bash - I'm having trouble performing arithmetic expressions in UNIX -

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i have following script:

  #!/bin/sh   r=3   r=$((r+5))   echo r

however, error:

syntax error @ line 3: $ unexpected.

i don't understand i'm doing wrong. i'm following online guide letter http://www.unixtutorial.org/2008/06/arithmetic-operations-in-unix-scripts/

this sounds fine if you're using bash, $((r+5)) might not supported if you're using shell. /bin/sh point to? have considered replacing /bin/bash if it's available?


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