A conditional expression is a special kind of expression that has
three operands. It allows you to use one expression’s value to select
one of two other expressions.
The conditional expression in awk is the same as in the C
language, as shown here:
selector ? if-true-exp : if-false-exp
There are three subexpressions. The first, selector, is always
computed first. If it is “true” (not zero or not null), then
if-true-exp is computed next, and its value becomes the value of
the whole expression. Otherwise, if-false-exp is computed next,
and its value becomes the value of the whole expression.
For example, the following expression produces the absolute value of x:
x >= 0 ? x : -x
Each time the conditional expression is computed, only one of
if-true-exp and if-false-exp is used; the other is ignored.
This is important when the expressions have side effects. For example,
this conditional expression examines element i of either array
a or array b, and increments i:
x == y ? a[i++] : b[i++]
This is guaranteed to increment i exactly once, because each time
only one of the two increment expressions is executed
and the other is not.
See Arrays in awk,
for more information about arrays.
As a minor gawk extension,
a statement that uses ‘?:’ can be continued simply
by putting a newline after either character.
However, putting a newline in front
of either character does not work without using backslash continuation
(see awk Statements Versus Lines).
If --posix is specified
(see Command-Line Options), this extension is disabled.