How does shell redirection use dup2?

A strong answer

A strong candidate frames How does shell redirection use dup2? around the exact memory layout, compiler semantics, and hardware execution model covered in Descriptor Duplication and Redirection with dup and dup2. Explain how shells implement cmd > file by opening the file and dup2-ing its descriptor onto stdout (fd 1).

Standard library I/O functions like printf() and puts() are hardcoded to write directly to file descriptor 1 (STDOUT_FILENO).

When a shell executes ls -l > out.txt, the ls program does not accept a --redirect-output command-line argument. The source code of ls still calls printf().

In a production interview, always connect the specification rule to its concrete runtime consequence: state what lives on the stack versus the heap or static storage segment, how the compiler or operating system validates alignments, lifetimes, and error codes, and what happens under adversarial or boundary inputs (such as zero-length buffers, integer overflow, concurrent mutation, or partial I/O). Backing your explanation with a small trace of register/memory state transitions immediately separates senior systems engineers from candidates who only memorized syntax.

What the interviewer is listening for

• Mastery of core invariant: Duplicate file descriptors to target identical Open File Descriptions using dup and dup2.
• Mastery of core invariant: Implement standard stream redirection (stdout/stderr) for shell pipelines.
• Mastery of core invariant: Explain the race condition eliminated by the atomicity guarantee of dup2.
• Hardware and ABI awareness: Explaining how sizes, alignments, calling conventions, or kernel boundaries behave on real 64-bit targets rather than relying on implementation-defined assumptions.
• Defensive engineering: Naming the exact compiler flags (-Wall -Wextra -Werror), static analyzers, or runtime sanitizers (-fsanitize=address,undefined,thread) that verify correctness in CI.

Common mistakes

• Explaining only surface syntax without tracing the underlying memory addresses, ownership lifetime, or kernel/user-space state transitions.
• Assuming undefined behavior (such as signed overflow, uninitialized reads, strict-aliasing violations, or dangling pointers) will "just work" because it passed an unoptimized -O0 debug build.
• Ignoring return codes, partial reads/writes, alignment padding, or cleanup paths when an allocation or syscall fails midway through a function.