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Trying to translate:

What is a basic, elegant and fast way, to write factorial in Bash/in a shell script, purely. (Code) Please make suggestions. (Code)

Tem outra alternativa mais básica/elegante/rápida de como fazer em poucas linhas de código uma maneira de calcular um número fatorial em BASH (Shell Script) puro.

Ex.: 5! = 5*4*3*2*1 = 120

Essa é minha sugestão:

num=5
echo $(($(eval echo {$num..1} | sed 's/ /*/g')))
user unknown
  • 10,482
Helio Giroto
  • 1

1 Answers1

2

This works without eval: 

prod=$(echo {1..5}* 1); echo $((prod))

Here is a bash function, which takes an parameter, what isn't possible in the above solution:

fak () 
{ 
    echo $(($(seq -s'*' 1 $1)))
}

As cas pointed out in the comments, the Shell is limited to 64 bits, for bigger results, bc is worth a try, but that isn't puro, is it? Well - that argument count's for seq too; it's an external program.

echo {1..30}* 1 | bc
user unknown
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  • +1 for the {1..5}* 1, but it's probably better to pipe into bc rather than use bash's built-in arithmetic. bc can work with integers of any arbitrary precision, while shell has a maxint value (determined mostly by cpu architecture it was compiled on - e.g. 64 bit int on a 64 bit cpu). For example, prod=$(echo {1..30}* 1); echo $((prod)) prints -8764578968847253504 (clearly wrong), while echo {1..30}* 1 | bc prints 265252859812191058636308480000000 (presumably correct). – cas Feb 26 '18 at 02:11
  • Yes, bc reaches far wider, while not arbitrary wide - test echo "9^9^9 | bc". And afaik, bash calculated with 64 bits when I still used a 32bit PC, but I'm not 100% sure about that. I don't speak Brazilian/Portuguese, but "em BASH (Shell Script) puro" sounds much like "in purely Bash" doesn't it? (I updated the translation). And the example 5! didn't look that ambitious, too. :) – user unknown Feb 26 '18 at 03:08
  • the bc docs & man page say "arbitrary" so that's what i copied :) – cas Feb 26 '18 at 03:13
  • BTW, piping into bc works for at least up to 99999!. That took about 5 minutes to run on my machine, which was boring enough to discourage me from trying larger numbers. – cas Feb 26 '18 at 03:22
  • afaik echo "8^8^8 | bc" fails too, but "7^7^7 | bc" should work. :) – user unknown Feb 26 '18 at 03:28
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    {1..5}* 1 looks cool until you happen to have a file starting with one of these digits in the current directory: and bc responds with (standard_in) 1: syntax error – arielf Sep 26 '18 at 18:48