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varmod-assign-shell.mk revision 1.1 
      1 # $NetBSD: varmod-assign-shell.mk,v 1.1 2021/12/28 00:56:17 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 variable assignment operator '!=', which also assigns
      6 # when the shell command fails or crashes.
      7 #
      8 # The variable modifier '::!=' and its close relatives have been around since
      9 # var.c 1.45 from 2000-06-01.
     10 #
     11 # Before 2020.08.25.21.16.53, the variable modifier '::!=' had a bug for
     12 # unsuccessful commands, it put the previous value of the variable into the
     13 # error message instead of the command that was executed.  That's where the
     14 # counterintuitive error message 'make: "previous" returned non-zero status'
     15 # comes from.
     16 #
     17 # BUGS
     18 #	Even though the variable modifier '::!=' produces an error message,
     19 #	the exit status of make is still 0.
     20 #
     21 #	Having an error message instead of a warning like for the variable
     22 #	assignment operator '!=' is another unnecessary inconsistency.
     23 
     24 DIRECT=         previous
     25 DIRECT!=        echo output; false
     26 
     27 ASSIGNED=       previous
     28 _:=             ${ASSIGNED::!=echo output; false}
     29 
     30 all:
     31 	@echo DIRECT=${DIRECT:Q}
     32 	@echo ASSIGNED=${ASSIGNED:Q}
     33