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It's not a completely non special character: for instance in bash it's special inside braces in the syntax where "/{,usr/}bin" expands to "/bin /usr/bin". But the need to start that syntax with the open brace will remind you about the need to escape a literal comma there if you ever want one.
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What about using the filename in arrays in bash/sh?
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But Bash arrays don’t use comma, what’s the problem?
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Oh, that might be true, I do remember encountering some escaping issues when creating a more complex POSIX (or bash) script that involved lists and iterating through stuff.

I see Bash only uses commas in Brace expansions:

file{1,2,3}.txt # file1.txt file2.txt file3.txt

I guess it would only be a problem if you want to expand

    file,.txt   
    file,,.txt   
    file,,,.txt
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Imagine seeing this code:

    echo file{",",",,",",,,"}.txt
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Have you met Bash? It’s a shrine to space-delimited everything lol
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I reworded my comment for clarity now.
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You never used CVS/RCS with its “,v” files?
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Until someone forces you to use a file system that cannot tolerate commas...
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Which file system would that be?
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