The sed command in Linux is a powerful text-processing tool, often described as a "stream editor." It allows users to search, find, replace, insert, or delete text in files or streams. While it may seem complex at first, sed is an essential tool for automating tasks and handling large amounts of data efficiently. In this article, we’ll explore the basics of sed and include plenty of examples to help you get started.

What is sed?

sed, short for stream editor, reads text from standard input or a file, performs text transformations, and outputs the result. Unlike traditional text editors, sed operates non-interactively, meaning it processes text in a stream, making it ideal for scripting and automating repetitive tasks.

How to Use sed

The basic syntax of the sed command is as follows:

sed [options] 'script' file
  • [options]: Flags to modify sed behavior (e.g., -i for in-place editing).
  • 'script': Instructions for processing text (e.g., substitution, deletion).
  • file: Input file (optional; sed can also read from standard input).

Common Use Cases and Examples

1. Substituting Text

The s command is used for substitution. To replace all occurrences of "foo" with "bar":

sed 's/foo/bar/' input.txt
  • Example Output: If input.txt contains "foo is great", the result will be "bar is great".

To replace text globally (all occurrences in a line), use the g flag:

sed 's/foo/bar/g' input.txt

2. In-Place Editing

Edit a file directly without creating a new file using the -i flag:

sed -i 's/foo/bar/g' input.txt

This replaces "foo" with "bar" across all lines of input.txt.

3. Deleting Lines

To delete specific lines, use the d command:

sed '2d' input.txt
  • Example Output: This deletes the second line of input.txt.

To delete all blank lines:

sed '/^$/d' input.txt

4. Printing Specific Lines

Print only specific lines using the p command:

sed -n '3p' input.txt
  • Example Output: This displays only the third line of input.txt.

5. Inserting or Appending Text

To insert text before a specific line, use the i command:

sed '2i\Inserted line' input.txt

This adds "Inserted line" before the second line.

To append text after a specific line, use the a command:

sed '2a\Appended line' input.txt

6. Replacing Text on Matching Lines

Replace text only on lines that match a pattern:

sed '/pattern/s/foo/bar/' input.txt
  • Example: Replace "foo" with "bar" only in lines containing "pattern".

7. Working with Ranges

To apply commands to a range of lines:

sed '2,4d' input.txt

This deletes lines 2 through 4 in input.txt.

Or, you can match based on patterns:

sed '/start/,/end/d' input.txt

This deletes all lines between "start" and "end" (inclusive).

Combining Multiple Commands

You can chain multiple commands using a semicolon:

sed 's/foo/bar/; 2d' input.txt

This replaces "foo" with "bar" and deletes the second line.

Alternatively, you can use the -e flag:

sed -e 's/foo/bar/' -e '2d' input.txt

Why Use sed?

sed shines when handling repetitive tasks, processing large files, or automating text manipulation in scripts. Whether you’re a system administrator, developer, or data analyst, sed can save you time and effort.

Conclusion

The sed command is a versatile tool in the Linux world. Its ability to manipulate text efficiently makes it invaluable for anyone working with Linux. Start simple, experiment with the examples above, and soon you’ll be wielding sed like a pro.