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 $ bang() ( IFS=\*; let N=$1-1 k="$*" && bang $N $k || echo ${2-1} )

Nov. 25, 2019, 8:06 a.m.CK

Explanation

A recursive function that leverages the arithmetic expansions evaluated by the let command, and the significance of the field separator when arguments are expanded using "$*".

  • IFS=\* concatenates the arguments "$*" using a * symbol, which let interprets as a multiplication operator;

  • let N=$1-1 k="$*" assigns arithmetic values to variables N and k. N will have a value one less than the first argument; k will be the product of all the arguments;

  • && performs the command immediately to its right iff the expression immediately to its left evaluates to true;

  • bang $N $k calls itself with the first argument being one less than it was, and the second argument being the cumulative product of the arguments thus far across all iterations;

  • || performs the command immediately to its right iff the command immediately to its left wasn't performed;

  • echo ${2-1} prints the second argument, which—if there is one—will be the factorial, otherwise it prints 1.

Limitations

If a positive integer is not the one and only argument passed to it when invoked by the user, the result will be nonsense or an error will be thrown.