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Tutorial 8 - Links / Process Management

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=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===

{|width="100%" cellspacing="0" cellpadding="10"

|- valign="top"

|colspan="2" style="font-size:16px;font-weight:bold;border-bottom: thin solid black;border-spacing:0px;"|Course Notes

|colspan="2" style="font-size:16px;font-weight:bold;border-bottom: thin solid black;border-spacing:0px;padding-left:15px;"|Concepts / Commands

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

|colspan="2" |Course Notes:<ul><li>[https://matrix.senecacollege.ca/~osl640/slides/OSL640-Week8.pdf PDF] | [https://matrix.senecacollege.ca/~osl640/slides/OSL640-Week8.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]

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

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

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

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

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

* [https://man7.org/linux/man-pages/man1/sleep.1.html sleep]

* [http://linuxcommand.org/lc3_man_pages/aliash.html alias] , [https://man.linuxreviews.org/man1p/unalias.1p.html unalias]

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

|}

= KEY CONCEPTS =

<!--

<table align="right"><tr valign="top"><td>[[Image:inode-number-1.png|thumb|right|300px|Each inode stores the attributes and disk block locations of the object's data. (Image licensed under [https://creativecommons.org/licenses/by-sa/3.0/ cc])]]</td><td>[[Image:inode-number-2.png|thumb|right|360px|The '''inode number''' is like a '''finger-print''', and is considered to be '''unique''' for each file on the Unix / Linux file system.]]</td></tr></table>

''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

-->

===i-node (index) ID Number of a File===

[[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===

[[Image:hard-link-1.png|thumb|right|300px|A '''Hard link''' is a file which is created that shares the '''same i-node number''' with the original file (Image licensed under [https://creativecommons.org/licenses/by-sa/3.0/ 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:''' If only one hard link remains (even if original file has been removed), the '''data in that hard linked file is NOT lost'''. The data in hard linked files are '''automatically updated''' when original file are updated.

'''Disadvantages:''' Hard links '''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:''

ln myfile.txt myfile1.hard.lnk ln myfile.txt ~/backups/myfile.hard.lnk '''

===Symbolic Links===

[[Image:symbolic-link-1.png|thumb|right|300px|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 [https://creativecommons.org/licenses/by-sa/3.0/ 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:''' symbolic links 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:''' Symbolic links are NOT good for backup purposes since a symbolic link can point to a nonexistent file (referred to as a "broken link").

''Examples:''

ln -s otherfile.txt otherfile1.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 (''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:

<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>Displays snapshot information about processes. ''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>'''ctrl-c'''</td><td>'''Terminates''' a process running in the foreground</td></tr><tr valign="top"><td>'''ctrl-z'''</td><td>Sends a process running in the foreground into the '''background'''.</td></tr><tr valign="top"><td>fg</td><td>Moves a background job from the current environment into the foreground. ''Example: ''fg %job-number</td></tr><tr valign="top"><td>bg</td><td>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. ''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===

'''Aliases:'''

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:''

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:''

<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-run''' a most recent previously-issued command beginning with string "xxx"</td></tr></table>

= INVESTIGATION 1: LINKING FILES =

Jason Michael Carman

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