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ShellCheck plugin for SonarQube

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Use $((..)) for arithmetics, e.g. i=$((i + 2))
Problematic code
i=3
i=i+2

Correct code
i=3
i=$((i + 2))

Rationale
Unlike most languages, variable assignments (almost) always assigns strings and not expressions. In the example code, i will become the string i+2 instead of the intended 5.
To instead evaluate a mathematical expressions, use $((..)) as in the correct code.
Exceptions
If you wanted to assign a literal string, quote it:
description="friendly-looking"

ShellCheck (as of v0.5) doesn't recognize Bash/Ksh numeric variables created with declare -i where this syntax is valid. Using $((..)) still works, but you can also [[ignore]] this warning.
Related resources

Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!