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Tutorial8: Links / Process Management

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:* 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

[[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>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>fg'''ctrl-c'''</td><td>~~Moves a background job from the current environment into the foreground. ~~''~~Example:~~ 'Terminates'''~~fg %job-number~~a process running in the foreground</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>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===

'''~~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''': Depending on your ULI101 instructor, you may be required to complete this tutorial for '''marks''' in this course. Please refer to your instructor's course notes and lecture notes regarding evaluation for this course. The due date for successfully completing this tutorial (i.e. '''tutorial 8''') is by '''Friday by midnight''' next week (i.e. '''Week 8'''). If your instructor has NOT assigned marks for completing this tutorial, you can perform it for '''practice'''. 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.

# 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.

# 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.saul/myscripts checking-scripts ~~myscripts~~ [[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 ~~myscripts~~checking-scripts 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.

# 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 command history''' on your Matrix account.

# 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?

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