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スクリプト言語（Perl、Python、Ruby など）がシェル言語として適していないのはなぜですか? [closed] 質問する

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スクリプト言語（Perl、Python、Ruby など）がシェル言語として適していないのはなぜですか? [closed] 質問する

シェル言語の違いは何ですか？バッシュ（bash）、Zシェル（zsh）、魚（fish）そして上記のスクリプト言語はシェルにより適しているのでしょうか？

コマンドラインを使用する場合、シェル言語の方がはるかに簡単であるように思われます。たとえば、シェルプロファイルを使用するよりも、bashを使用する方がはるかにスムーズに感じられます。Python の、反対の報告にもかかわらず中規模から大規模のプログラミングの大部分はBashよりもPythonの方が簡単だということには、ほとんどの人が同意すると思います。私はPythonを最も使い慣れた言語として使っています。パールそしてルビー。

その理由を明確に説明しようとしましたが、両方で文字列の扱いが異なることが関係していると推測する以外には説明できませんでした。

この質問をする理由は、両方で使用できる言語を開発したいと考えているからです。そのような言語をご存知の場合は、ぜひ投稿してください。

としてS.ロット説明すると、質問にはいくらかの明確化が必要です。私はシェル言語とスクリプト言語の機能について尋ねています。したがって、比較はさまざまなインタラクティブ言語の特性についてではありません（再生産) 環境（履歴やコマンド ライン置換など）で使用できます。この質問の別の表現は次のようになります。

複雑なシステムの設計に適したプログラミング言語は、同時に、ファイル システムにアクセスしたりジョブを制御したりできる便利なワンライナーを表現できるでしょうか? プログラミング言語は、スケールアップだけでなくスケールダウンにも便利に対応できるでしょうか?

ベストアンサー1

私が思いつく違いはいくつかあります。ここでは、特に順序は決めずに、ただの考えの流れを述べます。

  1. Python とその仲間は、スクリプト作成に優れているように設計されています。Bash とその仲間は、スクリプト作成にのみ優れているように設計されており、一切の妥協はありません。つまり、Python はスクリプト作成と非スクリプト作成の両方に優れているように設計されており、Bash はスクリプト作成のみを重視しています。

  2. Bash などは型付けされていませんが、Python などは強く型付けされています。つまり、数値123、文字列123、ファイルは123まったく異なります。ただし、これらは静的に型付けされていないため、区別するためにはそれぞれに異なるリテラルが必要です。
    例:

                    | Ruby             | Bash    
-----------------------------------------
number          | 123              | 123
string          | '123'            | 123
regexp          | /123/            | 123
file            | File.open('123') | 123
file descriptor | IO.open('123')   | 123
URI             | URI.parse('123') | 123
command         | `123`            | 123

  3. Python & Co. are designed to scale up to 10000, 100000, maybe even 1000000 line programs, Bash & Co. are designed to scale down to 10 character programs.

  4. In Bash & Co., files, directories, file descriptors, processes are all first-class objects, in Python, only Python objects are first-class, if you want to manipulate files, directories etc., you have to wrap them in a Python object first.

  5. Shell programming is basically dataflow programming. Nobody realizes that, not even the people who write shells, but it turns out that shells are quite good at that, and general-purpose languages not so much. In the general-purpose programming world, dataflow seems to be mostly viewed as a concurrency model, not so much as a programming paradigm.

I have the feeling that trying to address these points by bolting features or DSLs onto a general-purpose programming language doesn't work. At least, I have yet to see a convincing implementation of it. There is RuSH (Ruby shell), which tries to implement a shell in Ruby, there is rush, which is an internal DSL for shell programming in Ruby, there is Hotwire, which is a Python shell, but IMO none of those come even close to competing with Bash, Zsh, fish and friends.

Actually, IMHO, the best current shell is Microsoft PowerShell, which is very surprising considering that for several decades now, Microsoft has continually had the worst shells evar. I mean, COMMAND.COM? Really? (Unfortunately, they still have a crappy terminal. It's still the "command prompt" that has been around since, what? Windows 3.0?)

PowerShell was basically created by ignoring everything Microsoft has ever done (COMMAND.COM, CMD.EXE, VBScript, JScript) and instead starting from the Unix shell, then removing all backwards-compatibility cruft (like backticks for command substitution) and massaging it a bit to make it more Windows-friendly (like using the now unused backtick as an escape character instead of the backslash which is the path component separator character in Windows). After that, is when the magic happens.

They address problem 1 and 3 from above, by basically making the opposite choice compared to Python. Python cares about large programs first, scripting second. Bash cares only about scripting. PowerShell cares about scripting first, large programs second. A defining moment for me was watching a video of an interview with Jeffrey Snover (PowerShell's lead designer), when the interviewer asked him how big of a program one could write with PowerShell and Snover answered without missing a beat: "80 characters." At that moment I realized that this is finally a guy at Microsoft who "gets" shell programming (probably related to the fact that PowerShell was neither developed by Microsoft's programming language group (i.e. lambda-calculus math nerds) nor the OS group (kernel nerds) but rather the server group (i.e. sysadmins who actually use shells)), and that I should probably take a serious look at PowerShell.

Number 2 is solved by having arguments be statically typed. So, you can write just 123 and PowerShell knows whether it is a string or a number or a file, because the cmdlet (which is what shell commands are called in PowerShell) declares the types of its arguments to the shell. This has pretty deep ramifications: unlike Unix, where each command is responsible for parsing its own arguments (the shell basically passes the arguments as an array of strings), argument parsing in PowerShell is done by the shell. The cmdlets specify all their options and flags and arguments, as well as their types and names and documentation(!) to the shell, which then can perform argument parsing, tab completion, IntelliSense, inline documentation popups etc. in one centralized place. (This is not revolutionary, and the PowerShell designers acknowledge shells like the DIGITAL Command Language (DCL) and the IBM OS/400 Command Language (CL) as prior art. For anyone who has ever used an AS/400, this should sound familiar. In OS/400, you can write a shell command and if you don't know the syntax of certain arguments, you can simply leave them out and hit F4, which will bring a menu (similar to an HTML form) with labelled fields, dropdown, help texts etc. This is only possible because the OS knows about all the possible arguments and their types.) In the Unix shell, this information is often duplicated three times: in the argument parsing code in the command itself, in the bash-completion script for tab-completion and in the manpage.

Number 4 is solved by the fact that PowerShell operates on strongly typed objects, which includes stuff like files, processes, folders and so on.

Number 5 is particularly interesting, because PowerShell is the only shell I know of, where the people who wrote it were actually aware of the fact that shells are essentially dataflow engines and deliberately implemented it as a dataflow engine.

Another nice thing about PowerShell are the naming conventions: all cmdlets are named Action-Object and moreover, there are also standardized names for specific actions and specific objects. (Again, this should sound familar to OS/400 users.) For example, everything which is related to receiving some information is called Get-Foo. And everything operating on (sub-)objects is called Bar-ChildItem. So, the equivalent to ls is Get-ChildItem (although PowerShell also provides builtin aliases ls and dir – in fact, whenever it makes sense, they provide both Unix and CMD.EXE aliases as well as abbreviations (gci in this case)).

But the killer feature IMO is the strongly typed object pipelines. While PowerShell is derived from the Unix shell, there is one very important distinction: in Unix, all communication (both via pipes and redirections as well as via command arguments) is done with untyped, unstructured strings. In PowerShell, it's all strongly typed, structured objects. This is so incredibly powerful that I seriously wonder why noone else has thought of it. (Well, they have, but they never became popular.) In my shell scripts, I estimate that up to one third of the commands is only there to act as an adapter between two other commands that don't agree on a common textual format. Many of those adapters go away in PowerShell, because the cmdlets exchange structured objects instead of unstructured text. And if you look inside the commands, then they pretty much consist of three stages: parse the textual input into an internal object representation, manipulate the objects, convert them back into text. Again, the first and third stage basically go away, because the data already comes in as objects.

However, the designers have taken great care to preserve the dynamicity and flexibility of shell scripting through what they call an Adaptive Type System.

Anyway, I don't want to turn this into a PowerShell commercial. There are plenty of things that are not so great about PowerShell, although most of those have to do either with Windows or with the specific implementation, and not so much with the concepts. (E.g. the fact that it is implemented in .NET means that the very first time you start up the shell can take up to several seconds if the .NET framework is not already in the filesystem cache due to some other application that needs it. Considering that you often use the shell for well under a second, that is completely unacceptable.)

私が言いたい最も重要な点は、スクリプト言語やシェルの既存の取り組みを見たい場合、UnixやRuby/Python/Perl/PHPファミリーだけに留まるべきではないということです。たとえば、Tclすでに言及しました。レックス別のスクリプト言語になります。エマックスリスプもう一つあります。シェルの領域には、OS/400 のコマンド ラインや DCL など、すでに述べたメインフレーム/ミッドレンジ シェルがいくつかあります。また、Plan9 の rc もあります。

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