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

The ifne command, part of the moreutils package, is a useful tool for conditionally executing commands based on the presence or absence of input from stdin. It simplifies shell scripting and command line operations by allowing you to execute commands only when there is input to process, or conversely, when there is no input at all.

Use case 1: Run the specified command if and only if stdin is not empty

Code:

cat file.txt | ifne grep 'pattern'

Motivation:

Imagine you are running a series of commands in a script, and one of them produces potentially no lines of output depending on previous operations. Let’s say you’re working with a file processing script that checks for a specific pattern within the content of a file. In some situations, this script might deal with empty files or files where the desired content isn’t present. Using ifne, you can ensure that the grep 'pattern' command runs only when there is actual content from stdin to work on. This prevents unnecessary command execution and potential errors or misleading results from operating on empty input.

Explanation:

• cat file.txt: This command reads the content of file.txt and pipes it to the next command.
• |: This pipe operator takes the output of the preceding command and uses it as the input for the subsequent command.
• ifne: The ifne command examines the input it receives from stdin, and runs the following command only if there is some content.
• grep 'pattern': This command searches for the specified pattern in the lines fed to it through stdin.

Example output:

If file.txt contains:

This is a sample text.
It includes several lines.
Find the pattern here.

And you search for 'pattern', the output would be:

Find the pattern here.

If file.txt is empty, there would be no output, and grep would not run.

Use case 2: Run the specified command if and only if stdin is empty, otherwise pass stdin to stdout

Code:

cat file.txt | ifne -n echo 'The file is empty'

Motivation:

There are scenarios in file processing or data pipeline tasks where acting upon empty input is crucial. For example, you might want to notify the user or log an entry when an expected file does not contain any data. Using ifne -n ensures that you are notified if a file is unexpectedly empty while still preserving any non-empty input for other purposes.

Explanation:

• cat file.txt: Reads the content of file.txt for inspection and passes it downstream in the command line.
• |: This pipe operator continues the data stream from cat file.txt to the next command.
• ifne -n: The -n flag modifies the ifne behavior to execute the upcoming command (echo 'The file is empty') only if stdin is empty. Furthermore, if there is content in stdin, it passes the content through unchanged.
• echo 'The file is empty': This command is designed to notify the user or log a message that the file provided no input.

Example output:

If file.txt contains data, the output will be the content of the file itself.

If file.txt is empty, the output will be:

The file is empty

Conclusion

The ifne command is a versatile utility for handling conditional executions based on input state. By using it, you can optimize command line workflows and script behaviors to respond properly to varying inputs, whether it’s avoiding unnecessary operations or flagging unexpected conditions like empty inputs. Understanding and applying ifne can significantly streamline task automation in Unix-like environments.