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lang-bash
SHELLis mandatory: it is not. POSIX merely states how the variable is interpreted when set. As for "correct", that aspect makes the question opinion-based (it will only attract opinions).makemay useSHELL, that is straightforward. But the paragraph beginning "If it were only used", etc., undermines the question.SHELLis special in that respect (at least for gnu make), it isn't initialized that way.$SHELLis the only environment variable thatmakeis not allowed to import. This is because users could have a shell likecshthat is not compatible.systemcontains "One reviewer suggested that an implementation of system() might want to use an environment variable such as SHELL to determine which command interpreter to use. The supposed implementation would use the default command interpreter if the one specified by the environment variable was not available. This would allow a user, when using an application that prompts for command lines to be processed using system(), to specify a different command interpreter. Such an implementation is discouraged."