3

I know from call /?, the ~ in variable (e.g. %~d1) is used to parse a part of file-path (driver here), but the tilde is used in another context here: https://www.tutorialspoint.com/batch_script/batch_script_string_length.htm:

@echo off
set str = Hello World
call :strLen str strlen
echo String is %strlen% characters long
exit /b

:strLen
setlocal enabledelayedexpansion

:strLen_Loop
   if not "!%1:~%len%!"=="" set /A len+=1 & goto :strLen_Loop
(endlocal & set %2=%len%)
goto :eof

Here, what is the meaning of this variable expansion: "!%1:~%len%!"? And how does it calculate the length of string via comparing it to empty string? What is the purpose of tilde here? Moreover, this example will get me into infinite loop where the output is if not "!str:~136!" == "" set /A len+=1 & goto :strLen_Loop (where the number !str:~n grows).

  • In this context, it is used to perform substring modification of a variable using an offset from the first character in the variables content, as explained in the help output of Set /?. – T3RR0R Jul 16 at 16:40
3

The tilde (~) sign is used in different ways in batch files:

  • Argument quote removal. A tilde sign before an command-line argument (such as "%~1") indicates to remove the surrounding quotes from the parameter. Such if the value for %1 is "Hi" then %~1 will expand to only Hi.

  • Substring processing. There is a way to extract substring from a string variable in batch file. The syntax is: %variable:~num_chars_skip,num_chars_keep%. Num chars skip means the point where to start in the string, or to exclude how much characters preceding the string variable. And num chars to keep indicates the number of characters after the start point. Num chars to keep is optional, but first is mandatory. If num chars to keep is not specified, only that num chars to keep'th character will be parsed.

You should read these in command prompt for more help:

  • call /?
  • for /?
  • set /?

Hope that helps

| improve this answer | |
  • You can use a tilde with FOR metavariables, e.g. if %%A is "egg" then %%~A will be egg. – Michael Harvey Jul 16 at 19:23
  • You can remove quotes from %var% variables FOR %%A in (%quotedv%) do set unquotedv=%%~A – Michael Harvey Jul 16 at 20:27
  • OK, but why does the batch from tutorial causes me an infinite loop? Should not stop the strLen_Loop once the %1 input arg is empty? Why does it still process (in my example 136, and more)? – Herdsman Jul 16 at 21:52
  • Thanks @MichaelHarvey for information. – Wasif Hasan Jul 17 at 0:29
  • @Herdsman the loop is not infinite. Look at the If statement, whenever it matches, then the loop is exited. – Wasif Hasan Jul 17 at 0:30
0
  • For "fix" this tutorial code and prevent your loop infinite, quoted in the commentary:

I see, but yet, in my machine I need to ^C to terminate it. Just look how big number I get - 136 - even though there is no length or word with 136 characters - @Herdsman

@echo off && setlocal enabledelayedexpansion

set "_str=Hello World" & call :strLen_Loop str

:strLen_Loop
if "%~1" == "" ( 
      endlocal & goto :EOF
    ) else if not "!_%~1:~%_len%!" == "" (
      set /A "_len+=1" & goto=:strLen_Loop
    ) else echo\String is !_len! characters long & exit /b
| improve this answer | |

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