Advanced sed Command Techniques in Linux: Replace, Edit, and Delete Text

The Linux sed command, short for stream editor, is one of the most powerful tools available for Text manipulation. Whether you want to search and replace Text, delete lines, or extract parts of a file, sed offers a variety of functions to handle Text processing in an efficient and non-interactive manner. This article will take you through everything you need to know about sed, from basic commands to advanced usage, including common issues and how to troubleshoot them.

1. What is sed?

Sed command line tool that allows you to perform fundamental Text modifications on an input data stream (typically a file or input from a pipeline). Unlike Text editors like Vim or Nano, which require manual interaction, sed can automate Text processing tasks. It’s widely used for:

• Searching for specific Text patterns
• Replacing Text within files
• Deleting lines from files
• Modifying Text based on patterns

The best part? You don’t need to open the file in an editor to make changes—sed can do it all from the command line.

2. Basic Syntax of sed

Let’s first understand the basic syntax of sed Before diving into examples:

sed [options] 'command' file

• Options: Flags that modify the behavior of sed.
• Command: The action you want to perform (like substitution, deletion, etc.).
• File: The file you wish to process.

Options in sed

Here are some useful options you will frequently use with sed:

• -n: Suppresses automatic printing of lines. Those lines which are modified by commands will be printed.
• -i: Edits the file in-place, saving changes directly to the file.
• -e: Allows you to run multiple sed commands in sequence.

3. Everyday Use Cases for sed

Let’s start with some simple examples that illustrate the power of sed for everyday text-processing tasks.

3.1. Replacing Text in a File

One of the most common uses of sed is to replace Text. The s (substitute) command allows you to replace occurrences of one string with another.

Syntax:

sed 's/old-text/new-text/' filename

Example: Replace the word “apple” with “orange” in a file:

sed 's/apple/orange/' fruits.txt

This action replaces the first occurrence of the word ‘apple’ on each line with ‘orange.’

Replace All Occurrences in a Line

To replace every occurrence of the word on a line, add the g (global) flag:

sed 's/apple/orange/g' fruits.txt

Replace Text on Specific Lines

You can also target a specific line number. For example, to replace the word “apple” with “orange” only on the second line:

sed '2s/apple/orange/' fruits.txt

3.2. Deleting Lines

sed can delete lines based on line numbers or patterns.

Syntax:

sed 'Nd' filename

• N: The line number you want to delete.

Example: To delete the third line from a file:

sed '3d' file.txt

Deleting a Range of Lines

You can also delete a range of lines:

sed '3,5d' file.txt

This command deletes lines from 3 to 5.

Deleting Lines Matching a Pattern

Then, you can remove lines that have the same pattern using this command:

sed '/pattern/d' file.txt

Example: Delete all lines containing the word “error”:

sed '/error/d' logfile.txt

3.3. Inserting and Appending Text

Using the i (insert) and a (append) commands, you can insert or append Text at a specific location in a file.

Insert Text Before a Line:

sed '2i\Inserted line' file.txt

Append Text After a Line:

sed '2a\Appended line' file.txt

Enter or add Text to the line numbers in order to change the file values.

4. Advanced sed Techniques

After you’ve learned the rudiments, you will crave knowledge of what more sed has to offer. Included are patching regular expressions, conditional replacements, and combining multiple commands.

4.1. Using Regular Expressions (Regex)

sed is capable of understanding and using regular expressions, which allows for more complex Text manipulation.

Example: Replace any sequence of digits with the word “number”:

sed 's/[0-9]\+/number/g' file.txt

This operation replaces all digit sequences in the file with the word ‘number.’

4.2. Performing Multiple Operations

You can perform multiple operations in one sed command by separating them with a semicolon (;):

sed 's/apple/orange/g; 5d' file.txt

This command replaces “apple” with “orange” globally and then deletes the fifth line.

4.3. Save Changes to the File

By default, sed works on the input file without making any changes to the original file, ensuring the original stays intact. However, you can use the -i flag to edit the file in place:

sed -i 's/old-text/new-text/g' file.txt

“This action saves the changes directly to the file without creating a new copy.”

Creating a Backup File

When using the—i option, you can specify a backup extension for the original file and have a copy of it as a backup.

sed -i.bak 's/old-text/new-text/g' file.txt

This process creates an original copy of the file by adding a .bak extension before making any changes.

Sed is definitely a versatile tool, but users may need help using it. Here’s how to resolve them:

5.1. Problem: sed Not Changing Anything

Solution: Make sure you don’t just print the output but actually modify the file.

sed -i 's/apple/orange/g' file.txt

5.2. Problem: Special Characters Causing Errors

Solution: Some characters like /, &, and \ are special in sed and need to be escaped with a backslash.

Example: To replace /home/user with /root:

sed 's/\/home\/user/\/root/' file.txt

Alternatively, you can use different delimiters:

sed 's|/home/user|/root|' file.txt

5.3. Problem: Blank Lines Not Being Deleted

Solution: Ensure you’re using the correct pattern to match blank lines. Use the following command to delete all blank lines:

sed '/^$/d' file.txt

5.4. Problem: sed Hanging or Freezing

Solution: It is usually caused when sed is trying to read from a very huge file. If you need to optimize your sed commands and you want to divide the file into pieces in order.

6. Summary of Important Commands

Here’s a quick overview of the most commonly used sed commands:

Command	Description
sed ‘s/old/new/’ file	Replace the first occurrence of “old.”
sed ‘s/old/new/g’ file	Replace all occurrences of “old” globally
sed ‘Nd’ file	Delete line number N
sed ‘/pattern/d’ file	Delete lines matching a pattern
sed -n ‘Np’ file	Print line number N
sed -i ‘s/old/new/g’ file	Edit the file in place
sed ‘Ni\new line’ file	Append Text after line N
sed ‘Na\new line’ file	Append text after line N
sed ‘/^$/d’ file	Delete all blank lines

Conclusion

Linux Text Processing is just not a process, but sed is a cornerstone. Small files, large files, whether used for repetitive tasks working with only a little bit of file or an awful lot of file, sed is your friend here. From slight Text substitution to advanced usage, like using regular expressions and solving an everyday problem, we’ve learned everything.

By mastering sed, you’ll be able to manipulate Text files quickly and precisely, making it invaluable for system administrators, developers, and anyone working with Text files in Linux.

About the writer

Vinayak Baranwal wrote this article. Use the provided link to connect with Vinayak on LinkedIn for more insightful content or collaboration opportunities.