CompTIA Linux+ XK0-005 - 3.1 - Common Script Utilities: sed

The sed command, short for stream editor, is a powerful tool for performing text transformations in shell scripting. It allows you to modify and manipulate text files by applying a set of rules or commands to each line of input. With its versatile syntax and range of editing capabilities, sed is widely used for tasks such as search and replace, line insertion and deletion, and text substitution.

Basic Syntax

The basic syntax of the sed command is as follows:

sed OPTIONS 'COMMAND' FILE
- OPTIONS: Specifies any desired options for the sed command.
- 'COMMAND': Contains the set of editing commands to be applied to each line of input.
- FILE: The name of the file to be processed. If not specified, sed reads from standard input.

Common sed Options

sed provides various options that modify its behavior and enable additional functionalities. Here are some commonly used options:

-n: Suppresses the default output behavior of sed. Only explicitly specified commands with print instructions will produce output.
-i: Modifies the input file(s) in-place. This option allows you to directly edit the file(s) rather than writing the changes to standard output.
-p: Automatically prints each line of input after applying the specified commands. This option is useful for debugging or verifying the changes made by sed.

Key Features and Functionality

The sed command offers several key features that make it a valuable tool in shell scripting:

Search and Replace: sed allows you to search for a specific pattern or regular expression and replace it with another value. You can perform global replacements or limit replacements to specific occurrences or lines.
Line Insertion and Deletion: With sed, you can add or remove lines from a file based on specified conditions. This includes inserting lines before or after a specific pattern, deleting lines that match a pattern, or removing empty lines.
Selective Printing: sed allows you to selectively print lines from a file based on specified conditions. You can print lines that match a pattern, print a range of lines, or print specific columns.

Example Usage

Let's explore a few examples of how to use the sed command:

Search and Replace

To replace all occurrences of a word in a file, you can use the following command:

sed 's/old_word/new_word/g' filename

This command searches for old_word and replaces it with new_word in the specified file.

Line Insertion

To insert a line before a specific pattern in a file, you can use the following command:

sed '/pattern/i new_line' filename

This command searches for the specified pattern and inserts new_line before it in the file.

Selective Printing

To print lines that match a pattern, you can use the following command:

sed -n '/pattern/p' filename

This command searches for the specified pattern and prints only the lines that match it.

In-Place Editing

To edit a file in-place and save the changes, you can use the -i option followed by an optional backup file extension:

sed -i.bak 's/pattern/replacement/' filename

This command replaces pattern with replacement in the specified file and creates a backup file with the .bak extension.

Conclusion

The sed command is a versatile tool for text editing and manipulation in shell scripting. It offers powerful features for search and replace, line insertion and deletion, and text substitution. By mastering the usage of sed and understanding its options, you can efficiently process and transform text files in various scripting scenarios.

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