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Choosing a Shell in DOS and Windows...................................Choosing a shell in MS-DOS and MS-Windows is much more complex than onother systems. On MS-DOS, if `SHELL' is not set, the value of the variable`COMSPEC' (which is always set) is used instead. The processing of lines that set the variable `SHELL' in Makefilesis different on MS-DOS. The stock shell, `command.com', isridiculously limited in its functionality and many users of `make' tendto install a replacement shell. Therefore, on MS-DOS, `make' examinesthe value of `SHELL', and changes its behavior based on whether itpoints to a Unix-style or DOS-style shell. This allows reasonablefunctionality even if `SHELL' points to `command.com'. If `SHELL' points to a Unix-style shell, `make' on MS-DOSadditionally checks whether that shell can indeed be found; if not, itignores the line that sets `SHELL'. In MS-DOS, GNU `make' searches forthe shell in the following places: 1. In the precise place pointed to by the value of `SHELL'. For example, if the makefile specifies `SHELL = /bin/sh', `make' will look in the directory `/bin' on the current drive. 2. In the current directory. 3. In each of the directories in the `PATH' variable, in order. In every directory it examines, `make' will first look for thespecific file (`sh' in the example above). If this is not found, itwill also look in that directory for that file with one of the knownextensions which identify executable files. For example `.exe',`.com', `.bat', `.btm', `.sh', and some others. If any of these attempts is successful, the value of `SHELL' will beset to the full pathname of the shell as found. However, if none ofthese is found, the value of `SHELL' will not be changed, and thus theline that sets it will be effectively ignored. This is so `make' willonly support features specific to a Unix-style shell if such a shell isactually installed on the system where `make' runs. Note that this extended search for the shell is limited to the caseswhere `SHELL' is set from the Makefile; if it is set in the environmentor command line, you are expected to set it to the full pathname of theshell, exactly as things are on Unix. The effect of the above DOS-specific processing is that a Makefilethat contains `SHELL = /bin/sh' (as many Unix makefiles do), will workon MS-DOS unaltered if you have e.g. `sh.exe' installed in somedirectory along your `PATH'.
在DOS 或者 Windows中选择shell
...................................Choosing a shell in MS-DOS and MS-Windows is much more complex than on
other systems.在 MS-DOS 或者 MS-Windows 上选择一个shell， 比在其他系统上复杂多了。
在 MS-DOS ， 如果 SHELL 没有设定，那么变量 COMSPEC 会被使用。
在 makefile里面的，对SHELL 的设定方式， 在 MS-DOS 上的表现，也有所不同。
陈旧的shell--command.com'，被荒谬地限制在自己的功能里，很多make用户想要安装一个替代性的shell。
因此，在 MS-DOS, make 检查 SHELL 的值，根据它指向Unix 类型的shell 还是 DOS 类型的shell， 来决定自身的行为。
这可以保证合理的功能，甚至在 SHELL 指向了 command.com 时也是如此。如果 SHELL 指向了一个 Unix 类型的shell, 那么在 MS-DOS 系统上的 make 要检查是否此shell 真的存在；如果没有，它就忽略那些使用SHELL 的行。在MS-DOS中， GNU make 搜索如下的地方:
1. 在SHELL 的值所精确指定的位置，例如，如果makefile 指定了 SHELL=/bin/sh，make 会搜索 当前驱动器的/bin目录。
2. 在当前目录。
3. 按照顺序搜索PATH 变量中指定的路径。
在每一个它所检查的目录，make 会先看看特定的文件(上述例子中的sh)， 如果没有找到，它也会看看那些可知兴文锦，如.exe， .com，.bat，.btm，.sh 之类。
如果以上任何一个努力得到了成功，SHELL 的值回被设定为所找到的SHELL的全路径名。但是，如果没有找到，SHELL的值不会改变，设置它的行就会被无视。这也是 如果确实存在一个shell在此系统中存在，make 会仅仅支持 UNIX 类型的特定功能的原因。
请注意这个对shell的扩展搜索仅限于SHELL 被Makefile设置的场合；如果它被在环境变量或者命令行设置，你需要设置其完整路径名，就像那些在Unix中的一样。
上述DOS特定的处理效果是，一个包含 SHELL=/bin/sh 的makefile，如果你有诸如sh.exe 文件在你的PATH中，将要在MS-DOS 上正常无损地工作。
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本文转自健哥的数据花园博客园博客，原文链接：http://www.cnblogs.com/gaojian/archive/2012/09/29/2707602.html，如需转载请自行联系原作者