 +.. R11 -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``. +Let us now understand the operators. The plus operator adds the specified modes +to the specified classes. The minus operator removes the specified modes from +the specified classes. And finally the equal-to operator is used where modes +specified are to be made the exact modes for the specified classes. .. L12 -:: - head -n 1 wonderland.txt +{{{ Switch to slide,Symbolic modes... }}} -.. R13 +.. R12 -It prints only the first line of the file. Similarly, we could print only -the last line of the file. +We shall now learn the function of each mode. ``r`` stands for read which reads +a file or lists a directory’s contents. ``w`` is for write by which we can +write to a file or a directory. ``x`` stands for execute. As the name suggests, +it executes a file or recurse a directory tree. + +.. L13 -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. +{{{ Switch to the terminal }}} +{{{ Highlight the command, chmod ug+x foo.sh }}} -.. L13 -:: +.. R13 - tail -f /var/log/dmesg +In the command, the parameter ``ug+x`` is the mode parameter to the +``chmod`` command. It specifies the changes to be made to the +permissions of the file ``foo.sh``. +The ``u`` and ``g`` stand for the user and group, respectively. The ``x`` +stands for the execute permission and ``+`` stands for adding the +specified permission. So, essentially, we are asking ``chmod`` command to +add the execute permission for the user and group. The permission of others +will remain unchanged. .. R14 -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``. which will stop any process that is running from your -current shell. - -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. +So, if we wished to add the execute permission to all the users, instead of +adding it to just the user and group, we would have instead said .. L14 :: - cat /etc/passwd + chmod a+x foo.sh .. R15 -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 of all the users in the -file, how do we do it? +or .. L15 :: - - cut -d : -f 1 /etc/passwd -.. R16 + chmod ugo+x foo.sh -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 want to choose. -You can print multiple fields, by separating the field numbers with a -comma. +.. R16 Pause the video here, try out the following exercise and resume the video. @@ -220,155 +217,152 @@ Pause the video here, try out the following exercise and resume the video. .. L17 -{{{ Show slide with exercise 3 }}} +{{{ Show slide with exercise }}} .. R17 -Print only the first, fifth and the seventh fields of the file ``/etc/passwd``. +Change the permissions of a directory along with all of its +sub-directories and files. -.. R18 +.. L18 -Switch to the terminal for solution +{{{ Show slide with solution }}} -.. L18 +.. R18 -{{{ continue from paused state }}} -{{{ Switch to the terminal }}} +To change the permissions of a directory along with all of its +sub-directories and files, recursively, we use the ``-R`` option +with the chmod command as shown -:: - - cut -d : -f 1,5,7 /etc/passwd + chmod go-r -R <directory name>/ .. R19 -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, +It is important to note that the permissions of a file can only be changed +by a user who is the owner of a file or the superuser. +The superuser or the ``root`` user is the only user +empowered to a certain set of tasks and hence is called the superuser. +What if we wish to change the ownership of a file? The ``chown`` command is +used to change the owner and group. +By default, the owner of a file (or directory) is the user that +created it. The group is a set of users that share the same access +permissions i.e., read, write and execute. +For instance, to change the user and the group of the file +``wonderland.txt`` to ``alice`` and ``users``, respectively, we say, .. L19 :: - cut -c 1-4 /etc/passwd + chown alice:users wonderland.txt .. R20 -The end limits of the ranges can take sensible default values, if they are -left out. For example, +We get an error saying, the operation is not permitted. +We have attempted to change the ownership of a file that we own, to a +different user. Logically, this shouldn't be possible, because, this can +lead to problems, in a multi-user system. +Only the superuser is allowed to change the ownership of a file from one +user to another. The command above would have worked, if you did login as +the superuser and then changed the ownership of the file. .. L20 -:: - cut -c -4 /etc/passwd +.. L21 + +{{{ Show Summary slide }}} .. R21 -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. +This brings us to the end of the tutorial.In this tutorial, we have learnt to, -.. L21 -:: + 1. Chane the permissions of files using the ``chmod'' command. + #. Use the ``chown'' command to change the ownership of files. + +.. L22 - cut -c 10- /etc/passwd +{{{ Show self assessment questions slide }}} .. R22 -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. +Here are some self assessment questions for you to solve -.. L22 + 1. For a given file, change mode to r, w, x for all (user, group, others) + + 2. What changes, on specifying only an owner in the "chown" command? + + - Only the owner of the file + - The group ownership of the file + - Neither the owner nor the group .. L23 -{{{ Show slide with, paste }}} +{{{ Solution of self assessment questions on slide }}} .. R23 -For instance, given a file containing the names of students in a file, -students.txt, and another file with the marks of the students,marks.txt, - -.. R24 - -we wish to view the contents, side by side. The ``paste`` command allows -us to do that. +And the answers, -.. L24 +1. The required result can be obtained as, :: - paste students.txt marks.txt - paste -s students.txt marks.txt - -.. R25 - -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. + chmod ugo+rwx wonderland.txt -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 -then do, 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. +2. For ``chown'' command, if only an owner (a username or numeric user ID) + is given, then, that user is made the owner of each given file, and the + files' group is not changed. -.. L25 -.. L26 +.. L24 -{{{ Switch to summary slide }}} +{{{ Show the SDES & FOSSEE slide }}} -.. R26 +.. R24 -This brings us to the end of this tutorial. -In this tutorial, we have learnt to, +Software Development techniques for Engineers and Scientists - SDES, is an +initiative by FOSSEE. For more information, please visit the given link. - 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. - #. Display specific contents of file using the ``head`` and - ``tail`` commands. - #. Use the ``cut``, ``paste`` and ``wc`` commands. - -.. L27 +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. -{{{ Show self assessment questions slide }}} +.. L25 -.. R27 +{{{ Show the ``About the Spoken Tutorial Project'' slide }}} -Here are some self assessment questions for you to solve +.. R25 -1. How to view lines from 1 to 15 in wonderland.txt? +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. -2. In ``cut`` command, how to specify space as the delimiter? +.. L26 -.. L28 +{{{ Show the `` Spoken Tutorial Workshops'' slide }}} -{{{ Solution of self assessment questions on slide }}} +.. R26 -.. R28 +The Spoken Tutorial Project Team conducts workshops using spoken tutorials, +gives certificates to those who pass an online test. -And the answers, +For more details, contact contact@spoken-tutorial.org -1. We can use the head command as, -:: +.. L27 - head -15 wonderland.txt +{{{ Show the ``Acknowledgements'' slide }}} -2. We use the -d option with the command as, -:: +.. R27 - cut -d " " <filename> +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. -.. L29 +.. L28 -{{{ Show the Thank you slide }}} +{{{ Show the Thankyou slide }}} -.. R29 +.. R28 Hope you have enjoyed this tutorial and found it useful. Thank you! |