Tutorial8: Links / Process Management

LINKING FILES / MANAGING PROCESSES

Main Objectives of this Practice Tutorial

• Understand the purpose and why links are used in Unix / Linux

• Define the term inode number as it relates to a file on Unix / Linux

• Define the terms: Hard Link and Symbolic Link

• Issue the ln command to create hard and symbolic links

• Define and understand the purpose of a process in Unix / Linux

• Run and terminate processes in the foreground and background

• Display and manipulate background and foreground processes

Tutorial Reference Material

Course Notes		Concepts / Commands		YouTube Videos
Course Notes: PDF | PPTX		Links Hard Links Symbolic Links Managing Processes Process Information Manipulating Processes Running commands / programs in background with &	Linux Commands ln ps top fg bg jobs kill	Brauer Instructional Videos: Inodes and Links Processes and Jobs

KEY CONCEPTS

Linking Files

Links are powerful and add flexibility to Linux filesystems because everything is a file.

There are two types of Linux filesystem links: hard and soft. The difference between the two types of links is significant, but both types are used to solve similar problems. They both provide multiple directory entries (or references) to a single file, but they do it quite differently.

Reference: https://opensource.com/article/17/6/linking-linux-filesystem

inode (index) Number of a File:

The inode number is like a finger-print, and usually is unique for each file on the Unix / Linux file system.

The inode (index node) is a data structure in a Unix-style file system that describes a file-system object such as a file or a directory. Each inode stores the attributes and disk block locations of the object's data. File-system object attributes may include metadata (times of last change, access, modification), as well as owner and permission data.

Reference: https://en.wikipedia.org/wiki/Inode

The inode number is like a finger-print, and usually is 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 inode number for each file. You can see that each file
(whether it is a directory or regular file) has its own unique inode number.

Hard Links:

Hard links share the same inode number. Hard links are very good for backup purposes.

A Hard link is a reference to the physical data on a file system.

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, a hard links will automatically change when a change to that original file or hard links occur since they share the same i-node number
and the Unix/Linux OS treats them all as the same file.

Disadvantages of hard links are that they take-up extra space,
you cannot hard link directory files, 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:

Symbolic links do NOT share the same i-node number. Symbolic links are very good for short-cuts to other files (including directories) but NOT for backup purposes.

A Symbolic Link is an indirect pointer to a file and are also known as soft link or symlink.

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 programs that are executing on a UNIX system are referred to as processes:

• Each process has an owner
• Each process has a unique ID (PID) Processes in UNIX can run in the foreground or background.

UNIX processes are hierarchical:

• The process structure has a root, parents, and children
• Creation of a new process is called forking or spawning
• Each process has its own PID (process ID number)
• The Parent process can spawn a child and children can spawn their own children
• Processes keep their PID for their entire life
• Usually a parent sleeps when a child is executing (the exception is when the child process is executing in the background)

Below are some common Linux commands involving processes:

Linux Command	Purpose
ps	The ps (process status) command 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
top	The top command provides a continuous update including resource usage NOTE: You can press ctrl-c to exit
fg	The fg (foreground) command moves a background job from the current environment into the foreground. Example:fg %job-number</td></tr>
bg</td>	the bg utility shall resume suspended jobs from the current environment. Examples:</td></tr>
jobs</td>	The jobs utility shall display the status of jobs that were started in the current shell environment Examples:</td></tr>
kill</td>	The command kill 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:</td></tr></table> INVESTIGATION 1: LINKING FILES In this section, you will learn how to ... Perform the Following Steps: x In the next investigation, you will ... INVESTIGATION 2: MANAGING PROCESSES In this section, you will learn how to ... Perform the Following Steps: x In the next investigation, you will ... 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: Write a single Linux command to create a hard link called ~/backup/myfile.txt.lnk to the existing file called ~/myfile.txt Write a single Linux command to display detailed information for those files above displaying their i-node numbers. In this case, will the inode numbers for those files above be the same or different? Write a single Linux command to create a symbolic link called ~/shortcuts/murray.saul.lnk to the existing directory called ~murray.saul Write a single Linux command to display detailed information for those files above displaying their i-node numbers. In this case, will the inode numbers for those files above be the same or different? What data is contained in the file called ~/shortcuts/murray.saul.lnk? What would be the size of the file called ~/shortcuts/murray.saul.lnk? Write a single Linux command to run the program called ~/clean.sh in the background. What command would you issue to place the previously issued program in the foreground? What command would you issue to confirm that this program is running in the background? What key-combination would you issue to send that program again into the background? Write a single Linux command to display running processes in “real-time”. Write a single Linux command to terminal a process that has the following PID: 22384 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 Write a single Linux command to bring the second-recently process placed in the background into the foreground. Write a single Linux command to terminate the job #3. Create a table listing each Linux command, useful options and command purpose for the following Linux commands: ln , ps , top , fg , bg , jobs , kill