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5.3.1 Using One Shell
---------------------

Sometimes you would prefer that all the lines in the recipe be passed to
a single invocation of the shell.  There are generally two situations
where this is useful: first, it can improve performance in makefiles
where recipes consist of many command lines, by avoiding extra
processes.  Second, you might want newlines to be included in your
recipe command (for example perhaps you are using a very different
interpreter as your 'SHELL').  If the '.ONESHELL' special target appears
anywhere in the makefile then _all_ recipe lines for each target will be
provided to a single invocation of the shell.  Newlines between recipe
lines will be preserved.  For example:

     .ONESHELL:
     foo : bar/lose
             cd $( ../$@

would now work as expected even though the commands are on different
recipe lines.

   If '.ONESHELL' is provided, then only the first line of the recipe
will be checked for the special prefix characters ('@', '-', and '+').
Subsequent lines will include the special characters in the recipe line
when the 'SHELL' is invoked.  If you want your recipe to start with one
of these special characters you'll need to arrange for them to not be
the first characters on the first line, perhaps by adding a comment or
similar.  For example, this would be a syntax error in Perl because the
first '@' is removed by make:

     .ONESHELL:
     SHELL = /usr/bin/perl
     .SHELLFLAGS = -e
     show :
             @f = qw(a b c);
             print "@f\n";

However, either of these alternatives would work properly:

     .ONESHELL:
     SHELL = /usr/bin/perl
     .SHELLFLAGS = -e
     show :
             # Make sure "@" is not the first character on the first line
             @f = qw(a b c);
             print "@f\n";

or

     .ONESHELL:
     SHELL = /usr/bin/perl
     .SHELLFLAGS = -e
     show :
             my @f = qw(a b c);
             print "@f\n";

   As a special feature, if 'SHELL' is determined to be a POSIX-style
shell, the special prefix characters in "internal" recipe lines will be
_removed_ before the recipe is processed.  This feature is intended to
allow existing makefiles to add the '.ONESHELL' special target and still
run properly without extensive modifications.  Since the special prefix
characters are not legal at the beginning of a line in a POSIX shell
script this is not a loss in functionality.  For example, this works as
expected:

     .ONESHELL:
     foo : bar/lose
             @cd $(@D)
             @gobble $(@F) > ../$@

   Even with this special feature, however, makefiles with '.ONESHELL'
will behave differently in ways that could be noticeable.  For example,
normally if any line in the recipe fails, that causes the rule to fail
and no more recipe lines are processed.  Under '.ONESHELL' a failure of
any but the final recipe line will not be noticed by 'make'.  You can
modify '.SHELLFLAGS' to add the '-e' option to the shell which will
cause any failure anywhere in the command line to cause the shell to
fail, but this could itself cause your recipe to behave differently.
Ultimately you may need to harden your recipe lines to allow them to
work with '.ONESHELL'.