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| == 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 |
== 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: |
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| (0<n AND n<=10) OR n=-1 | # Bourne if test A && test B || test C; then ... |
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| can be written as follows: | 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: |
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| 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 |
# Bourne(?) if test A && { test B || test C; }; then ... |
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| Note that the parentheses have to be quoted: \(, '(' or "(". | What we should ''not'' do is try to use the `-a` or `-o` operators of the `test` command, because the results are undefined. |
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| # Bash/ksh | |
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| # Bash/ksh |
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:
ArithmeticExpression for arithmetic expressions, and
NewTestCommand for string (and file) expressions.
Examples:
# Bash/ksh
if (( (n>0 && n<10) || n == -1 ))
then echo "0 < $n < 10, or n==-1"
fior
# Bash/ksh
if [[ ( -f $localconfig && -f $globalconfig ) || -n $noconfig ]]
then echo "configuration ok (or not used)"
fiNote that the distinction between numeric and string comparisons is strict. Consider the following example:
n=3
if [[ n>0 && n<10 ]]
then echo "$n is between 0 and 10"
else echo "ERROR: invalid number: $n"
fiThe 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.