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

The sed command, short for “stream editor,” is a powerful tool for text processing and manipulation in Unix and Unix-like operating systems. It allows users to perform basic text transformations on input streams without manual editing, making it a favorite among developers and system administrators for automating and scripting text processing tasks. sed can execute complex text modifications with minimal code and is often used for tasks such as find-and-replace operations, text-based data filtering, and advanced text manipulations.

Use case 1: Replace all apple occurrences with mango in all input lines

Code:

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

Motivation:

This use case is essential in scenarios where you need to update specific text patterns across multiple lines of text efficiently. Suppose you have a large dataset or log files containing the word “apple” numerous times, and you need to replace it with “mango” to reflect updated data or project names. Using sed for this task is advantageous as it allows batch processing of text, saving time and reducing the likelihood of errors that might occur with manual editing.

Explanation:

• command: This is a placeholder for any command that outputs lines of text you want to modify using sed.
• sed: Invokes the sed utility.
• 's/apple/mango/g': The core component of this command:
  • s: Stands for substitute, indicating that a substitution operation is taking place.
  • apple: The basic regex pattern you want to find in the text.
  • mango: The replacement text that will replace every occurrence of “apple”.
  • g: Stands for “global”, meaning that all occurrences of “apple” in each line will be replaced with “mango”.

Example output:

If the input to the command were:

I have an apple.
Apple pie is delicious.
She bought an apple and an apple.

The output after the sed command execution would be:

I have a mango.
Apple pie is delicious.
She bought a mango and a mango.

Use case 2: Execute a specific script file and print the result

Code:

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

Motivation:

When faced with complex text processing tasks, writing an extensive sed command line might become cumbersome. In such cases, storing your sed commands in a separate script file is beneficial. This approach allows you to maintain and document various operations you wish to perform, promoting better organization and easier reuse of scripts for similar tasks in the future.

Explanation:

• command: Represents any command that produces text as output.
• sed: Launches the sed stream editor.
• -f path/to/script.sed: This option tells sed to read and execute commands from the specified script file. The file script.sed contains one or multiple sed commands that will be applied to the input stream.

Example output:

Assuming script.sed contains s/pear/banana/g and the input is:

I like pear juice.
Pear trees are common.
A pear a day keeps the doctor away.

The output will become:

I like banana juice.
Pear trees are common.
A banana a day keeps the doctor away.

Use case 3: Print just the first line

Code:

command | sed -n '1p'

Motivation:

Sometimes, you might want to quickly verify the header of a file or check the first line of content without scrolling or manually opening the file for viewing. Extracting the first line using sed is a convenient way to efficiently access specific information contained at the top of a file or data stream, which might include metadata, titles, or basic file descriptors.

Explanation:

• command: This placeholder stands for any text-outputting command.
• sed: Triggers the sed stream editor.
• -n: Suppresses automatic printing of pattern space, which is the default behavior of sed.
• '1p': A sed command that tells sed to print only the first line (line number 1).

Example output:

Given an input sample of:

First line of a file.
Second line of a file.
Third line of a file.

The output produced by the command will be:

First line of a file.

Conclusion:

The sed command is a versatile tool that enables effective and efficient text manipulation in a command-line environment. Its powerful ability to perform substitutions, execute scripted sequences of commands, and extract specific text pieces makes it an invaluable asset in any Unix-like operating system. With the examples illustrated, the potential for integrating sed into automation scripts and text processing workflows is vast, offering a streamlined approach to manage and transform text data with precision and agility.