Previous: Numeric tests, Up: test invocation [Contents][Index]
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.