Powerful Pattern Matching with `rargs` (with examples)

The rargs command is a versatile tool designed to facilitate the execution of commands for each line of input. It’s an enhanced version of xargs, equipped with the ability to support pattern matching. This capability makes it especially useful for processing text and file operations in a more advanced manner. By offering a more flexible approach to standard input processing, rargs can significantly streamline complex command operations.

Use case 1: Execute a command for every line of input

Code:

command | rargs command {0}

Motivation:

The primary motivation for this use case is to execute a particular command on each line received as input. This is particularly beneficial when dealing with a list of files, URLs, or strings that require the same operation. For instance, if you need to append text to multiple files named in a list, this usage of rargs simplifies the task by automatically iterating over each entry.

Explanation:

• command: Represents any command that generates a list of outputs, often filenames or strings, separated by newlines.
• |: Pipes the output of the first command into rargs.
• rargs: Invokes the pattern matching and iterative power of the tool.
• command {0}: The specific operation you wish to perform on each input line, where {0} serves as the placeholder for each line of input fed into the command.

Example Output:

Suppose you have a command that lists file names, and you want to add “_backup” to each:

file1
file2
file3

Using rargs, each file is processed, resulting in:

file1_backup
file2_backup
file3_backup

Use case 2: Do a dry run, which prints the commands that would be run instead of executing them

Code:

command | rargs -e command {0}

Motivation:

A dry run is essential for verification and debugging purposes, allowing you to see the exact commands that rargs would execute without making any changes to the system. This is particularly useful when you’re testing a new script or set of operations and want to ensure they behave as expected.

Explanation:

• -e: This flag tells rargs to execute a dry run. Instead of running the commands, it prints them to the terminal.
• command {0}: The intended command format for processing each line, displayed as-is due to the dry run option.

Example Output:

With a list of files and operations planned, a dry run might show:

mv file1 file1_backup
mv file2 file2_backup
mv file3 file3_backup

This output allows you to confirm the correctness of the script before actual execution.

Use case 3: Remove the .bak extension from every file in a list

Code:

command | rargs -p '(.*).bak' mv {0} {1}

Motivation:

Managing file extensions programmatically is a common task in maintenance scripts. For instance, when cleaning up backup files, it’s often necessary to revert them to their original names. rargs offers a pattern-matching solution that simplifies this operation considerably.

Explanation:

• -p '(.*).bak': This pattern captures any file name ending with .bak, storing the base name of the file in {1}.
• mv {0} {1}: Uses the mv command to rename {0} (full filename with .bak) to {1} (filename without .bak).

Example Output:

For files named:

document1.bak
spreadsheet.bak
presentation.bak

The renaming process would output:

document1
spreadsheet
presentation

Use case 4: Execute commands in parallel

Code:

command | rargs -w max-procs

Motivation:

When dealing with a large batch of operations, executing tasks in parallel can significantly reduce processing time. This is particularly useful in scenarios where multiple independent tasks can be run simultaneously without impacting each other, such as image processing or data analysis.

Explanation:

• -w max-procs: This option sets rargs to run the operations using the maximum number of concurrent processes supported, determined by the system’s capability.

Example Output:

Assuming a set of images are processed in parallel, output might be as follows, with the order of completion varying due to parallel execution:

Processed image1
Processed image3
Processed image2

Use case 5: Consider each line of input to be separated by a NUL character

Code:

command | rargs -0 command {0}

Motivation:

Certain data sources, especially binary or those involving null-separated records, require meticulous handling to avoid misinterpretation. Using NUL as a separator ensures accurate reading and processing of such data, often seen when dealing with filenames that include newline characters.

Explanation:

• -0: This flag interprets input lines as being separated by NUL characters, accommodating inputs that might otherwise be split erroneously by newline.

Example Output:

For NUL-separated filenames:

file\0anotherfile\0lastfile

The command would correctly parse and process:

Processing file
Processing anotherfile
Processing lastfile

Conclusion:

rargs is a powerful command for those who need more sophisticated input handling than xargs can offer. Its pattern matching and additional execution options facilitate a wide variety of tasks, from batch processing and file manipulation to configuring complex pipelines with increased flexibility and efficiency. Whether you’re preparing a set of files for processing or managing iterations across diverse data, rargs streamlines command execution, providing a rich set of features for both novice and experienced users.