Difference between revisions of "Tutorial5: Redirection"

From CDOT Wiki
Jump to: navigation, search
(INVESTIGATION 2: REDIRECTION USING PIPES)
(INVESTIGATION 1: BASICS OF REDIRECTION)
 
(100 intermediate revisions by the same user not shown)
Line 5: Line 5:
 
:* Define the terms '''Standard Input''' ('''stdin'''), '''Standard Output''' ('''stdout'''), and '''Standard Error''' ('''stderr''')
 
:* Define the terms '''Standard Input''' ('''stdin'''), '''Standard Output''' ('''stdout'''), and '''Standard Error''' ('''stderr''')
  
:* Understand the purposes of redirection symbols '''>''', '''>>''', '''2>''', '''2>>''', '''|''' and define their purpose in terms of redirection
+
:* Understand the purposes of redirection symbols '''>''', '''>>''', '''2>''', '''2>>''', and  '''|'''  
  
:* Understand the purpose of the additional file manipulation commands: '''cut''', '''tr''', and '''wc'''
+
:* Define the following file manipulation commands: '''cut''', '''tr''', and '''wc'''
  
:* Define the term '''pipeline command''' and explain how a pipeline command works in terms of redirection
+
:* Define the term '''pipeline command''' and explain how a pipeline command functions
  
:* Define the term '''filter''' and how it relates to redirection using pipeline commands
+
:* Define the term '''filter''' and how it relates to '''redirection''' using pipeline commands
  
:* Understand how to use the '''semicolon''' symbol "''';'''" to issue multiple Unix / Linux commands
+
:* Use the '''semicolon''' ";" and '''grouping''' "( )" symbols issue multiple Unix / Linux commands in a single line
  
:* Understand how to use '''grouping''' using the symbols "'''( )'''" and how it is used to issue multiple Unix / Linux commands
+
:* Use the '''backslash''' "\" symbol to spread-out long Unix/Linux commands over multiple lines
 
<br>
 
<br>
  
Line 68: Line 68:
 
'''Standard input''' ('''stdin''') is a term which describes from where a command receives input.<br>
 
'''Standard input''' ('''stdin''') is a term which describes from where a command receives input.<br>
 
This would apply only to Unix/Linux commands that accept stdin input (like ''cat'', ''more'', ''less'', ''sort'', ''grep'', ''head'', ''tail'', etc.).<br>
 
This would apply only to Unix/Linux commands that accept stdin input (like ''cat'', ''more'', ''less'', ''sort'', ''grep'', ''head'', ''tail'', etc.).<br>
With the examples below, standard input is being sent into the command from the text file.
+
 
  
 
''Examples:''
 
''Examples:''
Line 76: Line 76:
  
 
<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>
 
<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 it's output.<br>In the examples below, output from a command is sent to the '''monitor''', unless it is sent to a '''regular file'''.<br>We will discuss another redirection technique called '''pipelines''' later.
+
'''Standard output''' ('''stdout''') describes where a command sends it's output.<br>In the examples below, output from a command is sent to the '''monitor''', unless it is sent to a '''text file'''.
 +
 
  
 
''Examples:''
 
''Examples:''
  
 
<span style="font-family:courier;font-weight:bold">ls -l<br>ls -l > detailed-listing.txt<br>ls /bin >> output.txt</span>
 
<span style="font-family:courier;font-weight:bold">ls -l<br>ls -l > detailed-listing.txt<br>ls /bin >> output.txt</span>
 +
  
 
<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>
 
<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.<br>In the examples below, error messages from a command are sent to a file or are redirected to a file that acts as a "garbage can".
+
'''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'''.
 +
 
  
''Examples (remember, Unix/Linux is case sensitive and issuing PWD in caps will generate an error message):''
+
''Examples:''
  
 
<span style="font-family:courier;font-weight:bold">PWD<br>PWD 2> error-message.txt<br>PWD 2 >> error-messages.txt<br>PWD 2> /dev/null</span>
 
<span style="font-family:courier;font-weight:bold">PWD<br>PWD 2> error-message.txt<br>PWD 2 >> error-messages.txt<br>PWD 2> /dev/null</span>
  
 +
The '''/dev/null''' file (sometimes called the '''bit bucket''' or  '''black hole''') is a special system file<br>that discards all  data written into it. This is useful to discard unwanted command output.
 +
 +
 +
''Examples:''
 +
 +
<span style="font-family:courier;font-weight:bold;">LS 2> /dev/null<br>ls > /dev/null<br>find / -name "tempfile" 2> /dev/null</span>
 +
<br><br>
  
 
===Additional File Manipulation Commands===
 
===Additional File Manipulation Commands===
Line 96: Line 106:
 
These commands are displayed in the table below:
 
These commands are displayed in the table below:
  
<table cellpadding="5"><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).'''<br><br>''Examples:''<br><span style="font-family:courier;font-weight:bold;">cut -f2 filename</span> - extract 2nd field from all records in file, using tab as delimiter (default)<br><span style="font-family:courier;font-weight:bold;">cut -d' ' -f2,5 filename</span> - extract 2nd and 5th field, using space as delimiter<br><span style="font-family:courier;font-weight:bold;">cut -d' ' -f1-3,5 filename</span> - extract 1st through 3rd and 5th fields, using space as delimiter<br><span style="font-family:courier;font-weight:bold;">cut -c3-5 filename</span> - extract 3rd to 5th characters</td></tr><tr valign="top"><td>'''tr'''</td><td>'''Used to translate characters to different characters.'''<br><br>''Examples:''<br><span style="font-family:courier;font-weight:bold;">tr a A < filename</span> - translate all characters "a" to "A"<br><span style="font-family:courier;font-weight:bold;">tr "[a-z]" "[A-Z]" < filename</span> - translate lowercase "a" through "z" to uppercase<br><span style="font-family:courier;font-weight:bold;">tr "a-z" "A-Z" < filename</span> - translate lowercase "a" through "z" to uppercase, different syntax (non-System V)<br><span style="font-family:courier;font-weight:bold;">tr ':' ' ' < filename</span> - translate all colons to spaces<br><span style="font-family:courier;font-weight:bold;">tr ' ' '\n' < filename</span>  - translate all spaces to newline characters<br><span style="font-family:courier;font-weight:bold;">tr 'abc' 'A' < filename</span> - translate 'a', 'b', and 'c' to 'A', the last character in the "to" string repeats<br></td></tr><tr valign="top"><td><span style="font-family:courier;font-weight:bold;">wc</span></td><td>'''Displays various counts of the contents of a file.'''<br><br>''Examples:''<br><span style="font-family:courier;font-weight:bold;">wc -l filename</span> - displays number of lines in file<br><span style="font-family:courier;font-weight:bold;">wc -c filename</span> - displays number of characters in file<br><span style="font-family:courier;font-weight:bold;">wc -w filename</span> - displays number of words in fil<br></td></tr></table>
+
<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.<br><br>''Examples:''<br><span style="font-family:courier;font-weight:bold;">cut -f2 filename</span> - extract 2nd field from all records in file<br><span style="font-family:courier;font-weight:bold;">cut -d' ' -f2,5 filename</span> - extract 2nd and 5th field<br><span style="font-family:courier;font-weight:bold;">cut -d' ' -f1-3,5 filename</span> - extract 1st to 3rd and 5th fields<br><span style="font-family:courier;font-weight:bold;">cut -c3-5 filename</span> - extract 3rd to 5th characters</td></tr><tr valign="top"><td>'''tr'''</td><td>Used to translate characters to different characters.<br><br>''Examples:''<br><span style="font-family:courier;font-weight:bold;">tr "[a-z]" "[A-Z]" < filename</span> - translate lower to upper case<br><span style="font-family:courier;font-weight:bold;">tr "a-z" "A-Z" < filename</span> - same as above (non-System V servers)<br><span style="font-family:courier;font-weight:bold;">tr ':' ' ' < filename</span> - translate all colons to spaces<br><span style="font-family:courier;font-weight:bold;">tr ' ' '\n' < filename</span>  - translate all spaces to newline characters<br><br></td></tr><tr valign="top"><td><span style="font-family:courier;font-weight:bold;">wc</span></td><td>Displays various counts of the contents of a file.<br><br>''Examples:''<br><span style="font-family:courier;font-weight:bold;">wc -l filename</span> - displays number of lines in file<br><span style="font-family:courier;font-weight:bold;">wc -c filename</span> - displays number of characters in file<br><span style="font-family:courier;font-weight:bold;">wc -w filename</span> - displays number of words in fil<br></td></tr></table>
 
 
 
 
The '''/dev/null''' file (sometimes called the '''bit bucket''' or  '''black hole''') is a special system file<br>that discards all  data written into it. This is useful to discard unwanted command output.
 
 
 
''Example:''
 
 
 
<span style="font-family:courier;font-weight:bold;">find / -name "tempfile" 2> /dev/null</span>
 
 
<br><br>
 
<br><br>
  
Line 109: Line 112:
  
 
[[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.]]
 
[[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''' <u>directly</u> to '''standard input''' of other  commands without having to use temporary files.
+
'''Pipeline Command:''' Having commands send their s'''tandard  output''' <u>directly</u> to '''standard input''' of other  commands WITHOUT having to use '''temporary''' files.
  
A few simple commands can be combined to form a more powerful command line.<br>
+
A few simple commands can be '''combined''' to form a more <u>powerful</u> command line.<br>
  
  
Pipes that are used in a pipeline command are represented by the | symbol.
+
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 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
  
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 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
 
  
 
''Examples:''<br>
 
''Examples:''<br>
Line 125: Line 129:
 
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.
 
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.
  
The reason for the name "tee" is that the splitting of the flow of information resembles a capital T.
+
The reason for the name "'''tee'''" is that the splitting of the flow of information resembles a capital T.
 +
 
  
 
''Examples:''
 
''Examples:''
Line 149: Line 154:
  
  
Commands may also be '''split over multiple lines''', making it easier (for humans) to interpret a long command.<br>
+
Commands may also be '''spread-out over multiple lines''', making it easier (for humans) to interpret a long command.<br>
You can add a '''quote''' or '''"escape" the newline character''' at the end of a line, to get rid of the special meaning<br>of newline (to end a command line)
+
You can add a '''backslash''' symbol "\" at the end of a line, to get rid of the special meaning<br>of newline (to end a command line)
  
  
Line 172: Line 177:
 
# Save editing changes and exit the text editor.<br><br>
 
# Save editing changes and exit the text editor.<br><br>
 
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">tr 'a-z' 'A-Z' < data.txt</span><br><br>What this command do?<br><br>
 
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">tr 'a-z' 'A-Z' < data.txt</span><br><br>What this command do?<br><br>
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">tail -2 data.txt > output.txt</span><br><br>What does this command do? Check the contents of the '''output.txt''' file to confirm.<br><br>
+
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">tr 'a-z' 'A-Z' < data.txt > output.txt</span><br><br>What this command do? What are the contents of the file output.txt?<br><br>
 +
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">tr 'a-z' 'A-Z' > output.txt < data.txt</span><br><br>What this command do? Is there any difference in terms of this command and the previous command issued?<br><br>
 +
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">tr 'a-z' 'A-Z' >> output.txt < data.txt</span><br><br>What happens to the content of the output.txt file? Why?<br><br>
 +
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">tail -2 < data.txt > output.txt</span><br><br>What does this command do? Check the contents of the '''output.txt''' file to confirm.<br><br>
 
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">tail -2 > output2.txt < data.txt </span><br><br>Why does this command render the same results as the previous command?<br>Try explaining how the command works in terms of '''stdin''' and then '''stdout'''.<br><br>
 
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">tail -2 > output2.txt < data.txt </span><br><br>Why does this command render the same results as the previous command?<br>Try explaining how the command works in terms of '''stdin''' and then '''stdout'''.<br><br>
 
# Issue the following Linux command to create a file: <span style="color:blue;font-weight:bold;font-family:courier;">cat > output3.txt </span><br><br>
 
# Issue the following Linux command to create a file: <span style="color:blue;font-weight:bold;font-family:courier;">cat > output3.txt </span><br><br>
Line 180: Line 188:
 
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">cp ~murray.saul/uli101/cars .</span><br><br>
 
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">cp ~murray.saul/uli101/cars .</span><br><br>
 
# Issue the '''cat''' command to view the contents of the '''cars''' file.<br><br>
 
# Issue the '''cat''' command to view the contents of the '''cars''' file.<br><br>
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">cut -c1-10 uli101/cars</span><br><br>What did this command do?<br><br>
+
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">cut -c1-10 cars</span><br><br>What did this command do?<br><br>
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">cut -f5 uli101/cars > field5.txt</span><br><br>What did this command do?<br><br>
+
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">cut -f5 cars > field5.txt</span><br><br>What did this command do?<br><br>
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">cut -f1-3 uli101/cars > field123.txt</span><br><br>What did this command do?<br><br>
+
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">cut -f1-3 cars > field123.txt</span><br><br>What did this command do?<br><br>
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">cut -f1,5 uli101/cars > field15.txt</span><br><br>What did this command do?<br><br>
+
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">cut -f1,5 cars > field15.txt</span><br><br>What did this command do?<br><br>
 
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">wc cars > count1.txt</span><br><br>What information does the count1.txt file contain?<br><br>
 
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">wc cars > count1.txt</span><br><br>What information does the count1.txt file contain?<br><br>
 
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">wc cars > count2.txt</span><br><br>What information does the count2.txt file contain?<br><br>
 
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">wc cars > count2.txt</span><br><br>What information does the count2.txt file contain?<br><br>
Line 189: Line 197:
 
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">pwd > listing.txt</span><br><br>What happenned to the original contents of the file called '''listing.txt'''?<br><br>
 
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">pwd > listing.txt</span><br><br>What happenned to the original contents of the file called '''listing.txt'''?<br><br>
 
# Issue the following Linux command (use 2 greater-than signs): <span style="color:blue;font-weight:bold;font-family:courier;">date >> listing.txt</span><br><br>What information does the '''listing.txt''' file contain?<br><br>
 
# Issue the following Linux command (use 2 greater-than signs): <span style="color:blue;font-weight:bold;font-family:courier;">date >> listing.txt</span><br><br>What information does the '''listing.txt''' file contain?<br><br>
 +
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">cat listing.txt cars > combined.txt</span><br><br>What information does the '''combined.txt''' file contain?<br><br>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".<br><br>
 
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">cat listing.txt cars murray 2> result.txt</span><br><br>What information does the '''result.txt''' file contain?<br><br>
 
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">cat listing.txt cars murray 2> result.txt</span><br><br>What information does the '''result.txt''' file contain?<br><br>
 
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">cat listing.txt cars murray > myoutput.txt 2> result.txt</span><br><br>What is displayed on the monitor? what do those files contain?<br><br>
 
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">cat listing.txt cars murray > myoutput.txt 2> result.txt</span><br><br>What is displayed on the monitor? what do those files contain?<br><br>
# Make certain that your current directory is '''your home directory'''.<br><br>
 
# Issue the following Linux command run checking script:<br><span style="color:blue;font-weight:bold;font-family:courier;">bash /home/murray.saul/scripts/week5-check-1</span><br><br>
 
# If you encounter errors and re-run the checking script until you receive a congratulations message, and proceed to the next section.<br><br>
 
  
The problem with using redirection to create files, you have these files taking up space, which requires you remove them. In the next investigation, you will be learning how to issue pipeline commands which can provide information by issuing several Linux commands without creating temporary files.<br><br>
+
:The problem with using redirection to create files, you have these files taking up space, which requires you remove them. In the next investigation, you will be learning how to issue pipeline commands which can provide information by issuing several Linux commands without creating temporary files.<br><br>
  
 
=INVESTIGATION 2: REDIRECTION USING PIPES =
 
=INVESTIGATION 2: REDIRECTION USING PIPES =
Line 205: Line 211:
 
'''Perform the Following Steps:'''
 
'''Perform the Following Steps:'''
  
# x<br><br>
+
# Change to your '''home''' directory and confirm that you are now in your home directory.<br><br>
 +
# Issue the '''ls''' command to view the contents of your '''~/redirect''' directory.<br><br>These are all temporary files that you created in your previous investigation.<br>The '''problem''' with creating temporary files, is that they take up space on your server,<br>and should be removed.<br><br>
 +
# Issue the following Linux command to remove all files in your ''redirect'' directory: <span style="color:blue;font-weight:bold;font-family:courier;">rm  -r ~/redirect</span><br>and confirm that you have removed this directory and its contents.<br><br>'''NOTE:''' You will be issuing a pipeline command which will use the pipe symbol "|"<br>that will send the stdout from a command as stdin into another command.<br><br>
 +
# Issue the follow Linux pipeline command: <span style="color:blue;font-weight:bold;font-family:courier;">ls /bin | more</span><br><br>What happened?<br><br>
 +
# Issue the following Linux pipeline command: <span style="color:blue;font-weight:bold;font-family:courier;">ls /bin | who</span><br><br>What happened? Why is the result different than antipated?[[Image:pipe-diagram-1.png|thumb|right|350px|]]<br><br>'''NOTE:''' When issuing pipeline commands, commands to the right of the pipe symbol must be designed to accept stdin. 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!<br><br>
 +
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">cp /bin/?? > listing.txt</span><br><br>
 +
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">sort -r listing.txt</span><br><br>
 +
# Issue the following Linux command to remove the listing file: <span style="color:blue;font-weight:bold;font-family:courier;">rm listing.txt</span><br><br>
 +
# Issue the following Linux pipeline command: <span style="color:blue;font-weight:bold;font-family:courier;">ls /bin/?? | sort -r</span><br><br>You should notice that the output from this pipeline command is the same output<br>from the command you issued in '''step #6'''<br><br>
 +
# Issue the following Linux pipeline command: <span style="color:blue;font-weight:bold;font-family:courier;">ls /bin/?? | sort -r | more</span><br><br>What is different with this pipeline command as opposed to the previous pipeline command?<br><br>
 +
# Issue the '''ls''' command.<br><br>You should notice that no files have been created. Let's get practice issuing more pipeline commands<br>using commands (previously learned or new) to be used as filters.<br><br>
 +
# Issue the following Linux pipeline command: <span style="color:blue;font-weight:bold;font-family:courier;">ls /bin/?? | sort -r | head -5</span><br><br>What did you notice?<br><br>
 +
# Issue the following Linux pipeline command: <span style="color:blue;font-weight:bold;font-family:courier;">ls /bin/???? | sort -r | grep r | tail -2</span><br><br>What did you notice?<br><br>
 +
# Issue the following Linux pipeline command: <span style="color:blue;font-weight:bold;font-family:courier;">ls /bin/???? | sort -r | grep r | cut -c1-3</span><br><br>
 +
# Issue the following Linux pipeline command:<br><span style="color:blue;font-weight:bold;font-family:courier;">ls /bin/???? | tee unsorted.txt | sort -r | tee sorted.txt | tee unmatched.txt | grep r | tail -2</span><br><br>What did you notice?<br><br>
 +
# Check the files that were created to see how the '''tee''' command was used in the previous pipeline command.<br><br>
 +
# Change to your home directory.<br><br>
 +
# Issue the '''rm''' command to <u>only</u> remove the files '''unsorted.txt''' , '''sorted.txt''' , and '''unmatched.txt'''<br><br>
  
In the next investigation, you will ...
+
:In the next investigation, you will learn various techniques to issue multiple Linux commands on the same line, or long Linux commands over multiple lines.
 +
<br><br>
  
 
=INVESTIGATION 3: ISSUING MULTIPLE UNIX/LINUX COMMANDS=
 
=INVESTIGATION 3: ISSUING MULTIPLE UNIX/LINUX COMMANDS=
  
In this section, you will learn how to ...
+
In this section, you will learn how to issue multiple Unix / Linux commands in a single line or over multiple lines.
 
 
  
  
 
'''Perform the Following Steps:'''
 
'''Perform the Following Steps:'''
  
# x<br>
+
# Confirm you are located in your '''home''' directory.<br><br>
 +
# Issue the following Linux commands (using semicolon to separate each command): <span style="color:blue;font-weight:bold;font-family:courier;">cal;pwd;date</span><br><br>Note the from the output the order of how each of those commands were processed.<br><br>
 +
# Issue the following Linux commands: <span style="color:blue;font-weight:bold;font-family:courier;">(cal;pwd;date)</span><br><br>Was there any difference in the output of this command as opposed to the previous command?<br><br>
 +
# Issue the following Linux pipeline command (using \ at the end of most lines):<br><span style="color:blue;font-family:courier;font-weight:bold">echo "This will be split over multiple \<br>lines.  Note that the shell will realize \<br>that a pipe requires another command, so \<br>it will automatically go to the next line" |tr '[a-z]' '[A-Z]'</span><br><br>Did the command work? What does this command do?<br><br>
 +
# 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'''.
 +
<br><br>
  
 
= LINUX PRACTICE QUESTIONS =
 
= LINUX PRACTICE QUESTIONS =

Latest revision as of 09:33, 4 February 2020

REDIRECTION: STANDARD INPUT / STANDARD OUTPUT / STANDARD ERROR


Main Objectives of this Practice Tutorial

  • Define the terms Standard Input (stdin), Standard Output (stdout), and Standard Error (stderr)
  • Understand the purposes of redirection symbols >, >>, 2>, 2>>, and |
  • Define the following file manipulation commands: cut, tr, and wc
  • Define the term pipeline command and explain how a pipeline command functions
  • Define the term filter and how it relates to redirection using pipeline commands
  • Use the semicolon ";" and grouping "( )" symbols issue multiple Unix / Linux commands in a single line
  • Use the backslash "\" symbol to spread-out long Unix/Linux commands over multiple lines


Tutorial Reference Material

Course Notes
Linux Command / Shortcut Reference
YouTube Videos
Course Notes:


Redirection

Multiple Commands

Redirection Filters Brauer Instructional Videos:

KEY CONCEPTS

Redirection (Standard Input, Standard Output, Standard Error)

... 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

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, etc.).


Examples:

tr 'a-z' 'A-Z' < words.txt
cat < abc.txt
sort < xyz.txt


The standard input (stdin) symbol with one greater than sign overwrites existing file content with command output
The standard input (stdin) symbol with two greater than signs add command's output to bottom of existing file's contents.

Standard output (stdout) describes where a command sends it's output.
In the examples below, output from a command is sent to the monitor, unless it is sent to a text file.


Examples:

ls -l
ls -l > detailed-listing.txt
ls /bin >> output.txt


The standard error (sterr) symbol with one greater than sign overwrites existing file content with command's error message.
The standard error (stderr) symbol with two greater than signs add command's error message to bottom of existing file's contents.

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.


Examples:

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 all data written into it. This is useful to discard unwanted command output.


Examples:

LS 2> /dev/null
ls > /dev/null
find / -name "tempfile" 2> /dev/null


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:

Linux CommandPurpose
cutUsed 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
trUsed 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

wcDisplays 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



Piping (Using Pipes)

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 standard output directly to standard input of other commands WITHOUT having to use temporary files.

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 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


Examples:
ls -al | more
ls | sort -r
ls | sort | more
ls -l | cut -d" " -f2 | tr 'a-z' 'A-z"
ls | grep Linux | head -5
head -7 filename | tail -2


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 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.

The reason for the name "tee" is that the splitting of the flow of information resembles a capital T.


Examples: ls | tee unsorted.txt | sort
ls | grep Linux | tee matched.txt | more
ls | head -5 | tee -a listing.txt


Multiple Commands Using Semicolon, Grouping, and Backquotes

Besides piping, there are other ways that multiple commands may be placed in one line:
commands may be separated by semi-colons.


Example:

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


Multiple commands can also be grouped by using parentheses.


Example:

(echo "Who is on:"; w) > whoson


Commands may also be spread-out over multiple lines, making it easier (for humans) to interpret a long command.
You can add a backslash symbol "\" at the end of a line, to get rid of the special meaning
of newline (to end a command line)


Example:

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]'


INVESTIGATION 1: BASICS OF REDIRECTION


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:

  1. Login your matrix account and issue a command to confirm you are located in your home directory.

  2. Issue the following Linux command to create the directory: mkdir ~/redirect

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

  4. 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


  5. Save editing changes and exit the text editor.

  6. Issue the following Linux command: tr 'a-z' 'A-Z' < data.txt

    What this command do?

  7. Issue the following Linux command: tr 'a-z' 'A-Z' < data.txt > output.txt

    What this command do? What are the contents of the file output.txt?

  8. Issue the following Linux command: tr 'a-z' 'A-Z' > output.txt < data.txt

    What this command do? Is there any difference in terms of this command and the previous command issued?

  9. 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?

  10. 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.

  11. 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.

  12. Issue the following Linux command to create a file: cat > output3.txt

  13. Enter the follow text displayed below:

    This is the file output3.txt

  14. Press ctrl-d to exit the command.

  15. Issue the cat command to view the contents of the file: output3.txt

  16. Issue the following Linux command: cp ~murray.saul/uli101/cars .

  17. Issue the cat command to view the contents of the cars file.

  18. Issue the following Linux command: cut -c1-10 cars

    What did this command do?

  19. Issue the following Linux command: cut -f5 cars > field5.txt

    What did this command do?

  20. Issue the following Linux command: cut -f1-3 cars > field123.txt

    What did this command do?

  21. Issue the following Linux command: cut -f1,5 cars > field15.txt

    What did this command do?

  22. Issue the following Linux command: wc cars > count1.txt

    What information does the count1.txt file contain?

  23. Issue the following Linux command: wc cars > count2.txt

    What information does the count2.txt file contain?

  24. Issue the following Linux command: ls -l > listing.txt

    What information does the count2.txt file contain?

  25. Issue the following Linux command: pwd > listing.txt

    What happenned to the original contents of the file called listing.txt?

  26. Issue the following Linux command (use 2 greater-than signs): date >> listing.txt

    What information does the listing.txt file contain?

  27. 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".

  28. Issue the following Linux command: cat listing.txt cars murray 2> result.txt

    What information does the result.txt file contain?

  29. 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 problem with using redirection to create files, you have these files taking up space, which requires you remove them. In the next investigation, you will be learning how to issue pipeline commands which can provide information by issuing several Linux commands without creating temporary files.

INVESTIGATION 2: REDIRECTION USING PIPES

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


Perform the Following Steps:

  1. Change to your home directory and confirm that you are now in your home directory.

  2. 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.

  3. Issue the following Linux command to remove all 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 command.

  4. Issue the follow Linux pipeline command: ls /bin | more

    What happened?

  5. Issue the following Linux pipeline command: ls /bin | who

    What happened? Why is the result different than antipated?
    Pipe-diagram-1.png


    NOTE: When issuing pipeline commands, commands to the right of the pipe symbol must be designed to accept stdin. 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!

  6. Issue the following Linux command: cp /bin/?? > listing.txt

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

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

  9. 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 #6

  10. 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?

  11. 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.

  12. Issue the following Linux pipeline command: ls /bin/?? | sort -r | head -5

    What did you notice?

  13. Issue the following Linux pipeline command: ls /bin/???? | sort -r | grep r | tail -2

    What did you notice?

  14. Issue the following Linux pipeline command: ls /bin/???? | sort -r | grep r | cut -c1-3

  15. Issue the following Linux pipeline command:
    ls /bin/???? | tee unsorted.txt | sort -r | tee sorted.txt | tee unmatched.txt | grep r | tail -2

    What did you notice?

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

  17. Change to your home directory.

  18. Issue the rm command to only remove the files unsorted.txt , sorted.txt , and unmatched.txt

In the next investigation, you will learn various techniques to issue multiple Linux commands on the same line, or long Linux commands over multiple lines.



INVESTIGATION 3: ISSUING MULTIPLE UNIX/LINUX COMMANDS

In this section, you will learn how to issue multiple Unix / Linux commands in a single line or over multiple lines.


Perform the Following Steps:

  1. Confirm you are located in your home directory.

  2. 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.

  3. Issue the following Linux commands: (cal;pwd;date)

    Was there any difference in the output of this command as opposed to the previous command?

  4. 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?

  5. 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.



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_week5_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).

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

Week5-dir.png











Review Questions:

  1. 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” directory (append output to existing file and use a relative pathname)
  2. 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” directory. (overwrite previous file’s contents and use only relative pathnames)
  3. 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.txt” to the “tests” directory. (overwrite previous contents for both files and use only relative-to-home pathnames).
  4. 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” 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).

  5. Write a single Linux pipeline command to display a detailed listing of the “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.
  6. Write a single Linux pipeline command to display the sorted contents (in reverse alphabetical order) of the “linux” directory. Use a relative pathname.
  7. Assume that the text file called “.answers.txt” contains 10 lines. Write a single Linux pipeline command to only displays lines 5 through 8 for this file. Use only relative pathnames.
  8. Write a single Linux pipeline command to only display the contents of the “assignments” directory whose filenames match the pattern “murray” (both upper or lowercase). Use an absolute pathname.
  9. Write a single Linux pipeline command to display the number of characters contained in the file called “.answers.txt”. Use a relative-to-home pathname.
  10. Write a single Linux pipeline command to display the number of lines contained in the file called “questions.txt”. Use a relative pathname.
  11. 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.
  12. 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.