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Tutorial8: Links / Process Management

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{{Admon/caution|DO NOT USE THIS VERSION OF THE LAB. This page will no longer be updated.|'''New version here:''' https://seneca-ictoer.github.io/ULI101/A-Tutorials/tutorial8 '''Andrew's students please go here:''' http://wiki.littlesvr.ca/wiki/OPS145_Lab_7}}

=LINKING FILES / MANAGING PROCESSES=

:* Define the term '''i-node''' as it relates to the Unix/Linux File System

:* Issue the '''ls -i''' command to view '''i-node''' ID (index) numbers associated with Unix/Linux files

:* Define the terms '''Hard''' and '''Symbolic''' Links

|- valign="top" style="padding-left:15px;"

|colspan="2" |~~Course Notes~~'''Slides:'''<ul><li>Week 8 Lecture 1 Notes: [https://~~ict~~wiki.cdot.senecacollege.ca/~~~murray~~uli101/slides/ULI101-8.1.pdf PDF] | [https://wiki.~~saul~~cdot.senecacollege.ca/uli101/slides/ULI101-~~Week8~~8.1.pptx PPTX]</li><li>Week 8 Lecture 2 Notes: [https://wiki.cdot.senecacollege.ca/uli101/slides/ULI101-8.2.pdf PDF] | [https://~~ict~~wiki.cdot.senecacollege.ca/~~~murray.saul~~uli101/~~uli101~~slides/ULI101-~~Week8~~8.2.pptx PPTX] </li></ul>

| style="padding-left:15px;" |'''Links:'''

* [http://www.linfo.org/hard_link.html#:~:text=A%20hard%20link%20is%20merely,created%20to%20other%20hard%20links. Hard Links]

* [https://www.computerhope.com/jargon/s/symblink.htm#:~:text=Alternatively%20referred%20to%20as%20a,or%20directory%20using%20its%20path.&text=In%20Linux%20and%20Unix%20symbolic,created%20using%20the%20mklink%20command. Symbolic Links]

'''Managing Processes:'''

* [https://en.wikipedia.org/wiki/Inode inode]

* [https://opensource.com/article/18/9/linux-commands-process-management Manipulating Processes]

| style="padding-left:15px;"|'''Linux Commands:'''

* [http://man7.org/linux/man-pages/man1/ln.1.html ln]

* [http://man7.org/linux/man-pages/man1/ps.1.html ps]

* [https://mediatemple.net/community/products/dv/204404624/using-the-history-command history]

|colspan="1" style="padding-left:15px;" width="30%"|'''Brauer Instructional Videos:'''<ul><li>[https://www.youtube.com/watch?v=tZ94-qH9unM&list=PLU1b1f-2Oe90TuYfifnWulINjMv_Wr16N&index=9 Inodes and Links]</li><li>[https://www.youtube.com/watch?v=q93POTgH-aQ&list=PLU1b1f-2Oe90TuYfifnWulINjMv_Wr16N&index=7 Processes and Jobs]</li></ul>

|}

[[Image:inode-number-3.png|thumb|right|450px|The '''i-node number''' is like a '''finger-print''', and is considered to be '''unique''' for each file on the Unix / Linux file system.]]

An '''i-node''' is a '''database''' containing information (e.g. ''file type'', ''owner'', ''permissions'', etc.) for all files that are created on the Unix/Linux filesystem. The '''i-node number''' is like a '''finger-print''', and is considered to be '''unique''' for each file on the Unix / Linux file system. Referring to the diagram on the far right, issuing the ~~'''~~ls~~'''~~ command with the ~~'''~~-i~~'''~~ option displays the i-node number for each file. You can see that each file (whether it is a directory or regular file) has its own unique i-node number.

===Hard Links===

'''Advantages:''' ~~of hard links are that if~~ If only one hard link remains (even if original file has been removed), the '''data in that hard linked file is NOT lost'''~~, as well as~~ . The data in hard linked files ~~will automatically be updated to that of the original file since they share the same~~ are '''~~i-node number~~automatically updated'''when original file are updated.

'''Disadvantages:''' ~~of hard~~ Hard links ~~are that they~~ '''take-up extra space''',~~ ~~you '''cannot hard link directories''', and you '''cannot hard link files from other Unix/Linux servers~~ ~~''' (since the inode number may already be used by the other Unix/Linux server).

''Examples:''

'''touch myfile.txt<br~~>ln myfile.txt myfile1~~.hard.lnk ln myfile.txt myfile2.hard.lnk ln myfile.txt ~/backups/myfile.hard.lnk '''

===Symbolic Links===

'''Advantages:''' of symbolic links ~~are that they~~ are '''shortcuts''' to other files, where the symbolic link only contains the pathname to the original file, you '''can create symbolic links''' '''on different Unix/Linux servers''', and that you '''can create symbolic links for directories'''.

'''Disadvantages:''' ~~of symbolic~~ Symbolic links ~~are that they~~ are NOT good for backup purposes since a symbolic link can point to a nonexistent file (referred to as a "broken link").

''Examples:''

'''touch otherfile.txt<br~~>ln -s otherfile.txt otherfile1~~.sym.lnk ln -s otherfile.txt otherfile2.sym.lnk ln -s otherfile.txt ~/backups/otherfile.sym.lnk '''

'''Characteristics of Processes:'''

* Each process has ~~an owner~~an '''owner'''* Each process has a unique ID ('''PID''')* Processes keep ~~their PID for~~ their ''PID'' for their '''entire life'''.* Usually a parent sleeps (~~i.e. suspended~~''suspends'') when a child is running (the exception is when the child process is running in the background)*UNIX / Linux processes are '''hierarchical'''. The process structure can have '''child processes''', '''great grandchild processes''', etc.

Users can '''manage processes''' to become more '''productive''' while working in the Unix / Linux Command-line environment. Processes that run in the terminal are known as '''foreground''' processes. You can run or send processes currently running in the ''foreground'' to the '''background''' to free-up your terminal (e.g. issue other Linux commands). Below are a listing of common '''Linux commands''' and '''keyboard shortcuts''' to manage foreground and background processes:

~~''Common Linux commands / keyboard shortcuts to manage processes:''~~ <table cellpadding="5" width="80%"><tr valign="top"><th width="25%" style="border-bottom: 1px solid black;text-align:left;">Linux Command / Key Combination</th><th style="border-bottom: 1px solid black;text-align:left;">Purpose</th></tr><tr valign="top"><td>ps</td><td>~~The '''ps''' (''process status'') command displays~~ Displays snapshot information about processes. ~~By default, the ps command displays information only about the current terminal ('''ps -l''' provides a detailed listing, '''ps -U''' username shows all) ~~ ''Examples: ''ps , ps -l , ps -ef , ps -u , ps aux </td></tr><tr valign="top"><td>'''top'''</td><td>The '''top''' command provides a realtime status of running processes. '''NOTE:''' You can press '''ctrl-c''' to exit </td></tr><tr valign="top"><td>fg'''ctrl-c'''</td><td>~~The~~ '''fgTerminates''' ~~(foreground) command moves~~ a ~~background job from the current environment into the foreground. The fg command issued without arguments will place the most recent~~ process running in the ~~background to the '''~~foreground~~'''. ''Example: ''fg %job-number~~</td></tr><tr valign="top"><td>'''ctrl-cz'''</td><td>Sends a process running in the foreground into the '''~~Terminates~~background''' ~~a process running in the foreground~~.</td></tr><tr valign="top"><td>~~'''ctrl-z'''~~fg</td><td>~~Sends~~ Moves a ~~process running in~~ background job from the ~~foreground~~ current environment into the foreground. ''Example: '~~background~~'~~''.~~fg %job-number</td></tr><tr valign="top"><td>bg</td><td>~~The '''bg''' utility '''resumes''' suspended jobs from the current environment. The bg command issued without arguments will run~~ Runs (starts) the most recent process that was placed into the background. ''Example: ''bg %job-number</td></tr><tr valign="top"><td>jobs</td><td>The '''jobs''' utility displays the '''status''' of jobs that were started in the current shell environment~~ ~~. ''~~Examples~~Example: ''jobs [1]+ Stopped vim a   <-- Job #1 (+ most recent process / background) [2]  Running sleep 200 &  <-- Job #2 [3]  Running sleep 300 &  <-- Job #3 [4]- Running sleep 400 &  <-- Job #4 (- second recent process / background) </td></tr><tr valign="top"><td>kill</td><td>The kill command sends the specified signal to the specified processes or process groups. If no signal is specified, the '''TERM''' signal is sent. The default action for this signal is to terminate the process. ''Examples:'' kill PID , kill -9 PID , kill %job-number , kill -9 %job-number</td></tr></table>

===Aliases / Command History===

'''~~Alias~~Aliases:''' ~~Assigns~~ An '''alias''' is a '''nickname ''' to an existing command or group of commands. An alias existing in '''system memory''' and will be '''lost''' when your current Linux session ends, unless the alias is set in a '''start-up file''' (e.g. ~/.bashrc. You will learn about using start-up files later in this course.

''Examples:''

'''Command History:'''

The filename ~~'''~~~/.bash_history~~'''~~ stores recently executed command lines

''Examples of commands that use command history:''

<table width="80%"><tr valign="top"><td width="30%">up arrow or down arrow </td><td>move to '''previous''' command or '''next''' command within Bash shell prompt</td></tr><tr><td>fc -l</td><td> display last '''16''' commands</td></tr><tr><td>history | more</td><td>display all stored commands</td></tr><tr><td>!num</td><td>'''re-execute''' an issued command number by command number (determined from ''history'' command)</td></tr><tr><td>!xxx</td><td>'''re-~~execute~~run''' an a most recent previously-issued command beginning with string "xxx"</td></tr></table>

=INVESTIGATION 1: LINKING FILES=

'''ATTENTION''': This online tutorial will be required to be completed by '''Friday in week 9 by midnight''' to obtain a grade of '''2%''' towards this course In this ~~section~~investigation, you will learn how to create '''hard links''' and '''symbolic links''' on your Matrix account, and observe the advantages and limitations of using both types of links.

# '''Login''' to your matrix account.

# Issue a Linux command to '''confirm''' you are located in your '''home''' directory. '''NOTE:''' You will remain in your '''home''' directory to get practice using pathnames.

# Issue the following Linux command to create a directory called '''~/links''': mkdir ~/links

# Issue the '''ls-ld''' command to confirm that the directory '''~/links''' exists.

# Use a text editor to create a file called ~/links/data-file.txt (i.e. without changing to the links directory).

# Enter the following text displayed below: This is line 1 This is line 2 This is line 3 [[Image:hard-link-inode1.png|thumb|right|450px|'''Hard links''' share the same '''i-node''' with regular files on a Unix / Linux filesystem.]]

# Save your editing session and exit your text editor.

# Issue the following Linux command: ls -li ~/links/data-file.txt View the '''i-node''' number for this file. What does this ''i-node'' number represent? We will now create a '''hard link''' file ~~and~~ to demonstrate how creating hard links are useful for '''back-ups'''.

# Issue the following Linux command to create the following '''hard link''' in the same directory: ln ~/links/data-file.txt ~/links/data-file.hard.lnk

# Issue the following Linux command to display ''i-node'' ID numbers for both files: ls -li ~/links/data-file.txt ~/links/data-file.hard.lnk What do you notice about both of those file's ''i-node'' numbers?

# Use a text editor to edit ~/links/data-file.txt and '''add some lines of text''' to the bottom of that file.

# Save your editing session and exit your text editor.

# Issue the following Linux command: cat ~/links/data-file.hard.lnk You should notice that the hard linked file also contains the additional line(s) that you added to the original file. This is very useful for backing up your files without using the '''cp''' command!

# Use a text editor to edit the hard-linked file ~/links/data-file.hard.lnk and add some lines to the bottom of this file.

# Save your editing session and exit your text editor.

# Issue the following Linux command: cat ~/links/data-file.~~hard.lnk~~txt What happened to this ~~hard-linked~~'''original''' file? file? What does this mean in terms of creating hard-linked files for back-ups?

# Issue the following Linux command to create a hard-linked file in your '''home''' directory: ln ~/links/data-file.txt ~/data-file.hard.lnk

# Issue the following Linux command to compare all file's ''i-node'' numbers: ls -li ~/links/data-file.txt ~/links/data-file.hard.lnk ~/data-file.hard.lnk What do you notice about all of those file's ''i-node'' numbers?

# Issue the following Linux command to check that you created those hard links: ~~bash /home/murray.saul/scripts~~~uli101/week8-check-1 If you encounter errors, then view the feedback to make corrections, and then re-run the checking script. If you receive a congratulation message that there are no errors, then proceed with this tutorial.

# Issue the following Linux command to remove the '''~/links''' directory and its contents: rm -rf ~/links

# Issue a Linux command to confirm that the '''~/links''' directory has been removed.

# Issue the following Linux command to view the contents of your linked file in your '''home''' directory: cat ~/data-file.hard.lnk What do you notice? What does this tell you about hard links? We will now learn how to create '''symbolic links'''.

# Issue the following Linux command to create a directory called '''~/links2''': mkdir ~/links2 '''NOTE:''' You will remain in your '''home''' directory to get practice using pathnames. # Issue the ls-ld command to confirm that the directory called '''~/links2''' exists.

# Use a text editor to create a file called ~/links2/text-file.txt (i.e. without changing to the '''links2''' directory). [[Image:symbolic-link-inode1.png|thumb|right|500px|'''Symbolic links''' are '''pointers''' (i.e. pathnames) to '''regular files''' and '''directories'''. They do '''NOT''' share the same '''i-node'''.]]

# Enter the following text displayed below: This is line one This is line two This is line three

# Save your editing session and exit your text editor.

# Issue the following Linux command to view the contents of the '''symbolic''' linked file: cat ~/links2/text-file.sym.lnk What did you notice? Again, when you view the contents of the symbolic-linked file, you are redirected (via ''pathname'') to the original file.

# Issue the following Linux command to check that you created those symbolic links: ~~bash /home/murray.saul/scripts~~~uli101/week8-check-2 If you encounter errors, then view the feedback to make corrections, and then re-run the checking script. If you receive a congratulation message that there are no errors, then proceed with this tutorial.

# Issue the following Linux command to remove the '''~/links2''' directory: rm -rf ~/links2

# Issue a Linux command to confirm that the '''~/links2''' directory has been removed.

# Issue the following Linux command: ls -l ~/text-file.sym.lnk This output indicates a "'''broken link'''" and indicates this is not an effective method of backing up files.

# Issue a command to delete the ~/text-file.sym.lnk file which is a ''broken link''.

# Issue the following Linux command: ln -s ~~~murray~~jason.~~saul~~carman/~~scripts scripts~~example t8example [[Image:symbolic-link-inode2.png|thumb|right|500px|'''Symbolic links''' can be used to point to '''directories''' as well as regular files. Symbolic links can also point to files on '''other''' Unix/Linux filesystems.]]# Issue the following Linux command: ls -ld ~~scripts~~t8example What do you notice? Symbolic links are good for creating "short-cuts" to both '''regular files''' and '''directories'''.

=INVESTIGATION 2: MANAGING PROCESSES =

In this ~~section~~investigation, you will learn how to '''manage processes''' on a Unix / Linux server.

# Make certain that you are logged into your Matrix account.

# Issue a Linux command to confirm that you are located in your '''home''' directory. The '''sleep''' command '''pauses for a specified number of seconds''' before returning to the shell prompt. In ~~the~~ this tutorial, we will be using this command to '''simulate''' the management of "long-running" processes.

# Issue the following Linux command: sleep 700 Notice that this process will run for '''700 seconds''', and is forcing the user to '''wait''' until this process finishes. A process that is '''running in the terminal''' is referred to as a '''foreground processes'''. The Unix/Linux system is designed to allow users to send '''preemptive signals''' to manage those processes.

# Press the following '''key combination''' to '''terminate''' the command running on the terminal: ctrl-c You should notice that the process that was running in the foreground has been '''interrupted''' (i.e. terminated). '''NOTE:''' The '''ctrl-c''' key combination sends '''SIGINT''' ('''Signal Interrupt''' - which is signal '''#2''') to ''terminate'' a process that is running on the terminal (i.e. a '''foreground''' process).

# You should now see output similar to what is displayed below: [1]+ Stopped sleep 700 '''NOTE:''' This indicates that this process has been placed into the '''background'''. This is useful in order to "'''free-up'''" the terminal to run other Linux commands.

# Issue the following Linux command: jobs You should see the following output similar that was displayed above: [1]+ Stopped sleep 700 This display indicates that this process (that is now in the background) has '''stopped'''. In other words, the ''sleep'' command is NOT counting-down to zero to terminate. '''NOTE:''' You need to use the '''bg''' command to '''run''' that process that was sent into the '''background'''. [[Image:process-jobs2.png|thumb|right|300px|Using the '''bg''' command to '''run recent process''' that was placed into background from using '''ctrl-z''' keys.]]

# Issue the following Linux command: bg '''NOTE:''' You can use the bg command WITHOUT arguments to ~~specify the most~~ run recent ~~process that was placed into~~ in the background. From the '''jobs''' command, the process that has a plus sign "+" indicates the most recent process placed into the background.

# Issue the following Linux command: jobs You should see the following output similar that was displayed above: [1]+ sleep 700 & This display indicates that this process in the background is '''running in the background''' (indicated by the ampersand character "'''&'''"). Now this command has resume pausing until '''700 seconds'''. [[Image:process-jobs3.png|thumb|right|300px|Using the '''ampersand''' character '''&''' to run a '''series of processes''' in the background.]]

# Issue the following Linux command: fg You should notice that the ''sleep'' command is now running in the '''foreground'''.

In this ~~section~~investigation, you will learn how to '''manage aliases''' and ~~Linux~~ '''Linux command history''' on your Matrix account.

# Issue the following command to confirm that this newly-created alias is stored in memory: alias | grep "lh"

# Issue the following alias: lh What do you think this command does?

# '''Logout''' of your Matrix account~~, than log into~~ and then '''login''' to your Matrix account.

# Reissue the '''lal''' alias. What happened?

# Reissue the '''lh''' alias. What happened?

# Reissue the '''lh''' alias. What happened? Why?

# Reissue the '''lal''' alias. Why '''didn't''' this alias work? The checking script below is designed to act as a '''filter''' with a '''pipeline command'''. This will allow to check if your '''lh''' alias exists when it is checked in this program.

# Issue the following Linux pipeline command: alias | ~~bash /home/murray.saul/scripts~~~uli101/week8-check-3 If you encounter errors, then view the feedback to make corrections, and then re-run the checking script. If you receive a congratulation message that there are no errors, then proceed with this tutorial. We will complete this investigation by learning to execute p'''reviously issued commands''' by using '''command history'''.

# Issue the following Linux command: history | grep "lh" What do you notice?

# Type an exclamation mark '''!''' followed by the number by one of those commands listed in the '''history list''' and press ENTER What happened?

# Type the following: !unalias and press ENTER What happened?

# Issue the following Linux command: history | grep "lh" What happened?

# After you complete the Review Questions sections to get additional practice, then work on your '''online assignment 2''' and complete '''section4''' labelled: '''Linking files and directories'''.

= LINUX PRACTICE QUESTIONS =

simulate a quiz:

https://~~ict~~wiki.cdot.senecacollege.ca/~~~murray.saul~~uli101/~~uli101~~files/uli101_week8_practice.docx

Your instructor may take-up these questions during class. It is up to the student to attend classes in order to obtain the answers to the following questions. Your instructor will NOT provide these answers in any other form (eg. e-mail, etc).

# Aliases / History:<ol type="a"><li>Write a linux command to create an '''alias''' called '''ld''' that issues the command: '''ls -ld'''</li><li>Write a linux command to unset the '''alias''' created in the previous question.</li><li>Issue a Linux command to list '''history''' of commands that match the pattern called '''touch'''.</li></ol>

# Create a '''table''' listing each Linux command, useful options and command purpose for the following Linux commands: '''ln''' , '''ps''' , '''top''' , '''fg''' , '''bg''' , '''jobs''' , '''kill''' , '''alias''' , '''unalias''' , '''history'''

_________________________________________________________________________________

Author: Murray Saul

License: LGPL version 3

Link: https://www.gnu.org/licenses/lgpl.html

_________________________________________________________________________________

[[Category:ULI101]]

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