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Do not use redirection characters in your shell prompt

(tanelpoder.com)
217 points tanelpoder | 7 comments | 17 Mar 21 14:38 UTC | HN request time: 0.72s | source | bottom
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jandrese ◴[17 Mar 21 15:32 UTC] No.26492618[source]▶
This seems to be more of "don't paste garbage into a terminal, especially as root." With a sidenote that it might be safer if your custom application command interpreter didn't use > as the prompt character. I note that Bourne shell defaults to the safer % and # characters for the prompt. The # character for root is especially safe.
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rcarmo ◴[17 Mar 21 15:42 UTC] No.26492739[source]▶
Yeah. About the only relevant bit is that root prompts tend to use # as part of their prompt precisely to inject a comment character in case of mis-quotes/pastes.
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minitoar ◴[17 Mar 21 15:51 UTC] No.26492855[source]▶
Wow I never heard that! I always thought it was just some arbitrary convention I guess.
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1. taviso ◴[17 Mar 21 18:23 UTC] No.26494988[source]▶
I've heard this story before, but I don't think it's true.

I think the bourne shell copied # comments from csh, which had them first. When sh first implemented them, # comments only worked in scripts, not if you typed them at the shell (i.e. if isatty(0), then # wasn't special). The fact that the prompt came before comments, and that comments didn't work interactively really makes the story sound suspect!

I don't know the origin of # in csh, but I would guess it was chosen because it looks like a CPP directive.

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2. rcarmo ◴[17 Mar 21 18:38 UTC] No.26495178[source]▶
That's a fair point. But in the 90s as a System V and SunOS user, I saw some discussions happen on Usenet, and by the time X was commonplace, the myth/rationale was commonplace.
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3. mjevans ◴[17 Mar 21 19:14 UTC] No.26495629[source]▶
Even if # isn't / wasn't special, it's still exceedingly unlikely to be a valid program name, nor is it likely for a valid program name to begin with #. The practical effect should be the same as it having been special.
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4. taviso ◴[17 Mar 21 19:40 UTC] No.26495912[source]▶
Right, but the story sounds plausible because accidentally pasting '# foo > bar' would safely neuter everything else on the line.

If # isn't special, then the rest of the line is still parsed, so pasting '# foo || bar' or '# foo > baz' does do something.

5. dfox ◴[17 Mar 21 19:46 UTC] No.26495972[source]▶
I don't have reference for that but I always though that CPP directives are preceded by # because the character was previously used as comment delimiter by something (probably csh). In the grand scheme of things cpp is somewhat modern invention. IIRC the first book on Unix programming I had read mentioned in passing that cpp exists but the C code examples in there did not #include anything and if some declaration was required it was just written verbatim in the example code.
6. JdeBP ◴[18 Mar 21 00:23 UTC] No.26498466[source]▶
Even then, though, that was over a decade after the fact, and people were inventing explanations for things retroactively. Doug McIlroy has put the kibosh on this myth.

* https://minnie.tuhs.org/pipermail/tuhs/2017-April/011286.htm...

7. kragen ◴[21 Mar 21 06:30 UTC] No.26528534[source]▶
Hmm, wasn't # the default erase character for teletype-era Unix? And @ the "kill" character (like modern ^U)? That would have made them a little annoying to type—not annoying enough to keep them from being used for cpp, though, and https://github.com/memnoth/unix-v6/blob/master/sys/dmr/tty.c... suggests that you could type \#. (There was no "lnext" yet; it was just hardcoded to backslash.)

The actual input erase handling is on https://github.com/memnoth/unix-v6/blob/master/sys/dmr/tty.c.... The relevant struct member is declared in https://github.com/memnoth/unix-v6/blob/master/sys/tty.h#L35 and initialized to CERASE in each tty driver, for example, https://github.com/memnoth/unix-v6/blob/master/sys/dmr/kl.c#..., using the macro defined in https://github.com/memnoth/unix-v6/blob/master/sys/tty.h#L48.

So by default # was special, just to the kernel tty driver, not the shell.