this post was submitted on 15 Jan 2025
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    My heart goes out to shell programmers who have to support posix sh (lemmy.world)
    submitted 1 day ago by renzev to c/linuxmemes
    95 comments fedilink hide all child comments
     

    Explanation for newbies:

    • Shell is the programming language that you use when you open a terminal on linux or mac os. Well, actually "shell" is a family of languages with many different implementations (bash, dash, ash, zsh, ksh, fish, ....)

    • Writing programs in shell (called "shell scripts") is a harrowing experience because the language is optimized for interactive use at a terminal, not writing extensive applications

    • The two lines in the meme change the shell's behavior to be slightly less headache-inducing for the programmer:

      • set -euo pipefail is the short form of the following three commands:
        • set -e: exit on the first command that fails, rather than plowing through ignoring all errors
        • set -u: treat references to undefined variables as errors
        • set -o pipefail: If a command piped into another command fails, treat that as an error
      • export LC_ALL=C tells other programs to not do weird things depending on locale. For example, it forces seq to output numbers with a period as the decimal separator, even on systems where coma is the default decimal separator (russian, dutch, etc.).

    • The title text references "posix", which is a document that standardizes, among other things, what features a shell must have. Posix does not require a shell to implement pipefail, so if you want your script to run on as many different platforms as possible, then you cannot use that feature.

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    [–] [email protected] 87 points 1 day ago (4 children)

    set -euo pipefail is, in my opinion, an antipattern. This page does a really good job of explaining why. pipefail is occasionally useful, but should be toggled on and off as needed, not left on. IMO, people should just write shell the way they write go, handling every command that could fail individually. it's easy if you write a die function like this:

    die () {
  message="$1"; shift
  return_code="${1:-1}"
  printf '%s\n' "$message" 1>&2
  exit "$return_code"
}

# we should exit if, say, cd fails
cd /tmp || die "Failed to cd /tmp while attempting to scrozzle foo $foo"
# downloading something? handle the error. Don't like ternary syntax? use if
if ! wget https://someheinousbullshit.com/"$foo"; then
  die "failed to get unscrozzled foo $foo"
fi

    It only takes a little bit of extra effort to handle the errors individually, and you get much more reliable shell scripts. To replace -u, just use shellcheck with your editor when writing scripts. I'd also highly recommend https://mywiki.wooledge.org as a resource for all things POSIX shell or Bash.

    [–] eager_eagle 3 points 18 hours ago* (last edited 18 hours ago) (1 children)

    After tens of thousands of bash lines written, I have to disagree. The article seems to argue against use of -e due to unpredictable behavior; while that might be true, I've found having it in my scripts is more helpful than not.

    Bash is clunky. -euo pipefail is not a silver bullet but it does improve the reliability of most scripts. Expecting the writer to check the result of each command is both unrealistic and creates a lot of noise.

    When using this error handling pattern, most lines aren't even for handling them, they're just there to bubble it up to the caller. That is a distraction when reading a piece of code, and a nuisense when writing it.

    For the few times that I actually want to handle the error (not just pass it up), I'll do the "or" check. But if the script should just fail, -e will do just fine.

    [–] [email protected] 3 points 12 hours ago

    This is why I made the reference to Go. I honestly hate Go, I think exceptions are great and very ergonomic and I wish that language had not become so popular. However, a whole shitload of people apparently disagree, hence the popularity of Go and the acceptance of its (imo) terrible error handling. If developers don't have a problem with it in Go, I don't see why they'd have a problem with it in Bash. The error handling is identical to what I posted and the syntax is shockingly similar. You must unpack the return of a func in Go if you're going to assign, but you're totally free to just assign an err to _ in Go and be on your way, just like you can ignore errors in Bash. The objectively correct way to write Go is to handle every err that gets returned to you, either by doing something, or passing it up the stack (and possibly wrapping it). It's a bunch of bubbling up. My scripts end up being that way too. It's messy, but I've found it to be an incredibly reliable strategy. Plus, it's really easy for me to grep for a log message and get the exact line where I encountered an issue.

    This is all just my opinion. I think this is one of those things where the best option is to just agree to disagree. I will admit that it irritates me to see blanket statements saying "your script is bad if you don't set -euo pipefail", but I'd be totally fine if more people made a measured recommendation like you did. I likely will never use set -e, but if it gets the bills paid for people then that's fine. I just think people need to be warned of the footguns.

    EDIT: my autocorrect really wanted to fuck up this comment for some reason. Apologies if I have a dumb number of typos.

    [–] renzev 26 points 1 day ago

    I've been meaning to learn how to avoid using pipefail, thanks for the info!

    [–] [email protected] 16 points 1 day ago* (last edited 1 day ago) (1 children)

    Putting or die “blah blah” after every line in your script seems much less elegant than op’s solution

    [–] [email protected] 12 points 1 day ago (3 children)

    The issue with set -e is that it's hideously broken and inconsistent. Let me copy the examples from the wiki I linked.

    Or, "so you think set -e is OK, huh?"

    Exercise 1: why doesn't this example print anything?

    #!/usr/bin/env bash
set -e
i=0
let i++
echo "i is $i"

    Exercise 2: why does this one sometimes appear to work? In which versions of bash does it work, and in which versions does it fail?

    #!/usr/bin/env bash
set -e
i=0
((i++))
echo "i is $i"

    Exercise 3: why aren't these two scripts identical?

    #!/usr/bin/env bash
set -e
test -d nosuchdir && echo no dir
echo survived 

    #!/usr/bin/env bash
set -e
f() { test -d nosuchdir && echo no dir; }
f
echo survived

    Exercise 4: why aren't these two scripts identical?

    set -e
f() { test -d nosuchdir && echo no dir; }
f
echo survived

    set -e
f() { if test -d nosuchdir; then echo no dir; fi; }
f
echo survived

    Exercise 5: under what conditions will this fail?

    set -e
read -r foo < configfile

    And now, back to your regularly scheduled comment reply.

    set -e would absolutely be more elegant if it worked in a way that was easy to understand. I would be shouting its praises from my rooftop if it could make Bash into less of a pile of flaming plop. Unfortunately , set -e is, by necessity, a labyrinthian mess of fucked up hacks.

    Let me leave you with a allegory about set -e copied directly from that same wiki page. It's too long for me to post it in this comment, so I'll respond to myself.

    [–] [email protected] 1 points 19 hours ago* (last edited 19 hours ago)

    Woah, that ((i++)) triggered a memory I forgot about. I spent hours trying to figure out what fucked up my $? one day.

    When I finally figured it out: "You've got to be kidding me."

    When i fixed with ((++i)): "SERIOUSLY! WTAF Bash!"

    [–] [email protected] 8 points 1 day ago (1 children)

    From https://mywiki.wooledge.org/BashFAQ/105

    Once upon a time, a man with a dirty lab coat and long, uncombed hair showed up at the town police station, demanding to see the chief of police. "I've done it!" he exclaimed. "I've built the perfect criminal-catching robot!"

    The police chief was skeptical, but decided that it might be worth the time to see what the man had invented. Also, he secretly thought, it might be a somewhat unwise move to completely alienate the mad scientist and his army of hunter robots.

    So, the man explained to the police chief how his invention could tell the difference between a criminal and law-abiding citizen using a series of heuristics. "It's especially good at spotting recently escaped prisoners!" he said. "Guaranteed non-lethal restraints!"

    Frowning and increasingly skeptical, the police chief nevertheless allowed the man to demonstrate one robot for a week. They decided that the robot should patrol around the jail. Sure enough, there was a jailbreak a few days later, and an inmate digging up through the ground outside of the prison facility was grabbed by the robot and carried back inside the prison.

    The surprised police chief allowed the robot to patrol a wider area. The next day, the chief received an angry call from the zookeeper. It seems the robot had cut through the bars of one of the animal cages, grabbed the animal, and delivered it to the prison.

    The chief confronted the robot's inventor, who asked what animal it was. "A zebra," replied the police chief. The man slapped his head and exclaimed, "Curses! It was fooled by the black and white stripes! I shall have to recalibrate!" And so the man set about rewriting the robot's code. Black and white stripes would indicate an escaped inmate UNLESS the inmate had more than two legs. Then it should be left alone.

    The robot was redeployed with the updated code, and seemed to be operating well enough for a few days. Then on Saturday, a mob of children in soccer clothing, followed by their parents, descended on the police station. After the chaos subsided, the chief was told that the robot had absconded with the referee right in the middle of a soccer game.

    Scowling, the chief reported this to the scientist, who performed a second calibration. Black and white stripes would indicate an escaped inmate UNLESS the inmate had more than two legs OR had a whistle on a necklace.

    Despite the second calibration, the police chief declared that the robot would no longer be allowed to operate in his town. However, the news of the robot had spread, and requests from many larger cities were pouring in. The inventor made dozens more robots, and shipped them off to eager police stations around the nation. Every time a robot grabbed something that wasn't an escaped inmate, the scientist was consulted, and the robot was recalibrated.

    Unfortunately, the inventor was just one man, and he didn't have the time or the resources to recalibrate EVERY robot whenever one of them went awry. The robot in Shangri-La was recalibrated not to grab a grave-digger working on a cold winter night while wearing a ski mask, and the robot in Xanadu was recalibrated not to capture a black and white television set that showed a movie about a prison break, and so on. But the robot in Xanadu would still grab grave-diggers with ski masks (which it turns out was not common due to Xanadu's warmer climate), and the robot in Shangri-La was still a menace to old televisions (of which there were very few, the people of Shangri-La being on the average more wealthy than those of Xanadu).

    So, after a few years, there were different revisions of the criminal-catching robot in most of the major cities. In some places, a clever criminal could avoid capture by wearing a whistle on a string around the neck. In others, one would be well-advised not to wear orange clothing in certain rural areas, no matter how close to the Harvest Festival it was, unless one also wore the traditional black triangular eye-paint of the Pumpkin King.

    Many people thought, "This is lunacy!" But others thought the robots did more good than harm, all things considered, and so in some places the robots are used, while in other places they are shunned.

    The end.

    [–] [email protected] 2 points 1 day ago (1 children)

    This is great and thanks for taking the time to enlighten us 😄

    [–] [email protected] 1 points 1 day ago

    No worries! Bash was my first language, and I still unaccountably love it after 15 years. I hate it and say mean things about it, but I'm usually pleased when I get to write some serious Bash.

    [–] pivot_root 1 points 1 day ago (1 children)

    Exercise 6:

    set -e
f() { false; echo survived; }
if ! f; then :; fi

    That one was fun to learn.

    Even with all the jank and unreliability, I think set -e does still have some value as a last resort for preventing unfortunate accidents. As long as you don't use it for implicit control flow, it usually (exercise 6 notwithstanding) does what it needs to do and fails early when some command unexpectedly returns an error.

    [–] [email protected] 1 points 1 day ago

    I personally don't believe there's a case for it in the scripts I write, but I've spent years building the || die habit to the point where I don't even think about it as I'm writing. I'll probably edit my post to be a little less absolute, now that I'm awake and have some caffeine in me.

    One other benefit I forgot to mention to explicit error handling is that you get to actually log a useful error message. Being able to rg 'failed to scrozzle foo.* because service y was not available' and immediately find the exact line in the script that failed is so nice. It's not quite a stack trace with line numbers, but it's much nicer than what you have with bash by default or with set -e.

    [–] [email protected] 3 points 1 day ago (1 children)

    Yup, and set -e can be used as a try/catch in a pinch (but your way is cleaner)

    [–] [email protected] 7 points 1 day ago (1 children)

    I was tempted for years to use it as an occasional try/catch, but learning Go made me realize that exceptions are amazing and I miss them, but that it is possible (but occasionally hideously tedious) to write software without them. Like, I feel like anyone who has written Go competently (i.e. they handle every returned err on an individual or aggregated basis) should be able to write relatively error-handled shell. There are still the billion other footguns built directly into bash that will destroy hopes and dreams, but handling errors isn't too bad if you just have a little die function and the determination to use it.

    [–] [email protected] 3 points 1 day ago (1 children)

    "There are still the billion other footguns built directly into bash that will destroy hopes and dreams, but"

    That's well put. I might put that at the start of all of my future comments about bash in the future.

    [–] [email protected] 3 points 1 day ago

    Yep. Bash was my first programming language so I have absolutely stepped on every single one of those goddamn pedblasters. I love it, but I also hate it, and I am still drawn to using it.