Diff for "BashFAQ/017"

How can I group expressions, e.g. (A AND B) OR C?

The TestCommand [ uses parentheses () for expression grouping. Given that "AND" is "-a", and "OR" is "-o", the following expression

    (0<n AND n<=10) OR n=-1

can be written as follows:

    if [ \( $n -gt 0 -a $n -le 10 \) -o $n -eq -1 ]
    then
        echo "0 < $n <= 10, or $n=-1"
    else
        echo "invalid number: $n"
    fi

Note that the parentheses have to be quoted: \(, '(' or "(".

BASH and KornShell have different, more powerful comparison commands with slightly different (easier) quoting:

• ArithmeticExpression for arithmetic expressions, and

• NewTestCommand for string (and file) expressions.

Examples:

    if (( (n>0 && n<10) || n == -1 ))
    then echo "0 < $n < 10, or n==-1"
    fi

or

    if [[ ( -f $localconfig && -f $globalconfig ) || -n $noconfig ]]
    then echo "configuration ok (or not used)"
    fi

Note that the distinction between numeric and string comparisons is strict. Consider the following example:

    n=3
    if [[ n>0 && n<10 ]]
    then echo "$n is between 0 and 10"
    else echo "ERROR: invalid number: $n"
    fi

The output will be "ERROR: ....", because in a string comparision "3" is bigger than "10", because "3" already comes after "1", and the next character "0" is not considered. Changing the square brackets to double parentheses (( makes the example work as expected.

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