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DOS批处理 & 脚本技术（批处理室） » Ask the expert to explain the symbols < and the command IF NOT defined!

Floor1　fbwolf	Posted 2007-09-23 16:05
初级用户　Posts 22　Credits 56
As the title. Hope experts can inform in the way of examples, and I am very grateful!

Floor2　knoppix7	Posted 2007-09-23 18:00
银牌会员　Posts 634　Credits 1,287　From cmd.exe

Floor3　HAT	Posted 2007-09-23 21:33
版主　Posts 5,017　Credits 9,023
< represents input redirection IF NOT defined means "if not defined" C:\>set a=1 C:\>if not defined a (echo yes) else (echo no) no

Floor4　ngd	Posted 2007-12-21 12:39
中级用户　Posts 108　Credits 312
Using `if not defined` can delete duplicate lines for /f "delims=" %%i in (test.txt) do ( if not defined %%i set %%i=A & echo %%i>>test2.txt)

Floor5　everest79	Posted 2007-12-22 11:01
金牌会员　Posts 1,127　Credits 2,564
Originally posted by fbwolf at 2007-9-23 04:05 PM: As the title suggests I hope that the experts will inform me in an example way, and I am very grateful! C:\>echo abcd>a.txt C:\>set /p test=<a.txt C:\>echo %test% abcd C:\> C:\>set test=a C:\>if defined test (echo Variable test exists) else (echo Variable test does not exist) Variable test exists C:\>if not defined test (echo Variable test does not exist) else (echo Variable test exists) Variable test exists C:\>set test= C:\>if defined tset (echo Variable test exists) else (echo Variable test does not exist) Variable test does not exist C:\>if not defined tset (echo Variable test does not exist) else (echo Variable test exists) Variable test does not exist C:\>

Floor6　moniuming	Posted 2007-12-26 16:22
银牌会员　Posts 574　Credits 1,335　From 广西
Finally understand some, thank you very much for the 5th floor!

Floor7　ooaf	Posted 2009-02-07 20:31
中级用户　Posts 162　Credits 313
Using `if not defined` can delete duplicate lines for /f "delims=" %%i in (test.txt) do ( if not defined %%i set %%i=A && echo %%i>>test2.txt) In the line `if not defined %%i set %%i=A & echo %%i>>test2.txt）`, the `&` should be `&&`.