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 [email protected] to c/[email protected]
    94 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] 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.

    [–] jkercher 1 points 20 hours ago* (last edited 20 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

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

    [–] [email protected] 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.