lush/issues/07-redirection.md

07 — Implement I/O redirection for commands

Status: open Blocked by: #03

Syntax

`ls -l` > "output.txt"        -- redirect stdout to file
`ls -l` >> "output.txt"       -- append stdout to file
`cmd` 2> "err.txt"            -- redirect stderr to file
`cmd` 2>&1                    -- merge stderr into stdout
`cmd` < "input.txt"           -- redirect stdin from file

### Alternatively

`ls -l > output.txt`        -- redirect stdout to file
`ls -l >> output.txt`       -- append stdout to file
`cmd 2> err.txt`            -- redirect stderr to file
`cmd 2>&1`                    -- merge stderr into stdout
`cmd < input.txt`           -- redirect stdin from file

Implementation

• Before execvp() in the child process, use dup2() to redirect file descriptors
• Parse redirection operators as part of the command syntax
• Open target files with appropriate flags:
  • > — O_WRONLY | O_CREAT | O_TRUNC
  • >> — O_WRONLY | O_CREAT | O_APPEND
  • < — O_RDONLY
• 2>&1 — dup2(stdout_fd, STDERR_FILENO)

Challenge

> and >> conflict with Lua's greater-than and right-shift operators. Like piping, these operators must only be valid in command context. The parser can disambiguate because the left-hand side is a command expression.

Alternatives

• Although shell redirection isn’t available, simple redirection can be achieved using the io library.

file = io.open("OUTFILE.txt", "w")
file:write(`ls /`.stdout)
file:close()