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Nathan Manceaux-Panot

Just discovered in amazement that in Bash scripts, any variable named `SECONDS` will automatically increment every second. Bash is bonkers

25 July at 18:01 | Open on mas.to
43 comments
Nicolas :eisfunke_logo:

@Cykelero didn't want to believe you, so I tried it and now I'm staring at my screen in disbelief

#!/usr/bin/env bash

SECONDS=3

for i in $(seq 1 10); do
    echo $SECONDS
    sleep 1
done

yields

$ ./ohno.sh
3
4
5
6
7
8
9
10
11
12
25 July at 19:24 | Open on inductive.space
Thang Do

@eisfunke @Cykelero

Imagine the debugging madness this ensues

25 July at 22:52 | Open on social.linux.pizza
Steven Reed

@eisfunke @Cykelero one of the few pleasures of getting old is watching people significantly younger than me being weirded out by this sort of thing for the first time :)

26 July at 10:19 | Open on mas.to
Otte Homan - remember Geordie
Björn Gohla
Heals :heart_nb: (comms open)

@eisfunke @Cykelero @jakehamilton the actual use of the variable is to count the number of seconds since the bash instance was started - you don't have to assign anything to it but assigning a value resets it to that value (and it normally only works with the uppercase SECONDS).

Bash also offers $RANDOM (you can guess what it does) as well as $EPOCHSECONDS and $EPOCHREALTIME for the more exotic ones.

And.. many more (gnu.org/software/bash/manual/b)

26 July at 11:10 | Open on indiepocalypse.social
David Cantrell 🏏

@Cykelero it took me ages to track down that bug in one of my scripts a few months back. And when I say "track down" I mean "ineffectually whine for hours WHY DO YOU HATE ME? WHY IS THE COMPUTER DOING RANDOM STUPID SHIT?" until I asked someone on the internet and got an answer within approximately six seconds. Or was it seven? eight? nine? ...

25 July at 19:28 | Open on fosstodon.org
chrisfinazzo
Nathan Manceaux-Panot

@DrHyde Exactly!! Their intent was good, but the implementation deeply unfortunate

25 July at 19:44 | Open on mas.to
Nik | Klampfradler 🎸🚲

@DrHyde @Cykelero

Huh… that's a pre-defined variable counting the runtime of the script. The core issue here is that you used a reserved variable. There is exactly one variable named SECONDS, so "bash incrementing any variable named SECONDS" doesn't make much sense.

tldp.org/LDP/abs/html/internal

26 July at 9:22 | Open on toot.teckids.org
Carl
Nathan Manceaux-Panot

@nik Right! That’s the technically-accurate explanation. But I think many people will write a script *thinking* they’re just creating a regular old variable, not realizing they’re stepping on a landmine of good intentions

26 July at 9:51 | Open on mas.to
chebra

@Cykelero The same could happen with any variable if it conflicts with a global variable. If you use variable USER, you might also not realize that it already contains a value. And the value of PWD also sometimes changes "unexpectedly".

26 July at 16:33 | Open on mstdn.io
Carl

@DrHyde Why would you do 5,6,7,8... SECONDS RTFM if you could spent hours of whining and debugging instead?! That does not seem right. @Cykelero

26 July at 9:46 | Open on chaos.social
David Cantrell 🏏

@carl @Cykelero no normal person would expect the name of a variable to have special meaning and so think to RTFM and see if the variable name had a special meaning.

26 July at 9:49 | Open on fosstodon.org
Nathan Manceaux-Panot

@carl @DrHyde pls no being mean in my mentions mister

26 July at 9:49 | Open on mas.to
aaron
Andrew Reid

@Cykelero
Holy crap.

If I randomly do "echo $SECONDS", it seems to start at the number of seconds since the shell was launched.

It does not show up in "env | grep SECONDS".

Bonkers, like the man said.

25 July at 19:56 | Open on mastodon.cloud
Paul Johnson 🎶

@Cykelero Zsh too, and in zsh it can be floating point, if you want.

Another reason to prefer lowercase for your local $variables

25 July at 21:45 | Open on mastodon.cloud
graydon
Brian Gainor
Daniel Terhorst-North

@Cykelero you are just referencing a magic variable called SECONDS built into bash (and zsh). There are a few others that do ‘interesting’ things too!

25 July at 22:07 | Open on mas.to
ralen
Grumpy Old Techie 🕊️

@tastapod @Cykelero /bin/sh on FreeBSD doesn’t do that. If we can believe the man page that version of the Bourne shell is with us since 1989

/bin/sh
$ SECONDS=3
$
$ for i in $(seq 1 10); do
> echo $SECONDS
> sleep 1
> done
3
3
3
3
3
3
3
3
3
3

26 July at 10:34 | Open on hostux.social
Daniel Terhorst-North

@grumpyoldtechie @Cykelero yep, it’s a bash-ism. Nothing to do with posix Bourne shell. Not sure about ksh.

26 July at 14:29 | Open on mas.to
Daniel Terhorst-North

@grumpyoldtechie @Cykelero looks like ksh also has SECONDS but theirs is more sensible, which is generally true of ksh vs bash in my experience:

unix.stackexchange.com/a/11499

26 July at 22:28 | Open on mas.to
Daniel Terhorst-North

@grumpyoldtechie @Cykelero I love that you didn’t trust the first few!

26 July at 14:30 | Open on mas.to
Elliot Shank

@Cykelero Have you ever looked at /dev/tcp/… and /dev/udp/… ? Non-existent "files" that allow you to do networking from Bash.

25 July at 23:19 | Open on mastodon.social
:(){:|:&};:

@Cykelero AHHHHHH bash is an esoteric language

25 July at 23:20 | Open on kolektiva.social
mtp

@Cykelero This was a good nerd snipe. Here I was convinced that this was yet more insanity that accreted into Bash (alone) over the years, but nope: parsimonious ksh and mksh have it, too!

26 July at 6:59 | Open on proud.social
Tobias

@Cykelero Just checked, true for `zsh` and `ksh`, too.
The latter even gives you fractional parts up to the millisecond…

26 July at 7:00 | Open on toot.berlin
release_candidate
xenos 🐧🔰🇵🇭

@Cykelero did you check the man page for bash? it's documented there under shell variables.

26 July at 9:33 | Open on mas.to
JayMoore
Ellie

@Cykelero how many variables named `SECONDS` can exist concurrently?

26 July at 12:21 | Open on ellieayla.net
mirabilos

@Cykelero by the way, you should not define a variable in your script that’s comprised solely of uppercase letters, digits and underscore: these may be reserved by the implementation.

Environment variable names used by the utilities in the Shell and Utilities volume of POSIX.1-2024 consist solely of uppercase letters, digits, and the ('_') from the characters defined in [4]Portable Character Set and do not begin with a digit. Other characters, and byte sequences that do not form valid characters, may be permitted by an implementation; applications shall tolerate the presence of such names. Uppercase and lowercase letters shall retain their unique identities and shall not be folded together. The name space of environment variable names containing lowercase letters is reserved for applications. Applications can define any environment variables with names from this name space without modifying the behavior of the standard utilities.

(A separate location in POSIX makes this logic mostly apply to shell variables.)

So, user variables in shell scripts should always be lowercase.

@Cykelero by the way, you should not define a variable in your script that’s comprised solely of uppercase letters, digits and underscore: these may be reserved by the implementation.

Environment variable names used by the utilities in the Shell and Utilities volume of POSIX.1-2024 consist solely of uppercase letters, digits, and the ('_') from the characters defined in [4]Portable Character Set and do not begin with a digit. Other characters, and byte sequences that do not form valid characters,...

26 July at 15:46 | Open on toot.mirbsd.org
:parrot:
ferricoxide

@Cykelero@mas.to

Primarily working on RHEL-based (and derivative) systems, a lot of the nifty BASHisms tend to be missing because the version of BASH is typically five or more years behind current.

#Linux

26 July at 18:57 | Open on evil.social
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