bad40fb65b
Rename resolved→done (#1-4, #11), closed→done (#13, #18), open (post-core)→open (#7), mark #9 as superseded by #21.
49 lines
1.5 KiB
Markdown
49 lines
1.5 KiB
Markdown
# 07 — Implement I/O redirection for commands
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**Status:** open
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**Blocked by:** #03
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## Syntax
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```lua
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`ls -l` > "output.txt" -- redirect stdout to file
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`ls -l` >> "output.txt" -- append stdout to file
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`cmd` 2> "err.txt" -- redirect stderr to file
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`cmd` 2>&1 -- merge stderr into stdout
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`cmd` < "input.txt" -- redirect stdin from file
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```
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### Alternatively
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```lua lush
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`ls -l > output.txt` -- redirect stdout to file
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`ls -l >> output.txt` -- append stdout to file
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`cmd 2> err.txt` -- redirect stderr to file
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`cmd 2>&1` -- merge stderr into stdout
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`cmd < input.txt` -- redirect stdin from file
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```
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## Implementation
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- Before `execvp()` in the child process, use `dup2()` to redirect file descriptors
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- Parse redirection operators as part of the command syntax
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- Open target files with appropriate flags:
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- `>` — `O_WRONLY | O_CREAT | O_TRUNC`
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- `>>` — `O_WRONLY | O_CREAT | O_APPEND`
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- `<` — `O_RDONLY`
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- `2>&1` — `dup2(stdout_fd, STDERR_FILENO)`
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## Challenge
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`>` 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.
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## Alternatives
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- Although shell redirection isn’t available, simple redirection can be achieved using the `io` library.
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```lua lush
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file = io.open("OUTFILE.txt", "w")
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file:write(`ls /`.stdout)
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file:close()
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```
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