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https://linuxcommand.org/lc3_man_pages/dbl_parensh.html

NAME (( … )) - Evaluate arithmetic expression.

SYNOPSIS (( expression ))

DESCRIPTION Evaluate arithmetic expression.

The EXPRESSION is evaluated according to the rules for arithmetic
evaluation.  Equivalent to "let EXPRESSION".

Exit Status:
Returns 1 if EXPRESSION evaluates to 0; returns 0 otherwise.

SEE ALSO bash(1)

IMPLEMENTATION GNU bash, version 5.0.17(1)-release (x86_64-redhat-linux-gnu) Copyright (C) 2019 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later http://gnu.org/licenses/gpl.html