Question

I do see certain statements in a pre-written Makefile target, where they are invoking the shell to execute certain commands. Following is the syntax of the one which I am puzzled about.

[ -z "$(ENV_VAR)" ] || \
echo Building $(ENV_VAR); \

So my questions in the above statement are:

(1) Is this a conditional statement that checks the existence of an environment variable?

(2) What does the -z option mean.

(3) As of I know “||” is a logical operator, how does it behave in such a use case.

Answer 1

1. Yes.
2. -z: Expression is true, if the length of following string has zero length
3. cmd1 || cmd2: Execution of cmd2 if cmd1 returns with a status not equal to zero (i.e., cmd1 fails)