Difference between revisions of "Sed-Stream-Editor/C2/Sed-substitute-command/English"

Learning Objectives

• Workflow of Sed command
• Introduction to Sed command
• Sed substitute command

System requirements

Prerequisites

If not, then go through the Linux spoken tutorials on this website.

Code files

• The files used in this tutorial are available in the Code Files link on this tutorial page.
• Please download and extract them
• Make a copy and then use them while practising

1. Read - Sed reads a line from the input stream.

It can be a file or standard input.

It is stored in an internal buffer called pattern buffer.

First Sed reads a line from the input stream.

It can be a file or standard input.

It is stored in an internal buffer called pattern buffer.

2. Execute – All Sed commands are executed sequentially on the pattern buffer.

3. Display- Sends the modified content to the output stream.

The pattern buffer will be empty after sending the data.

All Sed commands are executed sequentially on the pattern buffer.

Third step is Display.

It sends the modified content to the output stream.

The pattern buffer will be empty after sending the data.

Remember to press the Enter key after every command.

sed ‘s/Unix/Linux/’ Highlight according to narration

sed space within single quotes s forward slash Unix forward slash Linux forward slash >> Enter

Here ‘s’ indicates the substitute option.

substitute command will replace Unix with Linux.

Here we are using the Sed command without a file.

So it will process from standard input.

"Unix is the most powerful OS" >> Enter

Highlight the output

Press Ctrl+d

We can see the word Unix is replaced with Linux.

>“unix is the most powerful OS”

Type unix is the most powerful OS and press Enter.

Note that we have typed ‘u’ in lowercase in Unix.

Here the replacement didn’t take place.

Press “Ctrl+D" keys

Press “Ctrl+D” keys to return back to the command prompt.

../Downloads>cat message.txt

Highlight Downloads

Highlight message.txt

Go to the directory where you have saved the message.txt on your system.

Type cat space message.txt

I’ll be using this file for demonstration of sed commands.

Clear the screen.

Type sed space a pair of single quotes space message.txt

It prints the content of the entire file.

In our case there is no sed option.

So it prints the contents of the pattern buffer on the standard output.

sed [options]...[command] [File]

Example:

sed G message.txt

In this example, capital ‘G’ is the option to add line space to the file.

message.txt is the file where the sed command will work with.

If no filename is provided with sed command, then the command will work on standard input data.

> sed G message.txt

Highlight the output

Type sed space G space message.txt

We can observe line space is added to the output.

Substitute command

syntax:

sed ‘s/oldstring/newstring/’ file

sed ‘s/oldstring/newstring/’ file > output.txt

Here ‘s’ specifies the substitute command.

forward slash (/) is to delimit the two arguments.

oldstring is the string to be searched from the file.

newstring is the string to be replaced by the searched string.

file is the input text file.

The redirection symbol and a file name is used to redirect the output to another file.

By default sed outputs everything to standard output instead of saving it in a file.

Highlight where the replacement has not taken place

sed space s forward slash success forward slash victory forward slash space message.txt

This will replace the word success with the word victory in message.txt.

We can use sed commands without using single quotes.

>sed s/success/great victory/ message.txt

Here we got an error and we didn’t get the expected output.

Highlight great victory

So quotes are needed when we use space or a special shell character inside the command.

It is good practice to write sed commands using quotes.

>sed ‘s/success/victory/’ message.txt

Type the command as shown.

Sed replaces only the first match in each line with the substitute command.

It will not replace the second or third occurrence in the same line.

Also note that in certain lines it didn’t substitute with the word victory.

This is because Success word has capital ‘S’.

As we know, sed commands are case sensitive.

We can use the flag capital I to make the pattern match case insensitive.

Look at the output.

Highlight the last sentence

>sed ‘s/success/victory/ 2’ message.txt

Use the ‘g’ option to represent global replacement.

Type the command as shown.

Here, we have used the capital I and g flags together.

This will replace all the patterns globally with case insensitive.

Highlight ‘Albert’ at two places

>sed '/Albert/s/success/victory/' message.txt

We want to replace the word success with victory in the line which contains the text ‘Albert’.

Type the command as shown.

Let us check the output.

Highlight the output

sed within single quotes s forward slash within square brackets 0 hyphen 9 asterisk forward slash forward slash space message.txt

Here regular expression represents one or more occurrences of 0-9 characters.

I want to replace it with the empty string.

In the output we can see, all the line numbers are replaced with empty spaces.

Type the command as shown.

We can see the replacement in the first two lines

Here, replacement takes place from the fourth line to the last line.

For example, in this command caret (^)symbol represents the beginning of the line.

If the line begins with 2 dot, then on that line it will perform the substitute command.

Slide 8:

Summary

• Work flow of Sed command
• Introduction to Sed command
• Sed substitute command

Assignment

As an assignment, try the below commands and see the output.

1. sed 's/[0-9.]*/(*)/' message.txt
2. sed '/secrets/s/success/victory/g' message.txt
3. sed 's/-/:/' message.txt

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