Why are scripting languages (e.g. Perl, Python, and Ruby) not suitable as shell languages?

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What are the differences between shell languages like Bash (bash), Z shell (zsh), Fish (fish) and the scripting languages above that makes them more suitable for the shell?

When using the command line, the shell languages seem to be much easier. It feels for me much smoother to use bash for example than to use the shell profile in IPython, despite reports to the contrary. I think most will agree with me that a large portion of medium to large scale programming is easier in Python than in Bash. I use Python as the language I am most familiar with. The same goes for Perl and Ruby.

I have tried to articulate the reason, but I am unable to, aside from assuming that the treatment of strings differently in both has something to do with it.

The reason of this question is that I am hoping to develop a language usable in both. If you know of such a language, please post it as well.

As S.Lott explains, the question needs some clarification. I am asking about the features of the shell language versus that of scripting languages. So the comparison is not about the characteristics of various interactive (REPL) environments such as history and command line substitution. An alternative expression for the question would be:

Can a programming language that is suitable for design of complex systems be at the same time able to express useful one-liners that can access the file system or control jobs? Can a programming language usefully scale up as well as scale down?

perl shell ruby python bash
12 Answers

Since both are formally programming languages, what you can do in one, you can do in the other. Actually it is a design emphasis issue. Shell languages are designed for interactive use, while scripting languages aren't.

The basic difference in the design is the storage of data between commands and the scope of variables. In Bash, etc. you have to jump through hoops to store a value (for example, commands like set a='something'), while in languages like Python you simply use an assignment statement (a = 'something'). When using the values in a shell language you have to tell the language that your want the value of the variable, while in scripting languages you have to tell the language when you want the immediate value of the string. This has effects when used interactively.

In a scripting language where ls was defined as a command

a = some_value

ls a*b

(What does a mean? Does this mean some_value * (whatever b is) or do you mean 'a'anystring'b'?. In a scripting language the default is what is stored in memory for a.)

ls 'a*b'  Now means what the Unix ls a*b means.

In a Bash-like language

set a=some_value

ls a*b   means what the Unix ls a*b means.

ls $a*b  uses an explicit recall of the value of a.

Scripting languages make it easy to store and recall values and hard to have a transient scope on a value. Shell languages make it possible to store and recall values, but have a trivially transient scope per command.

user_5598240

Scalability and extensibility? Common Lisp (you can even run CLISP, and possibly other implementations, as a login shell in Unix environments).

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