The SED command is one of the most powerful commands used during the process of text processing in Linux/Unix operating systems. The SED command is typically invoked for executing operations such as replace and search, text manipulation, and stream editing.

With SED, you can manipulate text files with ease without having to open them in an editor, and that's why it's such a great fit to edit batch files, work with log files, and do rapid conversions on big files.

In this guide, we will walk you through the SED command syntax, use cases and most commonly used SED options to help you master this power tool. You'll also learn how to securely manipulate text, replace strings, delete lines, and automate tasks seamlessly in Linux/Unix environments.

What is the Sed Command

SED command (Stream Editor) is a text manipulation command used to execute simple text transformations and complex operations like find and replace, inserting text, deleting text, and replacing text. It is often utilized in bscripts to automate tasks (e.g., file editing).

Sed Command Syntax:

The basic syntax for using the SED command in Linux is:

sed [options] 'command' [inputfile...]

where,

• 'OPTIONS': These are optional flags that modify the behavior of the sed command.
• 'COMMAND': This defines the command or sequence of commands to execute on the input file.
• 'INPUTFILE': One or more input files to be processed.

Commonly Used SED Command Options

Below are some of the most frequently used SED command options, let's check them out:

Practical Examples of SED Command Usage

Here are some basic SED commands that will help you get started with text manipulation.

Consider the below text file as an input.

cat > geekfile.txt

unix is great os. unix is opensource. unix is free os.
learn operating system.
unix linux which one you choose.
unix is easy to learn.unix is a multiuser os.Learn unix .unix is a powerful.

1. Sample Commands

Replacing or substituting string: Sed command is mostly used to replace the text in a file. The below simple sed command replaces the word "unix" with "linux" in the file.

sed 's/unix/linux/' geekfile.txt

Output:

linux is great os. unix is opensource. unix is free os.
learn operating system.
linux linux which one you choose.
linux is easy to learn.unix is a multiuser os.Learn unix .unix is a powerful.

Here the "s" specifies the substitution operation. The "/" are delimiters. The "unix" is the search pattern and the "linux" is the replacement string. By default, the sed command replaces the first occurrence of the pattern in each line and it won't replace the second, third...occurrence in the line.

2. Replacing the nth Occurrence of a Pattern in a Line

To replace only the nth occurance of a word in a line, use the following syntax:

 sed 's/old_word/new_word/n' filename

Use the '/1', '/2' etc. flags to replace the first, second occurrence of a pattern in a line. The below command replaces the second occurrence of the word "unix" with "linux" in a line.

sed 's/unix/linux/2' geekfile.txt

Output:

unix is great os. linux is opensource. unix is free os.
learn operating system.
unix linux which one you choose.
unix is easy to learn.linux is a multiuser os.Learn unix .unix is a powerful.

3. Replacing all the Occurrence of the Pattern in a Line

Here, we will use the g flag to replace all the occurances of a pattern in a line. Let's check out the syntax below:

sed 's/old_word/new_word/g' filename

The substitute flag /g (global replacement) specifies the sed command to replace all the occurrences of the string in the line.

sed 's/unix/linux/g' geekfile.txt

Output:

linux is great os. linux is opensource. linux is free os.
learn operating system.
linux linux which one you choose.
linux is easy to learn.linux is a multiuser os.Learn linux .linux is a powerful.

4. Replacing from nth Occurrence to all Occurrences in a Line

Use the combination of /1, /2 etc and /g to replace all the patterns from the nth occurrence of a pattern in a line. The following sed command replaces the third, fourth, fifth... "unix" word with "linux" word in a line.

sed 's/unix/linux/3g' geekfile.txt

Output:

unix is great os. unix is opensource. linux is free os.
learn operating system.
unix linux which one you choose.
unix is easy to learn.unix is a multiuser os.Learn linux .linux is a powerful.

5. Parenthesize First Character of Each Word

This sed example prints the first character of every word in parenthesis.

echo "Welcome To The Geek Stuff" | sed 's/\(\b[A-Z]\)/\(\1\)/g'

Output:

(W)elcome (T)o (T)he (G)eek (S)tuff

6. Replacing String on a Specific Line Number

You can restrict the sed command to replace the string on a specific line number. An example is

sed '3 s/unix/linux/' geekfile.txt

Output:

unix is great os. unix is opensource. unix is free os.
learn operating system.
linux linux which one you choose.
unix is easy to learn.unix is a multiuser os.Learn unix .unix is a powerful.

The above sed command replaces the string only on the third line.

7. Duplicating the Replaced Line with /p flag

The /p print flag prints the replaced line twice on the terminal. If a line does not have the search pattern and is not replaced, then the /p prints that line only once.

sed 's/unix/linux/p' geekfile.txt

Output:

linux is great os. unix is opensource. unix is free os.
linux is great os. unix is opensource. unix is free os.
learn operating system.
linux linux which one you choose.
linux linux which one you choose.
linux is easy to learn.unix is a multiuser os.Learn unix .unix is a powerful.
linux is easy to learn.unix is a multiuser os.Learn unix .unix is a powerful.

8. Printing Only the Replaced Lines

Use the -n option along with the /p print flag to display only the replaced lines. Here the -n option suppresses the duplicate rows generated by the /p flag and prints the replaced lines only one time.

sed -n 's/unix/linux/p' geekfile.txt

Output:

linux is great os. unix is opensource. unix is free os.
linux linux which one you choose.
linux is easy to learn.unix is a multiuser os.Learn unix .unix is a powerful.

If you use -n alone without /p, then the sed does not print anything.

9. Replacing String on a Range of Lines

You can specify a range of line numbers to the sed command for replacing a string.

sed '1,3 s/unix/linux/' geekfile.txt

Output:

linux is great os. unix is opensource. unix is free os.
learn operating system.
linux linux which one you choose.
unix is easy to learn.unix is a multiuser os.Learn unix .unix is a powerful.

Here the sed command replaces the lines with range from 1 to 3. Another example is

sed '2,$ s/unix/linux/' geekfile.txt

Output:

unix is great os. unix is opensource. unix is free os.
learn operating system.
linux linux which one you choose.
linux is easy to learn.unix is a multiuser os.Learn unix .unix is a powerful

Here $ indicates the last line in the file. So the sed command replaces the text from second line to last line in the file.

10. Deleting Lines from a Particular File

SED command can also be used for deleting lines from a particular file. SED command is used for performing deletion operation without even opening the file

Examples:

1. To Delete a particular line say n in this example

Syntax:
sed 'nd' filename.txt

Example:
sed '5d' filename.txt

2. To Delete a last line

Syntax:
sed '$d' filename.txt

3. To Delete line from range x to y

Syntax:
sed 'x,yd' filename.txt

Example:
sed '3,6d' filename.txt

4. To Delete from nth to last line

Syntax:
sed 'nth,$d' filename.txt

Example:
sed '12,$d' filename.txt

5. To Delete pattern matching line

Syntax:
sed '/pattern/d' filename.txt

Example:
sed '/abc/d' filename.txt

For more: SED command in Linux | Set 2

Advanced SED Command Examples

Below are some of the advanced SED commands that can be used for handling various editing tasks. let's check them out:

1. Regular Expressions

SED supports regular expressions that allows it to handle more complex pattern matching. To enable regular expressions, you need to use -r option.

Example:

Here, we have matched any word start with 'u' and replacing it with "Linux". Let's check this out in the provided syntax below:

sed -r 's/\bu\w+/Linux/g' geekfile.txt

2. Insert Text

You can use this option to insert any text before or after any specific line. Here's the syntax provided below:

sed '3i\new text' filename  # Insert text before line 3
sed '3a\new text' filename  # Insert text after line 3

Example: To insert a new line before line2 in geekfile.txt

sed '2i\This is a new line' geekfile.txt

Best Practices

• Ensure to back up your files before applying for the changes. (especially while using -i)
• Be cautious before using the extended reular expressions to avoid any unintended substitutions.
• Always test your SED command on sample file first (to avoid unintentional changes).

Conclusion

The SED command in Linux/Unix is a versatile and powerful tool for automating text editing tasks, from simple find-and-replace operations to more complex pattern matching. Whether you're working with a single file or need to process multiple files in a script, sed offers an efficient solution for text manipulation.