Working with Output

Capturing, transforming, and processing command output is central to building effective data processing pipelines. proc provides multiple approaches depending on your data size and processing needs.

Choosing the Right Output Method

When you need all output as an array and you’re confident the output is small enough to fit in memory, use .lines.collect():

const lines = await run("ls").lines.collect(); // All lines in memory

For large outputs that you want to process line-by-line without loading everything into memory, use .lines with for-await loops:

for await (const line of run("cat", "huge.log").lines) {
  process(line); // Constant memory usage
}

When you just want to see the output in your console, .toStdout() prints directly without capturing:

await run("ls", "-la").toStdout(); // Prints directly to console

For single-line results, .first or .last properties give you exactly what you need:

const result = await run("git", "rev-parse", "HEAD").lines.first;

Getting Output

As Lines

import { run } from "jsr:@j50n/proc@0.24.6";

const lines = await run("ls", "-la").lines.collect();
// string[]

As Bytes

const bytes = await run("cat", "file.bin").collect();
// Uint8Array[]

First Line

const result = await run("git", "rev-parse", "HEAD").lines.first;
// Single string

Print to Console

await run("ls", "-la").toStdout();

Transforming Output

Map Lines

const uppercase = await run("cat", "file.txt")
  .lines
  .map((line) => line.toUpperCase())
  .collect();

Filter Lines

const errors = await run("cat", "app.log")
  .lines
  .filter((line) => line.includes("ERROR"))
  .collect();

Parse Output

const commits = await run("git", "log", "--oneline")
  .lines
  .map((line) => {
    const [hash, ...message] = line.split(" ");
    return { hash, message: message.join(" ") };
  })
  .collect();

Streaming Output

Process output as it arrives:

for await (const line of run("tail", "-f", "app.log").lines) {
  if (line.includes("ERROR")) {
    console.error(line);
  }
}

Counting Output

const lineCount = await run("ls", "-la").lines.count();

Finding in Output

const match = await run("ps", "aux")
  .lines
  .find((line) => line.includes("node"));

Real-World Examples

Parse JSON Output

const data = await run("curl", "https://api.example.com/data")
  .lines
  .map((line) => JSON.parse(line))
  .collect();

Extract Fields

const pids = await run("ps", "aux")
  .lines
  .drop(1) // Skip header
  .map((line) => line.split(/\s+/)[1])
  .collect();

Aggregate Data

const total = await run("du", "-sh", "*")
  .lines
  .map((line) => {
    const size = line.split("\t")[0];
    return parseInt(size);
  })
  .reduce((sum, size) => sum + size, 0);

Next Steps

Process Pipelines - Chain commands
Running Processes - More ways to run
Array-Like Methods - Transform output

Keyboard shortcuts

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