How to perform global substitution with sed?

Performing Global Substitution with sed

The sed (Stream Editor) command in Linux is a powerful tool for performing text manipulation, including global substitution. Global substitution is the process of replacing all occurrences of a pattern in a text with a new pattern.

The Syntax for Global Substitution

The basic syntax for performing global substitution with sed is as follows:

sed 's/pattern/replacement/g' file

Here's a breakdown of the syntax:

• s: This is the substitution command in sed.
• pattern: This is the regular expression or text pattern that you want to replace.
• replacement: This is the new text that will replace the pattern.
• g: This is the global flag, which ensures that all occurrences of the pattern are replaced, not just the first one.
• file: This is the name of the file you want to perform the substitution on.

Example: Replacing All Occurrences of a Word

Let's say you have a file named example.txt with the following content:

The quick brown fox jumps over the quick dog.

If you want to replace all occurrences of the word "quick" with the word "slow", you can use the following sed command:

sed 's/quick/slow/g' example.txt

The output will be:

The slow brown fox jumps over the slow dog.

Example: Replacing Patterns with Capture Groups

You can also use capture groups in the pattern and the replacement to perform more complex substitutions. Capture groups are enclosed in parentheses () and can be referenced in the replacement using the \1, \2, etc. syntax.

For example, let's say you have a file named names.txt with the following content:

John Doe
Jane Smith

If you want to swap the first and last names, you can use the following sed command:

sed 's/\([^]*\) \([^]*\)/\2 \1/g' names.txt

The output will be:

Doe John
Smith Jane

Here's how the command works:

• \([^]*\) captures the first name (everything before the space).
• \([^]*\) captures the last name (everything after the space).
• \2 \1 swaps the order of the captured groups in the replacement.

Visualizing the Substitution Process

Here's a Mermaid diagram that illustrates the global substitution process with sed:

The diagram shows how the sed command takes the input text, performs the global substitution based on the specified pattern and replacement, and outputs the modified text.

Conclusion

Global substitution with sed is a powerful and efficient way to perform text manipulation in the Linux command line. By understanding the syntax and using capture groups, you can perform complex text transformations to meet your needs. Remember to always test your sed commands on a copy of the file to ensure you get the desired results before applying them to the original file.