Pure sh bible – Posix sh alternatives to external processes

Re:Pure sh bible – Posix sh alternatives to external processes by emmelaich on Monday October 19, 2020 @08:49PM (#24832867)

This uses e.g. i=$(($i + 1)) to increment.

The simpler and possibly faster way is just

   ((i++))

or ((i+=1))

This syntax works in macos ksh, so pretty sure it's POSIX.

Also, ksh93 supports sequence generation.. so as an alternative to the loops, use

   start=0
   end=10
   for i in {$start..$end}; do echo $i; done

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Re:Pure sh bible – Posix sh alternatives to external processes by fratajcz on Monday October 19, 2020 @09:18PM (#24833065)

I don't think it's actually specified by POSIX, it only mentions arithmetic expansion with $(()) and there is no mention of (()) for arithmetic operations in the grammar: https://pubs.opengroup.org/onlinepubs/007904875/utilities/xc...

The example given for arithmetic expansion also uses x=$(($x-1)): https://pubs.opengroup.org/onlinepubs/007904875/utilities/xc...

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Re:Pure sh bible – Posix sh alternatives to external processes by emmelaich on Monday October 19, 2020 @10:38PM (#24833454)

Interesting! I wonder if any shells that support $((..)) do not support ((..) though.

The update 2017 standard mentions ((..)) but not in the obvious place. It's under 'compound commands/grouping' ...

https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V...

> If a character sequence beginning with "((" would be parsed by the shell as an arithmetic expansion if preceded by a '$', shells which implement an extension whereby "((expression))" is evaluated as an arithmetic expression may treat the "((" as introducing as an arithmetic evaluation instead of a grouping command. A conforming application shall ensure that it separates the two leading '(' characters with white space to prevent the shell from performing an arithmetic evaluation.

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Re:Pure sh bible – Posix sh alternatives to external processes by Crestwave on Tuesday October 20, 2020 @07:38AM (#24836204)

Shells such as dash and ash do not support it, which are the most common test cases for POSIX scripts. It should also be noted that pre-increment itself is considered optional by POSIX, although += should work.

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