Sed Command in Unix/Linux 

Suppose you have a text file named "text.txt" with the following content                                                         2.1 Using the sed command let's try to delete the empty lines present in the file.                        

let's break down the sed command sed '/^$/d' text2.txt into its components:

• sed: This is the command for the stream editor.
• /^$/: This part enclosed in slashes is a regular expression pattern. It specifically matches empty lines. Here's what it means:
  • ^: Matches the start of a line.
  • $: Matches the end of a line.
  • Combined, ^$ matches lines that have nothing between the start and end, in other words, empty lines.
• d: This is the sed command to delete lines.
• text2.txt: This is the name of the file you want to process. In this case, it's "text2.txt."                                                                                           

2.2  Deleting Lines by Line Number

Suppose you have a text file named "text3.txt" with the following content

Now let us delete the 5th line using the sed command.

Certainly, let's break down the sed command sed "5d" text3.txt into its components:

• sed: This is the command for the stream editor.
• 5d: This part is a sed command that deletes the 5th line.
• text3.txt: This is the name of the file you want to process. In this case, it's "text3.txt."

So, when you put it all together, the sed "5d" text3.txt command tells sed to delete the 5th line in the "text3.txt" file.

2.3 Deleting Lines in a Range

Let us try to delete lines 3 to 7 from the text3.txt using the sed command.

let's break down the sed command sed "3,7d" text3.txt into its components:

• sed: This is the command for the stream editor.
• 3,7d: This part is a sed command that deletes lines from the 3rd to the 7th line (inclusive).
• text3.txt: This is the name of the file you want to process. In this case, it's "text3.txt."

So, when you put it all together, the sed "3,7d" text3.txt command tells sed to delete lines 3 through 7 in the "text3.txt" file.

3. Add or Insert Text:

3.1 Insert Text at the Beginning of Each Line

Suppose you have a text file named "text3.txt" with the following content

Let us try to insert "Hello all " at the beginning of every line present in text3.txt using the sed command.

 

3.2 Insert Text After a Specific Pattern

Suppose you have a text file named "text3.txt" with the following content

Now let us try to insert line no after line in the entire file text3.txt using the sed command.

3.3 Insert Text After a Specific Line

Suppose you have a text file named "text3.txt" with the following content 

Let us try to insert this is line-10 after the 9th line in text3.txt file.

let's break down the sed "9a\this is line-10" text3.txt command:

• sed: This is the command for the stream editor.
• "9a\this is line-10": This is the sed command.
  • 9: Specifies the line number (in this case, the 9th line).
  • a: Stands for "append." It tells sed to append text after the specified line.
  • \this is line-10: The text you want to insert, which is "this is line-10" in this case.
• text3.txt: This is the name of the file you want to process. In this command, it's "text3.txt."

So, when you run the sed "9a\this is line-10" text3.txt command, it inserts the line "this is line-10" after the 9th line in the "text3.txt" file, leaving the original contents unchanged. if you want to change the original contents of the file use the -i option.

4. Text Transformation:

Let us Convert Uppercase to Lowercase using the sed command. Suppose you have a file named "data.txt" with text in uppercase, and you want to convert it to lowercase. You can use sed to achieve this transformation:

Now let us try to Convert Uppercase to Lowercase using the sed command 

5. Filtering Lines Containing a Specific Word:

Suppose you have a file named "sample.txt," and you want to filter and display only the lines that contain the word "apple." You can use sed to achieve this:

let us try to print only the lines containing the word "apple"

As you can see, only the lines that match the pattern "apple" are displayed, thanks to the filtering capability of sed.

6. Conditional Editing:

Suppose you have a file named "data.txt" with lines of text, and you want to add the prefix "Processed: " to lines that contain the word "important." You can use sed with a conditional statement to achieve this:

When you run the sed '/important/s/^/Processed: /' data.txt command, it will modify the lines that contain "important" and add the "Processed: " prefix:

As you can see, the prefix "Processed: " was added only to the lines that contained the word "important," demonstrating how conditional editing in sed allows you to selectively modify text based on specific conditions or patterns.