Difference between revisions of "Linux-Ubuntu/C3/Introduction-to-the-Stream-Editor-sed/English"

Title Slide

Learning Objectives

• Use sed as a stream editor to process text.
• Print specific lines from a file.
• Apply line-based addressing techniques.
• Use context-based addressing with pattern matching.
• Redirect matched output to files.

System Requirements

• Ubuntu OS version 24 point 04 and
• Bash version 5 point 1 point 16.

Pre-requisites

https://EduPyramids.org

• Learners should have Ubuntu version 24 point 04 .
• Bash version 5 point 1 point 16 and
• Should know the basics of Linux terminal.
• For the prerequisite of Linux tutorials please visit this website.

Code Files

The following code files are required to practice this tutorial.

1. seddemo.txt.
2. sed-commands.txt

These files are provided in the Code Files link of this tutorial page.

• The following code files are required to practice this tutorial.
• These files are provided in the Code Files link of this tutorial page.

Introduction to sed

• sed is one of the most versatile tools available in Linux.
• It is a stream editor used to process text line by line.
• sed can function both as a display tool and an editing tool.
• It supports editing operations such as insertion, substitution and deletion.
• By default, sed sends its output to the standard output.
• The original file remains unchanged unless sed is explicitly instructed to modify it.

Highlight the lines, marks and student details.

We will see how to print lines using the sed command.

I have a file called sed demo dot t x t in the home directory.

This file is provided in the code files folder.

Please download it and move it to your Home directory.

Let us view its contents.

This file contains entries such as roll no, name, stream, marks, result and the stipend amount.

Type,

sed space single quote 2 p single quote space sed demo dot t x t.

Highlight p

Press Enter

highlight the output.

Hover your mouse on the second line of the output

The letter p specifies the action, which is printing.

Press Enter.

Observe the output.

It shows the entire file but the second line appears twice.

This is the default behavior of sed.

sed prints every line by default

The p option prints the specified line again.

sed -n '2p' seddemo.txt Press Enter

Highlight 2

Highlight p

Highlight sed demo.txt

• The hyphen n option suppresses the default output of sed.
• 2 specifies the line number to display or edit.
• p specifies the action to perform.
• It tells sed to print the selected line.
• sed demo dot t x t specifies the input file name.

This is the general syntax of the sed command.

Highlight -n

Highlight $ sign

Type this command and press Enter.

We see that the last line is displayed.

The hyphen n suppresses automatic printing of all lines.

Dollar refers to the last line of the file.

sed -n '3,6p' seddemo.txt press Enter

Highlight the output.

Type this command and press Enter.

The output displays the lines from the third line to the sixth line.

Press Enter

Show the file on one side and the output comparison

Type this command.

Any action can be reversed by placing an exclamation mark before the command.

This allows us to select the complementary set of lines.

Press Enter.

The output displays all lines except lines 3 to 6.

This method is known as line addressing.

Line addressing

It is one way to select specific lines for an action.

Another method is called context addressing.

Context addressing

• Context addressing selects lines based on their content.
• Lines are matched using a word, string, or pattern.
• Only the lines that match the pattern are processed.
• It is useful when line numbers are unknown or variable.
• Regular expressions can be used for flexible and powerful matching.
• Commonly used for searching and filtering text in files.

At the promptsed -n '/[cC]omputers/p' seddemo.txt

Highlight -n

highlight /[cC]omputers/

highlight p

Press Enter.

highlight /[cC]omputers/

Go to the Terminal.

Type the following command.

hyphen n suppresses default output.

This is the Context address.

It matches lines containing the word computers with small c and capital C.

p, prints the matched lines.

Press Enter.

Observe the output.

The lines containing the word computers are displayed.

We enclose parts of the pattern within square brackets.

This allows us to match any one character present inside the brackets.

When using a pattern in sed, it must be written between forward slashes.

sed -n '/[cC]omputers/w computer-student' seddemo.txt

press Enter.

Open file and show the computer-student

Type the following command.

and Press Enter.

Output is redirected to a file instead of getting displayed on the terminal.

This is useful for extracting specific lines into a separate file.

All matching lines are saved in the file computer hyphen student.

cat computer-student andPress Enter

type cat space computer hyphen student and press Enter.

We see that lines that match the given pattern are in the file.

Highlight -e

Highlight /electronics/w electro

Highlight /civil/w civil

Show the two files created

This allows us to write different matched lines to separate files.

Type this command and press Enter.

The hyphen e option allows combining multiple sed commands in a single line.

slash electronics slash w electro writes lines matching electronics to electro file.

slash civil slash w civil writes lines matching civil to civil file.

After running the command, two text files, electro and civil, are created with the matched lines.

Highlight the contents

cat civil

Press Enter

Highlight the contents

This displays the entries with the word electronics.

Let us see the contents of the file civil

Type cat space civil and press Enter.

This displays the entries having the word civil.

Summary

In this tutorial, we have learnt to:

• Use sed as a stream editor to process text
• Print specific lines from a file
• Apply line-based addressing techniques
• Use context-based addressing with pattern matching
• Redirect matched output to files

Let us summarise.

Assignment

As an assignment,

Using the text file seddemo.txt,

1. Print only the first five lines of the file.
2. Print all lines that do not contain the word "failed".
3. Write all lines containing the word student into a file named student-list.txt.

Thank you

Thank you.