> What is the basis for this? Shell script is a scripting language (more precisely, a set of scripting languages with similar features.) The difference between shell scripting languages and other scripting languages is that the former are optimized around the need to scale down to a convenient line-by-line way to work with the system in a REPL; while the others may support work in a REPL it is not what they are optimized for.

Disagree. The defining characteristic of shell scripting languages is that they are a shell that can be scripted. What's a shell? It's a program designed to be a layer around the OS, exposing all of the capabilities of the OS to the user in a convenient form.

So the only way a non-shell scripting language can become that powerful is to become a shell scripting language.

The primitives in shell scripts are the primitives of the operating system. The fundamental building blocks of your language are strings and files and processes, which makes it really convenient to work with strings and files and processes, AKA the operating system.

There's just certain types of tasks where you can more clearly express your intent in a shell script. If, for example, you need to spawn a ton of subprocesses, you can do that in any scripting language, but shell is designed from the ground up to launch subprocesses — it's most basic purpose is to launch programs.

And so while I can't give an example of something that can only be done in a shell script, here's a shell script that I wrote a few weeks ago that would have been a pain in the neck to express in any other language:

    #!/bin/sh
    cd "$(dirname "$0")"
    for test in *.in; do
        test="$(basename "$test" .in)"
        infile="$test.in"
        outfile="$test.out"
        output="$(./whofrom "$infile" 2>&1)"
        expected="$(cat "$outfile")"
        if [ "$output" != "$expected" ]; then
                echo "Failed test $test"
                echo "Expected: $expected"
                echo "Actual:   $output"
                echo
        fi
        if ! valgrind --error-exitcode=1 --leak-check=full ./whofrom "$infile" 2>/dev/null >/dev/null; then
                echo "Failed test $test"
                valgrind -q --leak-check=full ./whofrom "$infile"
                echo
        fi
    done

Install perl/python/ruby and nodejs -- requiring just the shell to be installed?

Snark aside (it's not only snark - pretty much every system will have something like a posix shell), proper posix portable shell is hard - it's an old gnarly language -- but it's what we've got. And with a bit of discipline and good practices -- it doesn't have to be bad. That said, a lot of real-world shell scripts are bad.

I tend to agree with the overall sentiment; shell isn't a great language. As soon as you start to mix awk (which awk is that, do you need GNU awk?), sed and perhaps a bit of egrep (or grep -E -- are both available? Does it accept only BSD-style parameters?) -- one should consider moving "up".

And for eg: setting up a python package/program -- I'd generally prefer a python script -- hopefully one that handles different file-paths (eg: / vs \ ) and other cross-platform stuff. If you already depend on python, why add dependency on shell?

Edit: I suppose it is possible to get some version of a repl with C++ (Cling?), Java (Beanshell, Java 9?), C# (Mono, but not currently .Net?). But each of those seems generally a bit harder to get access to than simply calling gdb on a binary with debugging symbols enabled.