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Arithmetic Conditionals

In Chapter 5, we saw how to compare strings by the use of [...] notation (or with the test built-in). Arithmetic conditions can also be tested in this way. However, the tests have to be carried out with their own operators. These are shown in Table 6-4.

Table 6-4. Test relational operators

Operator

Meaning

-lt

Less than

-gt

Greater than

-le

Less than or equal to

-ge

Greater than or equal to

-eq

Equal to

-ne

Not equal to

And as with string comparisons, the arithmetic test returns a result of true or false; 0 if true, 1 otherwise. So, for example, [ 3 -gt 2 ] produces exit status 0, as does [ \( 3 -gt 2 \) || \( 4 -le 1 \) ], but [ \( 3 -gt 2 \) && \( 4 -le 1 \) ] has exit status 1 since the second subexpression isn't true.

In these examples we have had to escape the parentheses and pass them to test as separate arguments. As you can see, the result can look rather unreadable if there are many parentheses.

Another way to make arithmetic tests is to use the $((...)) form to encapsulate the condition. For example: [ $(((3 > 2) && (4 <= 1))) = 1 ]. This evaluates the conditionals and then compares the resulting value to 1 (true).[12]

There is an even neater and more efficient way of performing an arithmetic test: by using the ((...)) construct.[13] This returns an exit status of 0 if the expression is true, and 1 otherwise.

The above expression using this construct becomes (( (3 > 2) && (4 <= 1) )). This example returns with an exit status of 1 because, as we said, the second subexpression is false.