How to use the command 'sed' (with examples)

The sed command (short for “stream editor”) is a powerful utility that allows users to edit text in a scriptable manner. It reads input line by line, applies specified operations to the text, and outputs the results. It is often used in conjunction with pipes (|) to process and transform text data.

Use case 1: Replace text occurrences (global substitution)

Code:

command | sed 's/apple/mango/g'

Motivation:

The motivation for using this example is to illustrate how to replace all occurrences of a word in a text file and print the modified text to the standard output. This can be useful when you want to make bulk changes to a file without manually editing each occurrence.

Explanation:

• command represents the command that generates the input text. This can be any command or process that produces text as output.
• sed is the command itself.
• 's/apple/mango/g' is the command argument. The s indicates a substitution operation. apple is the search pattern to be replaced, and mango is the replacement pattern. The g flag stands for global, meaning all occurrences of the search pattern will be replaced.

Example output:

Original Text:

I have an apple.
She has an apple pie.

Modified Text:

I have a mango.
She has a mango pie.

Use case 2: Execute a script file

Code:

command | sed -f path/to/script.sed

Motivation:

The motivation for using this example is to showcase how to execute a script file using the sed command. This can be useful when you have a collection of operations that need to be applied to a text file and you want to store them in a separate script for modularity and reusability.

Explanation:

• command represents the command that generates the input file. This can be any command or process that produces a file as output.
• sed is the command itself.
• -f path/to/script.sed is the command argument. The -f option specifies that the script to be executed is stored in a file. path/to/script.sed represents the path to the script file.

Example output:

This is the output of the script execution.

Use case 3: Print just the first line

Code:

command | sed -n '1p'

Motivation:

The motivation for using this example is to demonstrate how to print just the first line of a text file using the sed command. This can be useful when you want to quickly extract specific lines from a file without opening it manually.

Explanation:

• command represents the command that generates the input file. This can be any command or process that produces a file as output.
• sed is the command itself.
• -n is an option that tells sed to suppress automatic printing of lines. This is necessary to only display the lines we explicitly instruct sed to print.
• '1p' is the command argument. 1 represents the line number, and p is the command to print the line.

Example output:

This is the first line of the file.

Conclusion:

The sed command is a versatile utility for editing text files in a scriptable manner. It provides a wide range of functionalities, including global substitution, executing scripts, and selecting specific lines from a file. By mastering the various use cases of sed, users can efficiently process and manipulate text data.