Info Catalog make: Execution make: Execution make: Choosing the Shell

make: One Shell

 
 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 $(@D)
              gobble $(@F) > ../$@
 
 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
 _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'.
 

Info Catalog make: Execution make: Execution make: Choosing the Shell