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varmod-assign-shell.mk revision 1.3 
      1 # $NetBSD: varmod-assign-shell.mk,v 1.3 2022/01/09 18:22:31 rillig Exp $
      2 #
      3 # Tests for the variable modifier '::!=', which assigns the output of a shell
      4 # command to the variable, but only if the command exited successfully.  This
      5 # is different from the other places that capture the output of an external
      6 # command (variable assignment operator '!=', expression modifier ':sh',
      7 # expression modifier ':!...!'), which also use the output when the shell
      8 # command fails or crashes.
      9 #
     10 # The variable modifier '::!=' and its close relatives have been around since
     11 # var.c 1.45 from 2000-06-01.
     12 #
     13 # Before 2020.08.25.21.16.53, the variable modifier '::!=' had a bug for
     14 # unsuccessful commands, it put the previous value of the variable into the
     15 # error message instead of the command that was executed.  That's where the
     16 # counterintuitive error message 'make: "previous" returned non-zero status'
     17 # comes from.
     18 #
     19 # BUGS
     20 #	Even though the variable modifier '::!=' produces an error message,
     21 #	the exit status of make is still 0.
     22 #
     23 #	Having an error message instead of a warning like for the variable
     24 #	assignment operator '!=' is another unnecessary inconsistency.
     25 
     26 DIRECT=		previous
     27 DIRECT!=	echo output; false
     28 
     29 ASSIGNED=	previous
     30 _:=		${ASSIGNED::!=echo output; false}
     31 
     32 all:
     33 	@echo DIRECT=${DIRECT:Q}
     34 	@echo ASSIGNED=${ASSIGNED:Q}
     35