lush/issues/03-command-execution-runtime.md

03 — Implement direct command execution runtime (no shell)

Status: done Blocked by: #02 Blocks: #06, #07

Implement the C runtime that executes commands when a backtick expression is evaluated.

Critical constraint: do NOT spawn a shell (no bash, dash, sh, system(), popen()).

Return value

Backtick expressions return a table:

local r = `ls -lha`
r.code    -- exit code (integer)
r.stdout  -- captured stdout (string)
r.stderr  -- captured stderr (string)

Implementation approach

• fork() + execvp() on Unix/macOS
• Set up two pipes: one for stdout, one for stderr
• In child: dup2() pipes onto fd 1 and fd 2, then execvp(argv[0], argv)
• In parent: read both pipes into buffers, waitpid() for exit status
• Build a Lua table with code, stdout, stderr fields and push onto stack
• Depends on #04 (argv parsing) to tokenize the command string into argv[]

Error handling

• Command not found → set code to an error value, stderr to error message
• Permission denied → same
• fork() failure → raise a Lua error

Open questions

• New file (lcmd.c / lcmd.h) or add to an existing lib?
• Signal handling during child execution
• Binary output — return raw string bytes as-is?
• Set __tostring metamethod on result table?