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Tutorial5: Redirection

1,215 bytes added, 16:54, 14 October 2020

→‎INVESTIGATION 3: ISSUING MULTIPLE UNIX/LINUX COMMANDS

===Main Objectives of this Practice Tutorial===

:* Understand the purpose of the following file manipulation commands: '''cut''', '''tr''', and '''wc'''

:* Define the terms '''Standard Input''' ('''stdin'''), '''Standard Output''' ('''stdout'''), and '''Standard Error''' ('''stderr''')

:* Understand the purposes of the redirection symbols '''>''', '''>>''', '''2>''', '''2>>''', and '''|''' (pipe)

:* ~~Define~~ Understand the purpose of the the ~~following file manipulation commands:~~ '''~~cut~~/dev/null'''~~, '''tr''',~~ file and the '''wcHere Document'''

:* Define the term '''pipeline command''' and explain how a pipeline command functions

= KEY CONCEPTS =

===Additional File Manipulation Commands===

Before proceeding, there are some additional regular file manipulation commands that you can use with redirection (in addition to the other regular file manipulation commands introduced in week 2).

These commands are displayed in the table below:

<table cellpadding="5" width="55%"><tr><th style="border-bottom: 1px solid black;text-align:left;">Linux Command</th><th style="border-bottom: 1px solid black;text-align:left;">Purpose</th></tr><tr valign="top"><td>'''cut'''</td><td>Used to '''extract''' fields and characters from records. The option '''-c''' option is used to cut by a character or a range of characters. The '''-f''' option indicates the field number or field range to display (this may require using the '''-d''' option to indicate the field separator (delimiter) which is tab by default). ''Examples:'' cut -f2 filename - extract 2nd field from all records in file cut -d' ' -f2,5 filename - extract 2nd and 5th field cut -d' ' -f1-3,5 filename - extract 1st to 3rd and 5th fields cut -c3-5 filename - extract 3rd to 5th characters</td></tr><tr valign="top"><td>'''tr'''</td><td>Used to '''translate''' characters to different characters. ''Examples:'' tr "[a-z]" "[A-Z]" < filename - translate lower to upper case tr "a-z" "A-Z" < filename - same as above (non-System V servers) tr ':' ' ' < filename - translate all colons to spaces tr ' ' '\n' < filename - translate all spaces to newline characters </td></tr><tr valign="top"><td>wc</td><td>Displays various '''counts''' of the contents of a file. ''Examples:'' wc -l filename - displays number of lines in file wc -c filename - displays number of characters in file wc -w filename - displays number of words in fil </td></tr></table>

===Redirection (Standard Input, Standard Output, Standard Error)===

''~~...~~ '~~''standard~~ Standard streams''' are preconnected '''input and output communication channels ''' between a computer program and its environment when it begins execution. The three input/output (I/O) connections are called '''standard input''' ('''stdin'''), '''standard output''' ('''stdout''') and '''standard error''' ('''stderr'''). Originally I/O happened via a physically connected system console (input via keyboard, output via monitor), but standard streams '''abstract''' this. When a command is executed via an interactive shell, the streams are typically connected to the text terminal on which the shell is running, but can be changed with '''redirection''' or a '''pipeline'''. ''

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

[[Image:stdin-symbol.png|thumb|right|250px|The '''standard input''' ('''stdin''') symbol that describes where a Unix/Linux command receives '''input''']]

'''Standard input''' ('''stdin''') is a term which describes from where a command receives '''input'''. This would apply only to Unix/Linux commands that accept stdin input (like ''cat'', ''more'', ''less'', ''sort'', ''grep'', ''head'', ''tail'', ''tr'', ''cut'', ''wc'', etc.).

''Examples:''

<table align="right"><tr><td>[[Image:stdout-symbol-1.png|thumb|right|250px|The '''standard input''' ('''stdin''') symbol with one greater than sign '''overwrites ''' existing file content with command output]]</td><td>[[Image:stdout-symbol-2.png|thumb|right|250px|The '''standard input''' ('''stdin''') symbol with two greater than signs '''add ''' command's output to '''bottom ''' of existing file's contents.]]</td></tr></table>'''Standard output''' ('''stdout''') describes where a command sends itits '''s output'''. In the examples below, output from a command is sent to the '''monitor''', unless it is sent to a '''text file'''.

<table align="right"><tr><td>[[Image:stderr-symbol-1.png|thumb|right|250px|The '''standard error''' ('''sterr''') symbol with one greater than sign '''overwrites ''' existing file content with command's '''error message'''.]]</td><td>[[Image:stderr-symbol-2.png|thumb|right|250px|The '''standard error''' ('''stderr''') symbol with two greater than signs '''add ''' command's error message to '''bottom ''' of existing file's contents.]]</td></tr></table>

'''Standard Error''' ('''stderr''') describes where a command sends it's error messages. In the examples below we issue the pwd in capitals on purpose to generate an error message, which can be redirected to a '''text file'''.

PWD PWD 2> error-message.txt PWD 2 >> error-messages.txt PWD 2> /dev/null

The '''/dev/null''' file (sometimes called the '''bit bucket''' or '''black hole''') is a special system file that ~~discards~~ '''discard''' all data written into it. This is useful to discard unwanted command output.

[[Image:Here Document.png|thumb|right|~~150px~~175px|The '''Here Document''' allows a user to redirect stdin from within the command itself.]]

In Linux, the '''Here Document''' allows a user to redirect stdin from within the command itself.

cat <<+ Line 1 Line 2 Line 3 +

~~===Additional File Manipulation Commands===~~

~~There are some additional regular file manipulation commands that you can use with redirection (in addition to the other regular file manipulation commands introduced in week 2).~~

~~These commands are displayed in the table below:~~

<table cellpadding="5" width="55%"><tr><th style="border-bottom: 1px solid black;text-align:left;">Linux Command</th><th style="border-bottom: 1px solid black;text-align:left;">Purpose</th></tr><tr valign="top"><td>'''cut'''</td><td>Used to extract fields and characters from records. The option '''-c''' option is used to cut by a character or a range of characters. The '''-f''' option indicates the field number or field range to display (this may require using the '''-d''' option to indicate the field separator (delimiter) which is tab by default. ''Examples:'' cut -f2 filename - extract 2nd field from all records in file cut -d' ' -f2,5 filename - extract 2nd and 5th field cut -d' ' -f1-3,5 filename - extract 1st to 3rd and 5th fields cut -c3-5 filename - extract 3rd to 5th characters</td></tr><tr valign="top"><td>'''tr'''</td><td>Used to translate characters to different characters. ''Examples:'' tr "[a-z]" "[A-Z]" < filename - translate lower to upper case tr "a-z" "A-Z" < filename - same as above (non-System V servers) tr ':' ' ' < filename - translate all colons to spaces tr ' ' '\n' < filename - translate all spaces to newline characters </td></tr><tr valign="top"><td>wc</td><td>Displays various counts of the contents of a file. ''Examples:'' wc -l filename - displays number of lines in file wc -c filename - displays number of characters in file wc -w filename - displays number of words in fil </td></tr></table>

~~ ~~

===Piping (Using Pipes)===

[[Image:pipe-diagram-1.png|thumb|right|450px|A '''pipeline command''' sends a command's '''standard output''' directly to '''standard input''' of other command(s) without having to create temporary files.]]

'''Pipeline Command:''' Having commands send their s'''tandard output''' directly to '''standard input''' of other commands WITHOUT having to use '''temporary''' files.

Pipes that are used in a '''pipeline command''' are represented by the '''pipe''' "|" symbol.

A few simple commands can be '''combined''' to form a more powerful command line.

~~Pipes that are used in a '''pipeline command''' are represented by the '''pipe''' "|" symbol.~~ Commands to the '''right''' of the pipe symbol are referred to as '''filters'''. They are referred to as ''filters'' since those commands are used to '''modify''' the ~~stdin that was sent from~~ stdout of the previous command. Many commands can be "piped" together, but these commands (filters) must be chained in a specific order, depending on what you wish to accomplish

[[Image:tee-diagram-1.png|thumb|right|250px|The '''tee''' utility can be used to '''split ''' the flow of information. For example to save in a file as well as display on a screen. (Image licensed under [https://creativecommons.org/licenses/by-sa/3.0/ cc])]]

The '''tee''' utility can be used to split the flow of information. The tee option '''-a''' can be used to add content to the bottom of an existing file as opposed to overwriting the file's previous contents.

''Example:''

sleep 5; ls ~~(''Note: each command will be executed when the previous command has terminated'')~~

''Example:''

(echo "Who is on:"; w) > whoson ('''''Note:''' all command output is sent to a file'')

In this section, you will learn how to redirect ''standard input'', ''standard output '' and ''standard error '' when issuing Unix / Linux commands.

'''Perform the Following Steps:'''

# '''Login''' to your matrix account and issue a command to '''confirm''' you are located in your '''home''' directory. # Issue the following Linux command to create the following directory: mkdir ~/redirect

# Change to the '''~/redirect''' directory and confirm that you changed to that directory.

# Use a text editor to create a file in your current directory called '''data.txt''' and enter the following text displayed below: This is line 1 This is line 2 This is line 3

# Save editing changes and exit the text editor.

# Issue the following Linux command: tr 'a-z' 'A-Z' < data.txt What does this command do? # Issue the following Linux command: tr 'a-z' 'A-Z' < data.txt > output.txt What does this command do? What are the contents of the file ''output.txt''? # Issue the following Linux command: tr 'a-z' 'A-Z' > output.txt < data.txt What does this command do? Is there any difference in terms of this command and the previous command issued? # Issue the following Linux command: tr 'a-z' 'A-Z' >> output.txt < data.txt What happens to the content of the '''output.txt ''' file? Why?

# Issue the following Linux command: tail -2 < data.txt > output.txt What does this command do? Check the contents of the '''output.txt''' file to confirm.

# Issue the following Linux command: tail -2 > output2.txt < data.txt Why does this command render the same results as the previous command? Try explaining how the command works in terms of '''stdin''' and then '''stdout'''.

# Issue the following Linux command: cut -f1-3 cars > field123.txt What did this command do?

# Issue the following Linux command: cut -f1,5 cars > field15.txt What did this command do?

# Issue the following Linux command: wc cars > count1.txt What information does the '''count1.txt ''' file contain? # Issue the following Linux command: wc cars > count2.txt What information does the '''count2.txt ''' file contain? # Issue the following Linux command: ls -l > listing.txt What information does the '''count2.txt ''' file contain?

# Issue the following Linux command: pwd > listing.txt What happenned to the original contents of the file called '''listing.txt'''?

# Issue the following Linux command (use 2 greater-than signs): date >> listing.txt What information does the '''listing.txt''' file contain?

# Issue the following Linux command: cat listing.txt cars > combined.txt What information does the '''combined.txt''' file contain? '''NOTE''': The '''cat''' command stands for "'''concatenate'''" which means to '''combine''' contents of multiple files into a single file. This is why the command is called "cat".

# Issue the following Linux command: cat listing.txt cars murray 2> result.txt What information does the '''result.txt''' file contain?

# Issue the following Linux command: cat listing.txt cars murray > myoutput.txt 2> result.txt What is displayed on the monitor? what do those files contain? The '''Here Document''' allows you to redirect stdin from with the Linux command itself. Let's get some practice using the Here Document.

# Issue the following Linux command: cat <<+ line 1 line 2 line 3 + What do you notice?

# Issue the following Linux command: grep 2 <<+ line 1 line 2 line 3 + What do you notice? How does this differ from the previous command? Why?~~ ~~

=INVESTIGATION 2: REDIRECTION USING PIPES =

In this section, you will learn to issue '''pipeline commands ~~and learn how~~ ''' to ~~perform tasks using~~ issue Linux commands ~~with or~~ without ~~the generation of~~ having to generate temporary files.

'''Perform the Following Steps:'''

# ~~Change to your~~ Confirm that you are still located in the '''~~home~~~/redirect''' ~~directory and confirm that you are now in your home~~ directory. # Issue the '''ls''' command to view the contents of your '''~/redirect''' directory. ~~ These are all temporary files that you created in your previous investigation.~~ The '''problem''' with creating temporary files, is that they take up space on your server, and should be removed. # Issue the following Linux command to remove all temporary files in your ''redirect'' directory: rm -r ~/redirect and confirm that you have removed this directory and its contents. '''NOTE:''' You will be issuing a '''pipeline command ''' which will use the pipe symbol "|" that will send the stdout from a command as stdin into another commandwithout having to create temporary files. # Issue the follow Linux '''pipeline command''': ls /bin | more What happened? # Issue the following Linux '''pipeline command''': ls /bin | who What happened? Why is the result different than antipated?[[Image:pipe-diagram-1.png|thumb|right|350px|]] '''NOTE:''' When issuing pipeline commands, commands to the right of the pipe symbol must be designed to accept ~~stdin~~ '''standard input'''. Since the ''who '' command does not, you did NOT see the contents of the '''/bin''' directory but only information relating to the ''who'' command. Therefore, the '''order''' of which you build your pipeline command and the '''type of command''' that is used as a ''filter'' is extremely important! # Issue the following Linux command: cp ls /bin/?? > listing.txt

# Issue the following Linux command: sort -r listing.txt

# Issue the following Linux command to remove the listing file: rm listing.txt

# Issue the following Linux '''pipeline command''': ls /bin/?? | sort -r You should notice that the output from this pipeline command is the same output from the command you issued in '''step #67''' # Issue the following Linux '''pipeline command''': ls /bin/?? | sort -r | more What is different with this pipeline command as opposed to the previous pipeline command?

# Issue the '''ls''' command. You should notice that no files have been created. Let's get practice issuing more pipeline commands using commands (previously learned or new) to be used as filters.

# Check the files that were created to see how the '''tee''' command was used in the previous pipeline command.

# Change to your home directory.

'''Perform the Following Steps:'''

# Issue a Linux command to change to your home directory.

# Confirm you are located in your '''home''' directory.

# Issue the following Linux commands (using semicolon to separate each command): cal;pwd;date Note the from the output the order of how each of those commands were processed.

# Issue the following Linux commands: (cal;pwd;date) Was there any difference in the output of this command as opposed to the previous command? Let's see how grouping affects working with redirection. # Issue the following Linux commands: cal;pwd;date > output.txt # Issue a Linux command to view the contents of the file called '''output.txt''' What do you notice? Let's use grouping to make modification to the previous command # Issue the following Linux commands: (cal;pwd;date) > output.txt # Issue a Linux command to view the contents of the file called '''output.txt''' What do you notice? What did grouping the three Linux commands do?

# Issue the following Linux pipeline command (using \ at the end of most lines): echo "This will be split over multiple \ lines. Note that the shell will realize \ that a pipe requires another command, so \ it will automatically go to the next line" |tr '[a-z]' '[A-Z]' Did the command work? What does this command do?

# After you complete the Review Questions sections to get additional practice, then work on your '''online assignment 2 ''' and complete '''section3 ''' labelled: '''Redirection and Pipes'''.

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).

When answering Linux command questions, refer to the following Inverted Tree ~~diagram~~Diagram. The linux directory is contained in your home directory. Assume that you just logged into your Matrix account. Directories are underlined.

[[Image:week5-dir.png|thumb|left|300px|]]

'''Review Questions:'''

# Write a single Linux command to provide a detailed listing of all files in the '''/bin ''' directory, sending the output to a file called listing.txt in the ~~“projects”~~ “'''projects'''” directory (append output to existing file and use a relative pathname)# Write a single Linux command to redirect the stderr from the command: '''cat a.txt b.txt c.txt ''' to a file called '''error.txt ''' contained in the ~~“assignments”~~ “'''assignments'''” directory. (overwrite previous file’s contents and use only relative pathnames) # Write a single Linux command: '''cat ~/a.txt ~/b.txt ~/c.txt ''' and redirect stdout to a file called “good.txt” to the “tests” directory and stderr to a file called ~~“bad~~“'''bad.~~txt”~~ txt'''” to the ~~“tests”~~ “'''tests'''” directory. (overwrite previous contents for both files and use only relative-to-home pathnames).# Write a single Linux command to redirect the stdout from the command: '''cat a.txt b.txt c.txt ''' to a file called wrong.txt contained in the ~~“projects”~~ “'''projects'''” directory and throw-out any standard error messages so they don’t appear on the screen (append output to existing file and use only relative pathnames). # Write a single Linux '''pipeline command ''' to display a detailed listing of the ~~“projects “directory~~ '''projects''' directory but pause one screen at a time to view and navigate through all of the directory contents. Use a relative-to-home pathname.# Write a single Linux '''pipeline command ''' to display the sorted contents (in reverse alphabetical order) of the ~~“linux”~~ “'''linux'''” directory. Use a relative pathname.# Assume that the text file called “'''.answers.~~txt”~~ txt'''” contains 10 lines. Write a single Linux pipeline command to only displays lines 5 through 8 for this file. Use only relative pathnames.# Write a single Linux '''pipeline command ''' to only display the contents of the ~~“assignments”~~ “'''assignments'''” directory whose filenames match the pattern ~~“murray”~~ “'''murray'''” (both upper or lowercase). Use an absolute pathname.# Write a single Linux '''pipeline command ''' to display the number of characters contained in the file called “'''.answers.~~txt”~~txt'''”. Use a relative-to-home pathname.# Write a single Linux '''pipeline command ''' to display the number of lines contained in the file called ~~“questions~~“'''questions.~~txt”~~txt'''”. Use a relative pathname.# Write a single Linux '''pipeline command ''' to display only the first 10 characters of each filename contained in your current directory. Also, there is will be a lot of output, so also pause at each screenful so you can navigate throughout the display contents. Use a relative pathname.

# Create a '''table''' listing each Linux command, useful options that were mentioned in this tutorial for the following Linux commands: '''cut''' , '''tr''' , '''wc''' , and '''tee'''.

[[Category:ULI101]]

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