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Tutorial5: Redirection

1,764 bytes added, 09:33, 4 February 2020

→‎INVESTIGATION 1: BASICS OF REDIRECTION

These commands are displayed in the table below:

<table cellpadding="5" width="5055%"><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 ~~lowercase "a" through "z"~~ lower to ~~uppercase~~upper case tr "a-z" "A-Z" < filename - ~~translate lowercase "a" through "z" to uppercase, different syntax~~ same as above (non-System Vservers) tr ':' ' ' < filename - translate all colons to spaces tr ' ' '\n' < filename - translate all spaces to newline characters ~~tr 'abc' 'A' < filename - translate 'a', 'b', and 'c' to 'A', the last character in the "to" string repeats~~ </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>

[[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~~ 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

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:''

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:''

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

# Save editing changes and exit the text editor.

# Issue the following Linux command: tr 'a-z' 'A-Z' < data.txt What this command do?

# 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? # 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? # 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 to create a file: cat > output3.txt

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

# Issue the '''cat''' command to view the contents of the '''cars''' file.

# Issue the following Linux command: cut -c1-10 ~~uli101/~~cars What did this command do? # Issue the following Linux command: cut -f5 ~~uli101/~~cars > field5.txt What did this command do? # Issue the following Linux command: cut -f1-3 ~~uli101/~~cars > field123.txt What did this command do? # Issue the following Linux command: cut -f1,5 ~~uli101/~~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: 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 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.

'''Perform the Following Steps:'''

# ~~Confirm~~ Change to your '''home''' directory and confirm that you are ~~located~~ 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 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.

# 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. Since the who command does not, ~~an error message was displayed~~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 /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 #6''' # 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.

# Issue the following Linux pipeline command: ls /bin/?? | sort -r | head -5 What did you notice?

# Issue the following Linux pipeline command: ls /bin/???? | sort -r | grep r | tail -2 What did you notice?

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

# Change to your home directory.

# Issue the ~~following Linux~~ '''rm''' command to ~~remove the ~/redirect directory and its contents:~~ <~~span style="color:blue;font-weight:bold;font-family:courier;"~~u>~~rm -r ~/redirect~~only</~~span~~u>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=

Msaul

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