Replacing Text with `sed`: Practical Examples

The sed (stream editor) command in Unix-like operating systems is a powerful tool for manipulating text in data streams and files. An essential utility for system administrators and programmers, it allows for complex pattern matching, substitution, and more. In this article, we will focus on the specific application of sed for replacing text strings. We’ll cover some practical examples that you can use daily to enhance your work efficiency.

Basic Syntax of `sed`

Before diving into the examples, let’s understand the basic syntax of the sed command:

sed [options] 's/pattern/replacement/[flags]' file

Here, s signifies the substitution operation. The pattern is what you intend to replace, and the replacement is the new text you want to insert.

Example 1: Replacing the First Occurrence of a String in a File

Suppose you have a file named example.txt, and you want to replace the first occurrence of the word "apple" with "orange." You can achieve this with the following sed command:

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

By default, sed only replaces the first instance of the pattern in each line. If example.txt contains the word "apple" multiple times in a single line and you want to replace the very first occurrence throughout the file, use:

sed '0,/apple/s//orange/' example.txt

Example 2: Globally Replacing a String in a File

If you need to replace every occurrence of "apple" with "orange" within the entire file, you can add the g flag at the end of the command, which stands for "global":

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

This command will scan the entire file and replace every matching instance.

Example 3: Replacing Strings on Specific Lines

Sometimes, you might need to replace text only on specific lines. For example, to replace "apple" with "orange" only on the 3rd line of example.txt, use:

sed '3 s/apple/orange/' example.txt

You can also specify a range of lines. To replace "apple" with "orange" from line 2 to line 5:

sed '2,5 s/apple/orange/g' example.txt

Example 4: In-place File Editing

By default, sed outputs the modified content to standard output. To save the changes back into the file, use the -i option:

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

Be cautious with this option, as it directly modifies your file.

Example 5: Using Regular Expressions

sed supports regular expressions which provide powerful pattern matching capabilities. Suppose you want to replace any three-letter word that ends with "at" with "hat" in example.txt:

sed 's/\bat\>/hat/g' example.txt

External slashes (\) are used to denote word boundaries, ensuring that only whole words are matched.

Conclusion

These practical examples illustrate the versatility of sed in text processing and editing tasks. Whether you're working on scripts, configurational files, or large data sets, mastering sed can significantly streamline your workflow, allowing you to perform complex text manipulations with simple commands.

Always ensure to backup important files before using sed with in-place editing, as it makes permanent changes to files. Happy editing with sed!