Working with Output

Capture, transform, and process command output.

Choosing Your Approach

Use .lines.collect() when you need all output as an array (small outputs only):

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

Use .lines with for-await when processing large outputs line-by-line:

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

Use .toStdout() when you just want to see the output:

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

Use .first or .last when you only need one line:

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

Getting Output

As Lines

import { run } from "jsr:@j50n/proc@0.23.3";

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

proc