Tutorial 5: Redirection - Standard Input, Standard Output, and Standard Error
Main Objectives of this Tutorial
- Understand and use the cut, tr, and wc Linux commands
- Define the terms Standard Input (stdin), Standard Output (stdout), and Standard Error (stderr)
- Understand and use the >, >>, 2>, 2>> symbols with Linux commands
- Understand the purpose of the the /dev/null file and the Here Document
- Define the term pipeline command and explain how a pipeline command functions
- Define the term filter and how it relates to pipeline commands
- Use the semicolon ";" and grouping "( )" symbols to issue multiple Unix / Linux commands on a single line
- Use the backslash "\" symbol to spread-out long Unix/Linux commands over multiple lines
Tutorial Reference Material
Lecture Slides:
Redirection:
| Standard Input (stdin) | Standard Output (stdout) | Standard Error (stderr) | Pipeline Commands |
|---|
Multiple Commands:
Redirection Filters:
| more, less | head, tail | sort | uniq | grep | cut | tr | wc | tee |
|---|
Key Concepts
Additional File Manipulation Commands
Before proceeding, let's look at some additional commands used to manipulate content of text files.
Refer to the table below regarding these text file manipulation commands:
| Command | Description |
|---|---|
| tr | Used to translate characters to different characters. eg. tr 'a-z' 'A-Z' < filename |
| cut | Used to extract fields and characters from records. The option -c 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 seperator (delimeter) which is tab by default). eg. cut -c1-5 filename, cut -d":" -f2 filename |
| wc | Displays various counts of the contents of a file. The -l option displays the number of lines, the -w option displays the number of words, and the -c option displays the number of characters. eg. wc filename, wc -l filename, wc -w filename |
Redirection (Standard Input, Standard Output, Standard Error)
Redirection can be defined as changing the way from where commands read input to where commands sends output. You can redirect input and output of a command. Reference: https://www.javatpoint.com/linux-input-output-redirection
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.).
The standard input (stdin) symbol that describes where a Unix/Linux command receives input ^
Examples:
tr 'a-z' 'A-Z' < words.txt
cat < abc.txt
sort < xyz.txt
Standard output (stdout) describes where a command sends its output. In the examples below, output from a command is sent to the monitor, unless it is sent to a text file.
The standard out (stdout) symbol with one greater than sign overwrites existing file content with command output ^
The standard output (stdout) symbol with two greater than signs add command's output to bottom of existing file's contents. ^
Examples:
ls -l
ls -l > detailed-listing.txt
ls /bin >> output.txt
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.
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. ^
Examples:
PWD
PWD 2> error-message.txt
PWD 2 >> error-messages.txt
PWD 2> /dev/null
The /dev/null File
The /dev/null file (sometimes called the bit bucket or black hole) is a special system file that discard 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
The Here Document
In Linux, the Here Document allows a user to redirect stdin from within the command itself.
The Here Document allows a user to redirect stdin from within the command itself.
Example:
cat <<+
Line 1
Line 2
Line 3
+
Pipeline Commands
Pipeline Command: Having commands send their standard 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.
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 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
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 Command
The tee utility can be used to split the flow of standard output between a text file and the terminal screen.
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 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
Multiple commands can also be grouped by using parentheses.
Example:
(echo "Who is on:"; w) > whoson
(Note: all command output is sent to a file)
Commands may also be spread-out over multiple lines, making it easier (for humans) to interpret a long command.
The \ symbol “quotes-out” the meaning of the ENTER key as text (i.e. new-line as instead of running the command).
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
ATTENTION: The due date for successfully completing this tutorial (i.e. tutorial 5) is by July 5 @ 11:59 PM (Week 8).
In this investigation, 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 to create a file:
cat > output3.txt
- Enter the follow text displayed below:
This is the file output3.txt
- Press
ctrl-dto exit the command. - Issue the cat command to view the contents of the file: output3.txt
- Issue the following Linux command:
cp ~osl640/cars.txt .
- Issue the cat command to view the contents of the cars file.
- Issue the following Linux command:
cut -c1-10 cars.txt
- What did this command do?
- Issue the following Linux command:
cut -f5 cars.txt > field5.txt
- What did this command do?
- Check the contents in the file field5.txt to see what happened.
- Issue the following Linux command:
cut -f1-3 cars.txt > field123.txt
- What did this command do? (check file contents)
- Issue the following Linux command:
cut -f1,5 cars.txt > field15.txt
- What did this command do? (check file contents)
- Issue the following Linux command:
wc cars.txt > count.txt
- What information does the count.txt file contain?
- Issue the following Linux command:
wc -l cars.txt > count1.txt
- What information does the count1.txt file contain?
- Issue the following Linux command:
wc -w cars.txt > count2.txt
- What information does the count2.txt file contain?
- Issue the following Linux command:
ls -l > listing.txt
- What information does the listing.txt file contain?
- Issue the following Linux command:
pwd > listing.txt
- What happenned to the original contents of the file called listing.txt? Why?
- Issue the following Linux command (use 2 greater-than signs):
date >> listing.txt
- What information does the listing.txt file contain? Why?
- Issue the following Linux command:
cat listing.txt cars.txt > combined.txt
- What information does the combined.txt file contain? Why?
- 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.txt murray 2> result.txt
- What is displayed on the monitor? What information does the result.txt file contain? Why?
- Issue the following Linux command:
cat listing.txt cars.txt murray > myoutput.txt 2> /dev/null
- What is displayed on the monitor? What happened to the error message?
- Issue the following Linux command:
cat listing.txt cars.txt murray > myoutput.txt 2> result.txt
- What is displayed on the monitor? what do those files contain? Why?
- 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?
- Issue the following Linux command:
grep 2 > line2.txt <<+
line 1
line 2
line 3
+
- What do you notice? What is contained in the file line2.txt? Why?
- NOTE: You will now run a shell script to confirm that you properly issued Linux commands using redirection.
- Issue the following Linux command to run a checking script:
~osl640/week5-check-1
If you encounter errors, make corrections and re-run the checking script until you receive a congratulations message, then you can proceed.
Issue the ls command to see all of the temporary files that were created as a result of redirection.
- The problem with using these redirection symbols is that you create temporary text files that take up space on your file system.
Issue a Linux command (using Filename Expansion) to remove those temporary text files in the current directory.
Issue the following Linux command to check that you removed ALL of those temporary text files:
~osl640/week5-check-2
- If you encounter errors, make corrections and re-run the checking script until you receive a congratulations message, then you can proceed.
In the next investigation, you will be learning how to issue pipeline Linux commands which can accomplish tasks without creating temporary files.
Investigation 2: Redirection Using Pipeline Commands
In this investigation, you will learn to issue pipeline commands to to accomplish tasks without having to generate temporary files.
Perform the Following Steps:
Confirm that you are still located in the ~/redirect directory.
- The problem with creating temporary files, is that they take up space on your server, and should be removed. You actually did that in the previous investigation.
- 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 without having to create temporary files.
Issue the follow Linux pipeline command:
ls /bin | more
- What happened? Press q to exit display.
- Issue the following Linux pipeline command:
ls /bin | who
- What happened? Although this pipeline command provides output, it does not work properly as a pipeline command since the who command is NOT designed to accept standard input.
- NOTE: When issuing pipeline commands, commands to the right of the pipe symbol must be designed to accept 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:
ls /bin/?? > listing.txt
- Issue the following Linux command:
sort listing.txt
- Issue the following Linux command to remove the listing file:
rm listing.txt
- Issue the following Linux pipeline command:
ls /bin/?? | sort
- You should notice that the output from this pipeline command is the same output from the command you issued in step #5.
- Issue the following Linux pipeline command:
ls /bin/?? | sort | more
- What is difference with this pipeline command as opposed to the previous pipeline command? Press q to exit display.
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 | head -5
- What did you notice?
- Issue the following Linux pipeline command:
ls /bin/???? | sort | grep r | tail -2
- What did you notice? Could you predict the output prior to issuing this pipeline command?
- Issue the following Linux pipeline command:
ls /bin/???? | sort | grep r | cut -c1-6
- Try to explain step-by-step each process in the pipeline command (including filters) to explain the final output from this pipeine command.
- Confirm that you are still located in the ~/redirect directory.
- Issue the following Linux pipeline command:
ls /bin/???? | tee unsort.txt | sort | tee sort.txt | grep r | tee match.txt | head
Issue the ls command to view the contents of this redirectory.
- What did you notice?
View the contents of the text files that were created to see how the tee command was used in the previous pipeline command.
- What was the purpose of using the tee command for this pipeline command?
- You will now run a shell script to confirm that you properly issued that Linux pipeline command using the tee command and redirection.
Issue the following Linux command to run a checking script:
~osl640/week5-check-3
- If you encounter errors, make corrections and re-run the checking script until you receive a congratulations message, then you can proceed.
- Change to your home directory.
- Remove the ~/redirect directory and its contents.
In the next investigation, you will learn various techniques to issue multiple Linux commands on the same line, or issue a single Linux command over multiple lines.
Investigation 3: Issuing Multiple Unix/Linux Commands
In this investigation, you will learn how to issue multiple Unix / Linux commands in a single line or over multiple lines.
Perform the Following Steps:
- Confirm you are located in your home directory in your Matrix account.
- Issue the following Linux commands (using the semicolon character ";" to separate each Linux command):
cal;pwd;date
- Note the output as well as the order of what each Linux command results.
- 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
- What happened? Where is the output for the date command?
- Why isn't the output for the cal and pwd commands are NOT contained in that file?
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
- What did you notice?
Issue a Linux command to view the contents of the file called output.txt
- What does grouping do when issuing multiple Linux commands (separated by a semi-colon ";") that uses redirection?
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 is the purpose of issuing a Linux command in this way?
- Complete the Linux Practice Questions sections below to get additional practice.
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
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.
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” 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” 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.txt” to the “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” 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 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” directory. Use a relative pathname.
- 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.
- 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.
- 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.
- Write a single Linux pipeline command to display the number of lines contained in the file called “questions.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.
Author: Murray Saul
License: LGPL version 3 Link: https://www.gnu.org/licenses/lgpl.html