No such example; after all, you can spawn a shell subprocess in any of those.
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
> How do you find which module you need? Search on the web may be the best answer.
Experience, stdlib reference, web searches. Same as with bash.
> In shell at least you have "man -k".
I really disagree here. Since you took `glob` as an example, how do you get to the explanation of `for test in *.in`? Go on, try that with "man -k".
> Already python is telling me that the "man" documentation is going to be better
I'm not trying to say python is good here (well, the stdlib documentation on the web is actually pretty good, it's just not easily available from the console). But the idea that bash/man is more discoverable is just wrong... You can find the glob under "Pattern Matching" section which is all right, but you need to understand most of the expansion mechanism of shell to know it applies to "for". Then again "for" itself has a definition that belongs more to a CS material, than to a usage guide.
Since you took `glob` as an example, how do you get to the explanation of `for test in .in`?*
`man -k wildcard` points to `man 3am fnmatch` which points to `man 3 fnmatch`. Now the process for python and shell converge, and I still have to know to look at the "See Also" section of the manpage to find `man 7 glob` which finally gives me useful information.
The python workflow involved one fewer discrete step, but the user would have to know both help() in the python REPL and how to navigate man pages, while for shell scripts the user only needed to know how to navigate man pages.
I'm not trying to say python is good here (well, the stdlib documentation on the web is actually pretty good, it's just not easily available from the console). But the idea that bash/man is more discoverable is just wrong... You can find the glob under "Pattern Matching" section which is all right, but you need to understand most of the expansion mechanism of shell to know it applies to "for". Then again "for" itself has a definition that belongs more to a CS material, than to a usage guide.
I do very much agree with you here. Man pages are not very accessible. Some man pages (most that I work with, but I understand that's not everyone's experience) are very good, complete, and understandable. I'd be comfortable saying that python's and the shell's documentation features are roughly equally usefule ± some small amount.
Well I agree that glob was not the best example. For sure there are things you have to learn about shell scripting as well.. one of them is if "man for" doesn't work, try "help for" or "man bash". In fact there is no good reason for this and it should be improved. (not to mention that you already had to know what *.in does, but you could argue that this is fundamental shell syntax: man bash / pattern matching).
I'd argue that that's significantly worse at conveying the intent than the shell script version.
The shell script started as a workflow that was executed repeatedly, manually, from the shell, and then automated. Naturally, the shell script resembles very closely they commands typed in at the shell, with some added code to encode the part of the routine that was executed in the user's head.
The python script doesn't look anything like the routine that was previously entered at the shell. That makes it harder to tell at a glance that this script does the same thing.
It it horrible? No absolutely not. If you're working on a team where everyone knows python but not everyone is comfortable with shell scripts, then writing the script in python is the clear best option. But if you're working on a team where everyone is comfortable with both python and shell scripts, the shell script probably wins out.
> Can you give a shell script example of something that perl/python/ruby/nodejs can't do?
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?
> Install perl/python/ruby and nodejs -- requiring just the shell to be installed?
I just meant using any one of them, not all of them. And you can pretty much always rely on python and perl being installed.
But my point is that if you're doing something other than a one-off thing, then it belongs to some project and you're probably going to commit that script to that project's codebase. That means it has to be maintained. Do future you and whoever else has to maintain the project a favor and use one of the popular scripting languages that has reasonable syntax and semantics.
Many minimal distributions have only the shell installed - which make it (still) relevant for provisioning/bootstrap etc.
Personally I'd much rather maintain a shell script than a perl script - but that's just because I know shell better. Maybe shell is the first language people would program without learning it (js being the second)?
As a system shell? In theory, perhaps. But migrating a typical Linux/bsd distribution away from having any dependency in the shell would be a major undertaking. While some distributions already ship with only shell/busybox.
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: