A shell in Rust could be really cool [LWN.net]

A shell in Rust could be really cool

Posted Feb 14, 2025 23:51 UTC (Fri) by himi (subscriber, #340)
In reply to: A shell in Rust could be really cool by taladar
Parent article: Rewriting essential Linux packages in Rust

The constraints of shell arithmetic are a thing that has bitten me far too often (I've resorted to feeding stuff through `bc -l`, and occasionally even running calculations through python one-liners), but this is one I've never managed to run into - how exactly does this manifest?

A few simple experiments with bash suggest "08" used in an arithmetic expansion will error ('bash: 08: value too great for base (error token is "08")') - pretty much what I'd expect, though it'd take me by surprise the first time it happened (at least the error is nice and informative). Are you talking about something like iterating through 0-padded numeric months and hitting errors once you hit August?

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A shell in Rust could be really cool

Posted Feb 15, 2025 0:16 UTC (Sat) by Wol (subscriber, #4433) [Link]

Sounds like dates in Excel. If you paste *text* into a cell, and then try to interpret it as a date, it barfs when fed a "wrong" date. I've just yesterday had to fix that problem ...

Copy a column of date-times from one spreadsheet to another. Format the date-time as a time. Save as a csv. If you're not in America the csv will contain a "random" mix of times and date-times. But all the date-times will not be a valid American date, for example today - 15/02/2025 - will be a date-time, while fast-forward a fortnight to 01/03/2025 and it will be correctly converted to a time.

The fix is easy - explicitly format/force the text into a European date, and everything then works, but it's a pain in the arse. Especially if every time you hit the problem, you've forgotten the previous time ...

Cheers,
Wol

A shell in Rust could be really cool

Posted Feb 17, 2025 13:42 UTC (Mon) by taladar (subscriber, #68407) [Link] (1 responses)

You get exactly that error but if you e.g. write a script in e.g. March and test it only with dates occurring while you write it (and maybe 01 and 12 for over/underflow edge cases) you can easily end up with a script that breaks with that error in August or September (month 08 and 09).

The solution is easy once you know about it, you can simply prefix every variable in the arithmetic expression with 10# to force base 10 but you need to know about.

A shell in Rust could be really cool

Posted Feb 18, 2025 21:46 UTC (Tue) by himi (subscriber, #340) [Link]

Ah, thanks for that explanation, it makes sense, and adds one more thing to the list of gotchas for me to keep in mind with shell code . . .

I try to use unix timestamps ($(date +%s)) whenever I'm dealing with times and dates of any sort in shell scripts, and then use $(date -d @...) to convert to something human readable - not particularly portable, but I'm fortunate enough to only really deal with Linux systems (aside from an occasional Mac, which has caused me serious headaches at times).

I do sometimes use sub-second precision (e.g. $(date +%s.%N)) for higher resolution timing, which then necessitates using bc -l or similar for any calculations. That recently bit me, in combination with scoping issues - I was mixing second and sub-second precision and the (perfectly logical) variable name 'now', and getting errors from arithmetic expansions when a sub-second precision value was still active where I thought it should be second precision . . . Fixing that just required fixing the scoping and naming issues ('local' is one of my favourite keywords), but it took *far* to long to spot the bug.

And to bring this back on topic, that's one monstrosity of a shell script that *absolutely* needs to be rewritten - though probably not in Rust, sadly, since I'm the only person in my team who'd have any hope of working on a Rust version . . .