POSIX Shell Has This Awesome Thing Called An Until Loop
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- เผยแพร่เมื่อ 15 ก.ย. 2024
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Sometimes when you're writing a shell script you want some sort of action to keep attempting to run until it works and that's when a really cool POSIX compliant loop structure called an until loop comes into play.
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How do you stretch two loops to be a ten minute video?
TH-camr energy
but its 9:51
Yeah I know I should have stuck something on the end to get it to 10 minutes
ha ha, he still got 60+ likes so far
@@BrodieRobertson you could have just sang "loopy loopy loopy loop" for a minute.
If I remember correctly (at least in Pascal and Cobol) the until loop tests the condition at the end of the loop while tests at the top of the loop. I think this might be an important consideration in that an until loop will always execute at least once whereas a while loop may never execute.
In sh, `until` checks the condition at the top. In typical structural programming languages in widespread use today, there is no `until` loop, but a `do … while` loop, which checks the condition at the bottom.
@@humm535 Weird
You should definitely check out the tool entr, it's way better for the usecase you mentioned. It waits for filechanges to perform an action
That might be cool
so it's just `while` with an implicit negation? tbh i don't really see the use cases for `until` that couldn't just be a `while !`, unless i'm missing something here
It can help with readability in large and complicated shell scripts. Especially when negations are everywhere, "until X" is much easier to think about than "while not X", even though they technically are the same thing.
@@nmertsch8725 fair, although it's mostly the opposite in my experience, i find it easier to parse an explicit negation than to have to remember yet another loop type
It's basically just a more concise way of writing while ! which better describes the loop structure.
@@computer-love "untill true" sounds easier than "while not true" tho :p
incron is the best way to do what you describe in the beginning
Not exactly related to the video but i usually log the error output of a command to a separate file. For instance if normally the scripts "echoes" success messages to. local/log/script-output.log, then the standard errors of the script's commands go to local/log/script-err. log. This method makes debugging easier when you check your scripts after months of use.
I think there should also be a comment about how fast you named your patrons. You _must_ have practiced that a lot!
It's definitely not on the first take most of the time.
Pretty useful
and ... you're welcome =D
The "until" loop was in (Turbo)Pascal, BASIC and a few other old-school languages. Also, "until" is basically the same as " do {} while()" that's present in python, C etc...
It's a very different structure to a do while loop, you can make them act the same way but it's not the same structure what so ever
@@BrodieRobertson OK, you are right. Until does the check before executing the loop. I learned to code using Pascal (in high school a gogleplex years ago) , which has the "until" loop and my brain just linked it with the do-while for some reason.
Does this have a problem with large files? I noticed that if you write a file, it appears on the disk the moment it begins being written, not when it finishes. I think accessing a large file while it's being written can be dangerous.
When supplied with no status argument, the exit command will actually use the status code of the last command executed, not necessarily 0!
Cobol also has until loops
I've never touched COBOL before but that's cool
@@BrodieRobertson for a Completely Obsolete Bussines Oriented Language it has many cool features
Ruby also has it
Pascal has it too.
Great videos mate! Keep it up!
That's the plan
I think that you stretch out two loops to be a ten minute video for the people who are not up on scripting and who need the explainations. Thanks.
I've been trying to make it a useful 10 minutes, rather than just 10 minute of filler
Visual Basic has Until-Loops. QBasic might have it as well. (don't use that crap, learn a real programming language ;-P )