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What kind of strange shell syntax is hidden behind $((40-35)) and how come it calculates a value?

Example:

$> echo $((40-35))
5
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5  
Open bash manual page and search for ^ *Arithmetic Expansion –  enzotib Sep 1 '11 at 17:15
    
In tcsh, it just gives "Illegal variable name." –  barrycarter Sep 1 '11 at 20:57
2  
Why do you think it is "strange"? It's in the man page. 'man sh' –  Jonathan Cline IEEE Sep 1 '11 at 23:21
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2 Answers

up vote 15 down vote accepted

From the bash man page:

  ((expression))
          The expression is evaluated according  to  the  rules  described
          below  under ARITHMETIC EVALUATION.  If the value of the expres-
          sion is non-zero, the return status is 0; otherwise  the  return
          status is 1.  This is exactly equivalent to let "expression".

And further:

  Arithmetic Expansion
    Arithmetic expansion allows the evaluation of an arithmetic expression and
    the substitution of the result.  The format for arithmetic expansion is:

          $((expression))

Anything put into double parens becomes a math expression that bash will evaluate.

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This is arithmetic expansion. It's supported in all POSIX shells (it didn't exist in the original Bourne shell). The syntax of arithmetic expression is mostly the same as in the C language.

You can use variables with or without a $ in an arithmetic expression. Without a $, a variable stands as a variable, and can in particular be assigned to; for example echo $((a=4)) $a prints 4 4. With a $, the shell performs a simple text expansion; for example a=2+2; echo $(($a)) prints 4 whereas a=2+2; echo $((a)) is an error (but prints 4 on some shells anyway).

The exact rules for deciding whether $(( begins a command substitution ($(command) where the command happens to begin with an opening parenthesis) or an airthmetic expression ($((expression))) vary from shell to shell. If you want to perform a command substitution and the command begins with (, put a space to be safe: $( (echo hello)) (such commands are rare, which is why the syntax could be added without any practical danger of confusion).

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