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16.3.6 Connectives for test

Note it’s preferred to use shell logical primitives rather than these logical connectives internal to test, because an expression may become ambiguous depending on the expansion of its parameters.

For example, this becomes ambiguous when ‘$1’ is set to ‘'!'’ and ‘$2’ to the empty string ‘''’:

test "$1" -a "$2"

and should be written as:

test "$1" && test "$2"

Note the shell logical primitives also benefit from short circuit operation, which can be significant for file attribute tests.

! expr

True if expr is false. ‘!’ has lower precedence than all parts of expr. Note ‘!’ needs to be specified to the left of a binary expression, I.e., ‘'!' 1 -gt 2’ rather than ‘1 '!' -gt 2’. Also ‘!’ is often a shell special character and is best used quoted.

expr1 -a expr2

True if both expr1 and expr2 are true. ‘-a’ is left associative, and has a higher precedence than ‘-o’.

expr1 -o expr2

True if either expr1 or expr2 is true. ‘-o’ is left associative.