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Fixup and update cmd.exe shelling out docs #1605

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45 changes: 43 additions & 2 deletions book/coming_from_cmd.md
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# Coming from CMD.EXE

This table was last updated for Nu 0.67.0.
This table was last updated for Nu 0.99.1.
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@Kissaki Kissaki Nov 1, 2024 •
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I did not go over the command table. I was unsure whether it should remain the previous, be updated to the new, or split across concerns.

I chose to update it for simplicity, thinking issues can still be resolved either way when they come up/get reported.

But given that this refers to the table specifically, maybe it should remain the previous version?


| CMD.EXE | Nu | Task |
| ------------------------------------ | ----------------------------------------------------------------------------------- | --------------------------------------------------------------------- |
Expand Down Expand Up @@ -57,4 +57,45 @@ This table was last updated for Nu 0.67.0.
| `VERIFY` | | Verify that file writes happen |
| `VOL` | | Show drive information |

Before Nu version 0.67, Nu [used to](https://www.nushell.sh/blog/2022-08-16-nushell-0_67.html#windows-cmd-exe-changes-rgwood) use CMD.EXE to launch external commands, which meant that the above builtins could be run as an `^external` command. As of version 0.67, however, Nu no longer uses CMD.EXE to launch externals, meaning the above builtins cannot be run from within Nu, except for `ASSOC`, `CLS`, `ECHO`, `FTYPE`, `MKLINK`, `PAUSE`, `START`, `VER`, and `VOL`, which are explicitly allowed to be interpreted by CMD if no executable by that name exists.
## Forwarded CMD.EXE commands

Nu accepts and runs *some* CMD.EXE commands through `cmd.exe`.
Which ones, and under which precedence depends on the Nu version.

### Version 0.94 onward

Nu version 0.94 released *(on 2024-05-28)* with [a behavior change](https://github.com/nushell/nushell/pull/12921).

If the command is one of the forwarded CMD.EXE commands, `cmd.exe` is called with that command.

The forwarded commands have not changed since version 0.75.0 ([v0.94 src](https://github.com/nushell/nushell/blob/0.94.0/crates/nu-command/src/system/run_external.rs#L531-L538)):

`ASSOC`, `CLS`, `ECHO`, `FTYPE`, `MKLINK`, `PAUSE`, `START`, `VER`, `VOL`

Note that, contrary to earlier versions, the forwards take precedence over other external commands.

For example, with a `ver.bat` file in the current working directory, executing `^ver` leads to the CMD.EXE `VER` being executed, *NOT* the `ver.bat` file.

Executing `./ver` or `ver.bat` *will* execute the local bat file though.

### Version 0.67 until before 0.94

Nu version 0.67 released *(on 2022-08-16)* with [a behavior change](https://www.nushell.sh/blog/2022-08-16-nushell-0_67.html#windows-cmd-exe-changes-rgwood).

When a command is not found as an executable, if the command is one of the forwarded CMD.EXE commands, `cmd.exe` is called with that command.

The forwarded commands are:

* v0.67.0 onward ([src](https://github.com/nushell/nushell/blob/0.67.0/crates/nu-command/src/system/run_external.rs#L148)):\
`ASSOC`, `DIR`, `ECHO`, `FTYPE`, `MKLINK`, `START`, `VER`, `VOL`
* v0.68.0 onward ([src](https://github.com/nushell/nushell/blob/0.68.0/crates/nu-command/src/system/run_external.rs#L168-L169)):\
`ASSOC`, `CLS`, `DIR`, `ECHO`, `FTYPE`, `MKLINK`, `PAUSE`, `START`, `VER`, `VOL`
* v0.75.0 onward ([src](https://github.com/nushell/nushell/blob/0.75.0/crates/nu-command/src/system/run_external.rs#L207), [change](https://github.com/nushell/nushell/pull/7740)):\
`ASSOC`, `CLS`, `ECHO`, `FTYPE`, `MKLINK`, `PAUSE`, `START`, `VER`, `VOL`

## Prior to version 0.67

If a command is not found, `cmd.exe` is called with that command.
That means that effectively *all* CMD.EXE commands can be called from within Nu.

The disadvantage is that commands that do not resolve in `cmd.exe`, the command not found error message comes from `cmd.exe` rather than Nu.