Add stderr operators: 2>, 2>>, |&, 2>&1

[stalep]

Summary

Implement stderr redirection and piping operators. The OperatorType enum already defines these types but they are not wired in Executions.buildOperator() and the infrastructure for a separate stderr channel does not exist.

Operators to implement

Operator	Description	Example
2>	Redirect stderr to file (overwrite)	cmd 2> errors.log
2>>	Redirect stderr to file (append)	cmd 2>> errors.log
|&	Pipe both stdout and stderr	cmd |& grep error
2>&1	Merge stderr into stdout	cmd > out.txt 2>&1

Current state

• OperatorType defines PIPE_AND_ERROR, REDIRECT_OUT_ERROR, APPEND_OUT_ERROR, REDIRECT_OUT_ALL
• Executions.buildOperator() throws IllegalArgumentException for all of them
• Commands have no separate stderr channel — all output goes through Shell.write() / CommandInvocation.println()

Design considerations

1. Stderr channel for commands

Commands need a way to write to stderr separately from stdout:

// Option A: explicit method
commandInvocation.printErr("Error: file not found");

// Option B: stderr stream
commandInvocation.getStderr().write(...);

This requires:

• A stderr OutputDelegate on CommandInvocationConfiguration
• ShellOutputDelegate handling two output streams (stdout delegate + stderr delegate)
• Default behavior when no stderr redirection: write to the terminal (same as stdout)

2. Backward compatibility

Most existing commands use println() for everything including error messages. Options:

• println() remains stdout-only (breaking for commands that print errors via println)
• Add printErr() as the new explicit stderr path, println() unchanged
• Provide a System.err-like accessor for commands that want separation

3. Operator wiring

Each operator needs a case in Executions.buildOperator() and appropriate CommandInvocationConfiguration construction in buildExecution().

For 2>&1, stderr is merged into the stdout OutputDelegate. For |&, both stdout and stderr flow into the PipeOperator queue. For 2> / 2>>, only stderr goes to the file while stdout continues to the terminal.

4. Interaction with streaming pipes (#532)

The streaming pipe implementation uses PipeOutputDelegate for stdout. |& would need to route both stdout and stderr through the same queue. The QueueOutputStream is already thread-safe (uses BlockingQueue.put()), so both channels can write to it concurrently.

Dependencies

• Streaming pipe implementation (Streaming pipe implementation with concurrent pipeline execution #532) — completed
• No dependency on aesh-readline changes

Related

• Streaming pipe implementation with concurrent pipeline execution #532 — Streaming pipe implementation (Phase 1-3, completed)
• OperatorType.java — enum definitions already exist