View source for Linux-New/C2/Redirecting-Streams-and-Pipes/English

'''TITLE: Redirecting Streams and Pipes'''

'''Author:''' EduPyramids

'''Keywords:''' Standard input,standard output, standard error, redirection, pipes, file descriptors, input redirection, output redirection, error redirection, pipes, pipelining, wc, ls, more, Linux terminal, command chaining, EduPyramids, video tutorial.


{|border=1
|- 
|| '''Visual Cue'''
|| '''Narration'''
|- 
|| '''Slide 1'''

'''Title Slide'''
|| Welcome to this spoken tutorial on '''Redirecting Streams and Pipes.'''
|- 
|| '''Slide 2'''

'''Learning Objectives'''

In this tutorial, we will learn to:
* Identify stdin, stdout and stderr streams in Linux.
* Redirect standard output and error to files using '''redirection operators''' (>, >>, 2>, 2>>).
* Explain file descriptors and their role in redirection.
* Connect multiple Linux commands using pipes (|) to manipulate data efficiently.

|| In this tutorial, we will learn to:
* Identify standard input, standard output and standard error streams in Linux.
* Redirect standard output and error to files using '''redirection operators''' 
* Explain file descriptors and their role in redirection.
* Connect multiple Linux commands using pipes (|) to manipulate data efficiently.
|- 
|| '''Slide 3'''

'''System Requirements'''
|| To record this tutorial, I am using

'''Ubuntu OS version 24 point zero 4'''
|- 
|| '''Slide 4'''

'''Pre-requisites'''

[https://edupyramids.org/ https://EduPyramids.org]
|| To follow this tutorial,
* Learners should have '''Ubuntu version 24 point zero 4.'''
* For the prerequisite Linux tutorials please visit this website.
|-
|| '''Slide 5'''

'''Code files'''

The following code file is required to practice this tutorial
# '''rsp-commands.txt'''

This file is provided in the Code Files link of this tutorial page.
|| The following code file is required to practice this tutorial.

This file is provided in the '''Code Files''' link of this tutorial page.
|- 
|| Press''' Ctrl + Alt + T '''keys
|| Let us get started.

Open the terminal.
|- 
|| Open Terminal

Type '''cat aaa '''press''' Enter'''

Highlight Errored output.

'''cat: aaa: No such file or directory'''
|| Let’s type '''cat a a a''' and press '''Enter'''.

Since this file does not exist, terminal shows an error message.

Notice that errors are reported on the terminal.
|- 
  
|| '''Slide 6'''

'''Standard Streams'''

'''std-streams.png'''
|| '''Streams:''' are continuous flow of data used for input, output, and errors.

Linux commands handle '''input''', '''output''', and '''errors''' using '''streams''' and''' file descriptors'''.

Linux uses three standard streams:
* '''s t d in (0)''' for input
* '''s t d out (1)''' for output
* '''s t d e r r (2)''' for errors

By default, the standard streams are connected to the terminal.

Using '''redirection''', we can change this behavior.

Standard input can be redirected using the ''' less than(<)''' operator.
|- 
|| In a terminal type '''cat >test1.txt '''press''' Enter 

Type: Sample to learn Linux.

'''Press''' Enter.'''Press''' Ctrl '''and''' D '''keys together.
|| Now, let us learn to redirect standard input.

Let's create a file named''' test1 dot t x t''' in the current directory and add some text to it.

Users can pause the video and create the file '''test1 dot t x t'''.
|- 
|| Type '''wc < test1.txt'''Press '''Enter.'''

Point to the output on the terminal.
|| Now, type '''w c space less than space''' '''test1 dot t x t''' and press '''Enter.

'''w c''' displays the number of lines, words, and characters in the '''test1 dot t x t '''file.

This command reads the file using '''s t d in''', showing input redirection.
|- 
|| 
|| Now, let us learn to redirect '''standard output''' and '''standard error'''.

There are two ways to redirect them to a file.

Let us see this practically.
|- 
||Show the terminal output of '''wc command'''
|| Suppose we want to save the output to a file for later use. By default, '''w c''' sends output to '''s t d out''' 

Redirecting '''s t d out''' saves the output to a file instead.
|- 
|| Type''' wc test1.txt > wc-results.txt'''

Type '''cat wc-results.txt'''

Point to the output
|| Type: '''w c space test1 dot t x t space greater than space w c hyphen results dot t x t'''

Press '''Enter'''.

Now, to verify this, display the contents of '''w c hyphen results dot t x t''' using the '''cat''' command.

Type

'''cat space w c hyphen results dot t x t'''

And press '''Enter'''

The output from '''w c '''has been written to the file.
|- 
|| Type: '''echo "hello" > test2.txt''' press '''Enter'''

Show the output line with test2.txt

Type '''wc test2.txt > wc-results.txt'''

Type '''cat wc-results.txt '''press''' Enter'''
|| Let me create a new file by typing '''echo space “hello” space greater than space test2 dot t x t''' and press '''Enter.'''

We see the file''' test2 dot t x t''' is created.

Type '''w c space test2 dot t x t space greater than space w c hyphen results dot t x t'''.

The contents of '''w c hyphen results dot t x t ''' will be overwritten.

Let us verify using the '''cat '''command.
|- 
|| Type

'''wc test1.txt >> wc-results.txt ''' and '''Press Enter'''

Type '''cat wc-results.txt'''
|| Instead, if we run:'''w c space test1 dot t x t greater than greater than (>>) w c hyphen results dot t x t ''' and press '''Enter.'''

The new contents will not overwrite the existing contents of ''' w c hyphen results dot t x t.'''

Instead, the output will be appended to the file.

Let us verify using the '''cat '''command

Let me clear the terminal.
|- 
|| 
|| Redirecting '''standard error''' is done in a similar way.

The only difference is that we need to specify the file descriptor number of '''standard error.'''

This number comes before the '''greater than (>)''' or '''greater than greater than(>>)''' sign.
|- 
|| Type '''wc aaa '''press '''Enter 
|| Highlight error. '''There is no file named''' a a a'''.

Type: '''w c space a a a '''and press '''Enter.'''

The shell will display an error as '''No such file or directory.'''
|- 
|| 
|| If we don’t want error messages to show on the screen, we can redirect them to a file.
|- 
|| Type: '''wc aaa 2>errorlog.txt''' and press Enter
|| To redirect the error, type the following command:'''w c space a a a space 2 greater than(>) errorlog dot t x t''' and press '''Enter'''.

Now, the error will not appear on the terminal.

Instead, it will be written to the file '''errorlog dot t x t '''
|- 
|| Type: '''cat errorlog.txt '''press''' Enter'''
|| We can check the contents of '''errorlog dot t x t''' by typing, '''cat''' space''' errorlog dot t x t '''press''' Enter.

'''This will display the error message that was redirected from the terminal.
|- 
|| Type '''cat bbb 2> errorlog.txt 

Press Enter.

'''Type: '''cat errorlog.txt '''press''' Enter'''
|| Now, suppose we make another error by typing:'''cat space b b b space 2 greater than(>)space errorlog dot t x t '''and press''' Enter'''.

The previous contents of '''errorlog dot t x t''' will be overwritten.

The new error message will replace the old one.

Type '''cat space errorlog dot t x t '''and''' '''press''' Enter''' to see the changed error.
|- 
|| Type '''wc aaa 2>>errorlog.txt'''

Press '''Enter.'''
|| But what if we want to keep a record of all errors instead of overwriting them? 

We can append errors to a file using:'''w c space a a a space 2 greater than greater than(>>) errorlog dot t x t''' press '''Enter'''.

Now, all error messages will be added to '''errorlog dot t x t''' instead of replacing the previous ones.

To keep all errors instead of overwriting them, we can '''append''' them to a file. 
|- 
|| '''cat errorlog.txt''' press '''Enter'''
|| We can check all the accumulated errors by typing:'''cat''' space '''errorlog dot t x t '''and press''' Enter'''.  

This will display all the error messages that were appended to the file.
|- 
|| Type

'''wc aaaa.txt'''

Press '''Enter'''

Show the outputstdin , stdoutput, std error.
|| Now type:'''w c''' space '''a a a a dot t x t '''and press '''Enter'''.

The error caused by this command is displayed on the terminal.

If no redirection is used, future errors will also be shown on the terminal.

We saw how the three streams, '''standard output''', '''standard input''', and '''standard error''' can be redirected separately.
|- 
|| '''Slide 7'''

'''Pipelining'''

'''pipelining.png'''
|| We can gain more control if we can manipulate multiple streams together.

This process is called  '''pipelining'''.

'''Pipes''' link commands into a chain.The output of one command becomes the input of the next and so on.

A pipeline connects multiple commands using the pipe, that is, vertical bar symbol.
|- 
|| '''Image: pipe1.png'''

command1''' |''' command2 '''| '''command3''' | '''command4
|| A pipeline looks like this:

Here, the output of command1 is sent as input to command2, then its output is sent to command3, and so on, forming a chain of commands.
|- 
|| Back to terminal
|| Suppose we want to know the total number of files and directories in the current directory.

We can achieve this using a pipeline of commands.
|- 
|| 
|| Let me clear the terminal.
|- 
|| '''Type cat test1.txt''' press''' Enter'''
|| To see the contents of the file type:'''cat space test1 dot t x t '''and press '''Enter'''.

Type '''l s ''' and press '''Enter'''.This will display the list of files and directories.
|- 
|| Type: '''ls -l > test1.txt '''press''' Enter'''
|| To redirect type '''l s space hyphen l greater than (>) space test1 dot t x t '''and press '''Enter'''. 

The file list will be saved in '''test1 dot t x t''' instead of the terminal.
|- 
|| Type '''wc -l test1.txt''' press''' Enter'''
|| Now, to count the total lines, we can use:'''w c space hyphen l space test1 dot t x t''' and press '''Enter'''. 

This gives us the total number of files and directories.
|- 
|| Type '''ls -l | wc -l '''press '''Enter'''
|| We can do this more efficiently using a pipe:

'''l s space hyphen l space vertical bar space w c space hyphen l '''and press '''Enter.'''

Here, the output of the '''l s hyphen l''' command is sent directly as input to the ''' w c hyphen l''' command.

The output appears on the terminal, giving the total files and directories without an extra file.
|- 
|| Type''' cd /usr/bin '''and press''' Enter.'''

Highlight changed '''directory'''.
|| We can create longer chains of commands using '''pipes'''.

One common use of pipes is for reading multipage displays.

First, navigate to the ''' slash u s r slash bin''' directory.

Type ''' c d space slash u s r slash bin ''' and press''' Enter.'''

Now, we are in the '''slash u s r slash bin''' directory.
|- 
|| Type: '''ls -l press Enter'''

Type: '''ls -l | more '''press '''Enter'''
|| Now, run:'''l s space hyphen l ''' and press''' Enter '''.

The output is too long to view on the terminal.

Using a pipe with '''more''' lets us view it page by page:

Type''' l s space hyphen l space vertical bar space more''' and press '''Enter'''.

Here, the output of '''l s hyphen l''' is passed to '''more''', letting us view one screen at a time.
|- 
|| Press '''Enter''' 

Press '''q'''
|| Press '''Enter''' to scroll through the list one line at a time.

Press '''q''' to exit the '''more''' command and return to the terminal.
|- 
|| '''Slide 8'''

'''Summary'''

In this tutorial, you will learn to:
* Identify stdin, stdout and stderr streams in Linux.
* Redirect standard output and error to files using '''redirection operators''' (>, >>, 2>, and 2>>).
* Explain file descriptors and their role in redirection.
* Connect multiple Linux commands using pipes (|) to manipulate data efficiently.
|| With this we come to the end of this tutorial.

Let us summarise.
|- 
|| '''Slide 9'''

'''Assignment'''
# Create a file named '''mydata.txt '''and add 5 lines of text in it.
# Count the number of lines in '''mydata.txt''' and store the result in '''lines.txt'''.
# Display '''mydata.txt''' page by page in the terminal using '''more''' with a pipe.
# Display a file '''nofile.txt''' and redirect the error message to '''errorlog.txt'''.
# Count words in '''mydata.txt''' and show the result using a pipe.
|| As an assignment, please do the following.
|- 
|| '''Slide 10'''

'''Thank you'''

This Spoken Tutorial is brought to you by '''EduPyramids''' Educational Services Private Limited SINE IIT Bombay.
|| Thank you.
|-
|}