Difference between revisions of "Linux/C3/The-sed-command/English"

Learning objectives

System Requirement

Ubuntu Linux 12.04 Operating System and

GNU BASH version 4.2.24

Please note, GNU bash version 4 or above is recommended to practice this tutorial.

Prerequisites

You should know Basics of linux terminal

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Introduction Let us start with an introduction to sed.

Its a stream editor.

sed finds some pattern of text in a particular location of a file.

Then it behave as a display or editing function.

It performs editing like insertion, substitution, deletion of the matched text.

[enter]

We will see how to print using the sed command.

I have a file called seddemo.txt in home directory.

Let us view its content.

In this file we have some enteries like roll no, name, stream, marks, pass or fail and the stipend amount.

sed '2p' seddemo.txt [enter]

Hover your mouse on the second line

For this we need to oepn the terminal by pressing CTRL + ALT and T keys simultaneously on your keyboard.

Now Type:

sed space (in single quotes) ‘2p’ space seddemo.txt

Press Enter

p denotes the action, which is printing(p).

Look at the output.

It shows the entire file but see that the second line is printed twice.

This is the default behaviour of the action p.

sed -n '2p' seddemo.txt

[enter]

type:

sed space (minus) -n space (in single quotes) 2p space seddemo.txt

Press Enter.

-n

2

p

seddemo.txt

We see only the second line is printed.

Then we give the location in the stream that we want to edit or display.

We want to select the second line.

p indicates the action we want to take ie to print the second line.

And seddemo.txt is the name of the file

This is the general syntax of sed command.

sed -n '$p' seddemo.txt

[enter]

type:

sed space (minus) -n space (in single quotes) (dollar) $p space seddemo.txt

Press Enter.

We see that the last line is printed.

Now come back to the text editor.

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

[enter]

For this we need to type on the terminal:

sed space (minus) -n space (in single quotes) ‘3 (comma) ,6p’ space seddemo.txt

Press Enter.

The output is displayed from the third line to the sixth line.

sed -n '3,6!p' seddemo.txt

[enter]

Say if we had to print all lines except from 3^rd to 6^th we would type:

sed space (minus) -n space (in single quotes) ‘3 (comma) ,6 (exclamation mark) !p space seddemo.txt

Press Enter.

The output is displayed.

Let us switch back to the slides.

Line addressing and context addressing.

This is known as line addressing.

Address specified by the line numbers.

This is one way of addressing.

Another way of addressing is Context addressing

Lines that contain particular context say a particular word.

If we want to take actions on lines that contain a particular word we use context addressing.

Regular expressions can be used.

Let us see an example.

Come back to our editor.

sed -n '/[cC]omputers/p' seddemo.txt

[enter]

Come back to our temrinal.

type:

sed space (minus) -n space (in single quotes) (front slash)(opening square bracket) (opening square bracket) (front slash) ‘/[cC]omputers/p space seddemo.txt

Press Enter.

We see the lines with the word computers is displayed.

We write pattern within square brackets. This is to match any one or both of the characters within square brackets.

When we need to match patterns the pattern needs to be typed between front slashes.

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

[enter]

For this type:

sed space (minus) -n space (in single quotes) (front slash) (opening square bracket) (opening square bracket) (front slash) /[cC]omputers/w space computer_student space seddemo.txt

Press Enter.

Now all the matching lines would be transferred to the file computer_student.txt.

cat computer_student

[enter]

type:

cat space computer_student

Press Enter.

sed -n -e '/electronics/w electro’ -e

‘/civil/w civil' seddemo.txt”

[enter]

Type:

sed space (minus) -n space (minus) -e space (in single quotes) (front slash) ‘/electronics/w space electro.txt’ (minus) -e space (in single quotes) (front slash) ‘/civil/w space civil.txt’ space seddemo.txt

Press Enter

-e is used to combine multiple methods

Press Enter

This would create two files electro.txt and civil.txt.

[enter]

cat civil

[enter]

cat space electro.txt

This will display the enteries with the word electronics.

Let us see the content of civil file.

Then type:

cat space civil.txt

press Enter

This will display the enteries having the word civil.

I will use the same program.

Summary

In this tutorial we learnt,

sed.

To print using sed.

Line Addressing.

Context Addressing.

Use the same text file seddemo.txt.

Try to print records from 6th to 12th line.

Acknowledgement Slide

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Display the previous slide only and narrate this line.