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If grouping expressions inside [..](..), use ( .. ).

Problematic code:

[[ [ a || b ] && c ]]
[ [ a -o b ] -a c ]]

Correct code:

[[ ( a || b ) && c ]]
[ \( a -o b \) -a c ]]  # or  { [ a ] || [ b ]; } && [ c ]

Rationale:

[ .. ] should not be used to group subexpressions inside [[ .. ]] or [ .. ] statements.

For [[ .. ]], use regular parentheses.

For [ .. ], either use escaped parentheses, or preferably rewrite the expression into multiple [ .. ] joined with &&, || and { ..; } groups. The latter is preferred because [ .. ] is undefined for more than 4 arguments in POSIX.

Exceptions:

None

Related resources:

• Bash Pitfalls: if [ [ a = b ] && [ c = d ] ]; then ...
• POSIX test

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