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

The portable (POSIX or Bourne) way is to use multiple test (or [) commands:

    # Bourne
    if test A && test B || test C; then

The grouping is implicit in this case, because AND (&&) has a higher precedence than OR (||). If we need explicit grouping, then we can use curly braces:

    # Bourne(?)
    if test A && { test B || test C; }; then

What we should not do is try to use the -a or -o operators of the test command, because the results are undefined.

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


    # Bash/ksh
    if (( (n>0 && n<10) || n == -1 ))
    then echo "0 < $n < 10, or n==-1"


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

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

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

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.


2012-07-01 04:05