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Tutorial8: Links / Process Management

Revision as of 07:17, 12 March 2021 by Msaul (talk | contribs) (→‎Hard Links)

LINKING FILES / MANAGING PROCESSES


Main Objectives of this Practice Tutorial

  • Define the term i-node as it relates to the Unix/Linux File System
  • Issue the ls -i command to view i-node (index) numbers associated with Unix/Linux files
  • Define the terms Hard and Symbolic Links
  • Issue the ln command to create hard and symbolic links
  • Define term process as it relates to the Unix/ Linux operating system
  • Run and terminate processes in the foreground and background
  • Display and manipulate background and foreground processes
  • Use alias and history commands in Unix/Linux


Tutorial Reference Material

Course Notes
 Concepts / Commands
 YouTube Videos
Course Notes:


Links

Managing Processes

Linux Commands Brauer Instructional Videos:

KEY CONCEPTS

i-node (index) ID Number of a File

 
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

 
A Hard link is a file which is created that shares the same i-node number with the original file
(Image licensed under cc)
Image manipulated by author

A Hard link is a reference to the physical data on a file system.
It does this by creating a file that shares the same i-node number with the original file.


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

Disadvantages of 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
ln myfile.txt myfile1.hard.lnk
ln myfile.txt myfile2.hard.lnk
ln myfile.txt ~/backups/myfile.hard.lnk

Symbolic Links

 
A Symbolic Link is an indirect pointer to a file and are also known as soft link or symlink. The symbolic link file contains the pathname to the original file. (Image licensed under cc)

A Symbolic Link is an indirect pointer to a file and are also known as soft link or symlink. The symbolic link file contains the pathname to the original file.


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 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
ln -s otherfile.txt otherfile1.sym.lnk
ln -s otherfile.txt otherfile2.sym.lnk
ln -s otherfile.txt ~/backups/otherfile.sym.lnk

Managing Processes

All commands/programs (tasks) that are running on a Unix/Linux computer system are referred to as processes.

Characteristics of Processes:

  • Each process has an owner
  • Each process has a unique ID (PID)
  • Processes keep their PID for their entire life.
  • Usually a parent sleeps (i.e. suspended) 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.


Common Linux commands / keyboard shortcuts to manage processes:

Linux Command /
Key Combination		Purpose
psDisplays snapshot information about processes.
Examples: ps , ps -l , ps -ef , ps -u , ps aux
topThe top command provides a realtime status of running processes.
NOTE: You can press ctrl-c to exit
fgMoves a background job from the current environment into the foreground.
Example: fg %job-number
ctrl-cTerminates a process running in the foreground
ctrl-zSends a process running in the foreground into the background.
bgRuns (starts) the most recent process that was placed into the background.
Example: bg %job-number
jobsThe jobs utility displays the status of jobs that were started in the current shell environment. 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)

killThe 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

Aliases / Command History

Alias:

Assigns a nickname to an existing command or group of commands.

Examples:

alias (Alias command without an argument will display all the aliases currently set)

alias dir=ls
alias ls='ls -al'
alias clearfile='cat /dev/null >'

unalias alias-name (removes alias from memory)


Command History:

The filename ~/.bash_history stores recently executed command lines

Examples of commands that use command history:

up arrow or down arrow move to previous command or next command within Bash shell prompt
fc -l display last 16 commands
history | moredisplay all stored commands
!numre-execute an issued command number by command number (determined from history command)
!xxxre-execute an issued command beginning with string "xxx"


INVESTIGATION 1: LINKING FILES


In this section, 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.


Perform the Following Steps:

  1. Login to your matrix account.

  2. 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.

  3. Issue the following Linux command to create a directory called ~/links:
    mkdir ~/links

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

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

  6. Enter the following text displayed below:

    This is line 1
    This is line 2
    This is line 3

     
    Hard links share the same i-node with regular files on a Unix / Linux filesystem.
  7. Save your editing session and exit your text editor.

  8. 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 to demonstrate how creating hard links are useful for back-ups.

  9. 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

  10. 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?

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

  12. Save your editing session and exit your text editor.

  13. 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!

  14. 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.

  15. Save your editing session and exit your text editor.

  16. Issue the following Linux command:
    cat ~/links/data-file.txt

    What happened to this original file?</u> file?
    What does this mean in terms of creating hard-linked files for back-ups?

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

  18. 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?

  19. Issue the following Linux command to check that you created those hard links:
    bash /home/murray.saul/myscripts/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.

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

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

  22. 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.

  23. 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.

  24. Issue the ls -ld command to confirm that the directory called ~/links2 exists.

  25. Use a text editor to create a file called ~/links2/text-file.txt
    (i.e. without changing to the links2 directory).

     
    Symbolic links are pointers (i.e. pathnames) to regular files and directories.
    They do NOT share the same i-node.
  26. Enter the following text displayed below:

    This is line one
    This is line two
    This is line three

  27. Save your editing session and exit your text editor.

  28. Issue the following Linux command to create the following symbolic link in the same directory:
    ln -s ~/links2/text-file.txt ~/links2/text-file.sym.lnk

  29. Issue the following Linux command to display i-node numbers for both files:
    ls -li ~/links2/text-file.txt ~/links2/text-file.sym.lnk

    What do you notice about both of these file's i-node numbers?
    What do you notice about the size of the file ~/links2/text-file.sym.lnk?
    What pathname do you think this symbolic-linked file represents?

  30. Issue the following Linux command to create the following symbolic link in your home directory:
    ln -s ~/links2/text-file.txt ~/text-file.sym.lnk

  31. Issue the following Linux command to display i-node numbers for all of those files:
    ls -li ~/links2/text-file.txt ~/links2/text-file.sym.lnk ~/text-file.sym.lnk

    What do you notice about all of those file's i-node numbers?
    What is the file size of ~/text-file.sym.lnk?
    What pathname do you think this symbolic-linked file contains?

  32. Use a text editor to edit the symbolic link file called ~/links2/text-file.sym.lnk
    and add some lines to the bottom of that file.

  33. Save your editing session and exit your text editor.

  34. Issue the following Linux command to view the contents of the original file:
    cat ~/links2/text-file.txt

    What did you notice? This happened because when you edited the symbolic-linked file,
    you were redirected (via pathname) to the original file.

  35. Use a text editor to edit the original file called ~/links2/text-file.txt
    and add some lines to the bottom of that file.

  36. Save your editing session and exit your text editor.

  37. 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.

  38. Issue the following Linux command to check that you created those symbolic links:
    bash /home/murray.saul/myscripts/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.

  39. Issue the following Linux command to remove the ~/links2 directory:
    rm -rf ~/links2

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

  41. Issue the following Linux command to view the contents of the
    original file called ~/links2/text-file.txt:
    cat ~/text-file.sym.lnk

    What happened? Why did does this happen?

     
    Example of a broken link when a symbolic link points to a non-existent file.
  42. 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.

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

  44. Issue the following Linux command:
    ln -s ~murray.saul/myscripts checking-scripts

     
    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.
  45. Issue the following Linux command:
    ls -ld checking-scripts

    What do you notice? Symbolic links are good for creating "short-cuts" to both regular files and directories.


In the next investigation, you will learn how to manage processes on your Matrix server.

INVESTIGATION 2: MANAGING PROCESSES

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


Perform the Following Steps:

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

  2. 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 this tutorial, we will be using this command to simulate the management of "long-running" processes.

  3. 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.

  4. 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).

  5. Reissue the Linux command: sleep 700

  6. Press the key combination: ctrl-z

     
    Running a command in the terminal, pressing ctrl-z to place into the background, and issuing the jobs command to view processes in the background.
  7. 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.

  8. 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.

     
    Using the bg command to run recent process that was placed into background from using ctrl-z keys.
  9. Issue the following Linux command: bg

    NOTE: You can use the bg command WITHOUT arguments to run recent in the background. From the jobs command, the process that has a plus sign "+" indicates the most recent process placed into the background.

  10. 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.

     
    Using the ampersand character & to run a series of processes in the background.
  11. Issue the following Linux command: fg

    You should notice that the sleep command is now running in the foreground.

  12. Press the key combination to terminate the process running in the foreground:
    ctrl-c

    You can issue Linux commands with ampersand "&" in your terminal to run processes automatically in the background without having to issue ctrl-z and bg short-cut keys.

  13. Issue the following Linux commands:
    sleep 500 & sleep 600 & sleep 700 &

  14. Issue the jobs command. What do you notice?

    In the jobs command output, jobs that display a plus sign (+) indicates the most recent process
    placed in to the background, and a minus sign (-) indicates the second most recent process
    placed into the background.

    The kill command issued to terminate processes that are running in the foreground or background.
    Issuing the kill command without options would send the SIGTERM signal (eg. signal terminate - which is signal #15).

     
    Using the kill %1 command to terminate job #1.
  15. Issue the following Linux command to terminate the first job running in the background:
    kill %1

    NOTE: You can specify job number preceded by percent % with the
    kill, bg, and fg commands to specify the processes' job number.

  16. Issue the jobs command. What do you notice?

  17. Issue the following Linux commands:
    kill %2
    kill %3

  18. Issue the jobs command (you may have to issue the jobs command several times to get final result).
    What do you notice?

     
    Using round brackets to group a series of commands to be run as one process.
  19. Let's use grouping to run several commands in sequence within a single process.

  20. Issue the following Linux command:
    (sleep 400; sleep 500; sleep 600) &

  21. Issue the jobs command. What do you notice?
    You should notice all commands are run in a group as just one process.

  22. Issue the following Linux command to terminate the first job running in the background:
    kill %1

    NOTE: If issuing the kill command does not work, then you would need to send a STRONGER signal
    to "kill" (not "SIGTERM - which is signal #15") the process. The SIGKILL signal (signal #9)
    would be required to do this by issuing the kill command with the option: -9.

  23. Issue the jobs command and make certain there are no processes that are running in the background.

    You can also manipulate processes by their PID (process ID). Let's terminate our Matrix Bash shell process
    by using the kill command using that processes' PID.

  24. Issue the following Linux command: ps

  25. Note in the ps command output the PID of the process called bash.

    You will be using that PID when issuing the next Linux command.

  26. Issue the following Linux command (using the bash processes' PID number instead of "PID"):
    kill PID

    What did you notice?

    FYI: If the command did NOT work, issue the following Linux command (using the bash processes' PID number instead of "PID"):
    kill -9 PID

In the next investigation, you will learn how to create aliases and view command history on your Matrix server.

INVESTIGATION 3: ALIASES / COMMAND HISTORY


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


Perform the Following Steps:

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

     
    Issuing the alias command (without arguments) will display a list of existing aliases on your Unix / Linux system.
  2. Issue a Linux command to confirm that you are located in your home directory.

  3. Issue the following Linux command: alias | more

    Observe those existing aliases that have previously been declared. Take a few moments to run those aliases to see what happens.

  4. Issue the following to create an alias: alias lal='ls -al'

  5. Issue the following alias: lal

    What do you notice?

  6. Issue the following to create another alias (lowercase l and h):
    alias lh='ls --human-readable --size -1 -S --classify'

  7. Issue the following command to confirm that this newly-created alias is stored in memory:

    alias | grep "lh"

  8. Issue the following alias: lh

    What do you think this command does?

  9. Logout of your Matrix account and then login to your Matrix account.

  10. Reissue the lal alias. What happened?

  11. Reissue the lh alias. What happened?

  12. Issue the alias | grep lh command without any arguments to see if it is stored in memory.

  13. Reissue the command to create the lh alias in step #6.

  14. Run the lh alias to confirm that it is properly set in memory.

  15. Issue the following Linux command to edit your ~/.bashrc startup file:
    nano ~/.bashrc

  16. Add the following line at the bottom of this file:
    alias lh='ls --human-readable --size -1 -S --classify'

  17. Save your editing changes and exit your text editor.

  18. Logout of your Matrix account, then login to your Matrix account.

  19. Reissue the lh alias. What happened?

  20. Issue the following Linux command: unalias lh

  21. Run the lh alias to see what happens.

    What happenned?

  22. Logout of your Matrix account, then login to your Matrix account.

  23. Reissue the lh alias. What happened? Why?

  24. 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.

  25. Issue the following Linux pipeline command:
    alias | bash /home/murray.saul/myscripts/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 previously issued commands by using command history.

  26. Issue the following Linux command: history | grep "lh"

    What do you notice?

  27. Type an exclamation mark ! followed by the number by one of those commands
    listed in the history list and press ENTER

    What happened?

  28. Type the following: !unalias and press ENTER

    What happened?

  29. Issue the following Linux command: history | grep "lh"

    What happened?

  30. 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

The purpose of this section is to obtain extra practice to help with quizzes, your midterm, and your final exam.

Here is a link to the MS Word Document of ALL of the questions displayed below but with extra room to answer on the document to simulate a quiz:

https://ict.senecacollege.ca/~murray.saul/uli101/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).


Review Questions:

  1. Hard Links:
    1. What is the purpose of creating a hard-link?
    2. What is a limitation of a hard link?
    3. Write a single Linux command to create a hard link called ~/backup/myfile.txt.lnk for the existing file called ~/myfile.txt
    4. Write a single Linux command to display the i-node number for both files. Are the i-node numbers identical?

  2. Symbolic (Soft) Links:
    1. What is the purpose of creating a symbolic (soft) link?
    2. What is a limitation of a symbolic (soft) link?
    3. Write a single Linux command to create a symbolic link called ~/shortcuts/murray.saul.lnk
      to the existing directory called ~murray.saul
    4. Are the i-node numbers identical for both of those files?
    5. What data is contained in the file called ~/shortcuts/murray.saul.lnk?

  3. Background / Foreground Processes:
    1. Write a single Linux command to run the program called ~/clean.sh in the background.
    2. Write a single Linux command to place the previously issued program in the foreground.
    3. Write a single Linux command to confirm that this program is running in the background.
    4. What key-combination would you issue to send that program again into the background?
    5. Write a single Linux command to have that process sent into the background to continue running?

  4. Managing Background processes:
    Use the following diagram to answer the accompanying questions.
    Each of the following questions will use the diagram below and are treated as independent situations.

    [1]  Stopped vim a
    [2]- Stopped vim b
    [3]+ Stopped vim c
    1. Write a single Linux command to bring the second-recently process placed in the background into the foreground.
    2. Write a single Linux command to terminate job #3.

  5. Write a single Linux command to display running processes in “real-time”.
  6. Write a single Linux command to terminate a process that has the following PID: 22384

  7. Aliases / History:
    1. Write a linux command to create an alias called ld that issues the command: ls -ld
    2. Write a linux command to unset the alias created in the previous question.
    3. Issue a Linux command to list history of commands that match the pattern called touch.

  8. 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
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