Updating a command line parameter in a batch file

Question

Is it possible to update or replace a command line parameter (like %1) inside of a batch file?

Sample code:

rem test.cmd
@echo off
echo Before %1
IF "%1" == "123" (
    set %%1 = "12345678"
)
echo After %1

Desired Result:

C:/>Test 123
Before 123
After 12345678

Actual Result:

C:/>Test 123
Before 123
After 123

Answer 1

No. What you are trying is not possible.

Can be simulated passing original batch parameters to subrutine, or call the same cmd recursively with modified parameters, which again get %1, %2, ... the parameters provided in the call. But this is not what you ask.

rem test.cmd
@echo off
echo Before %1

if "%~1"=="123" (
    call :test %1234
) else (
    call :test %1
)

goto :EOF

:test

echo After %1

Answer 2

Argument variables are reserved, protected variables, you can't modify the content of one of those variables by yourself.

I suggest you to store the argument in a local variable then you can do all operations you want:

@echo off

Set "FirstArg=%~1"

Echo: Before %FirstArg%

IF "%FirstArg%" EQU "123" (
    Set "FirstArg=12345678"
)

Echo: After %FirstArg%

Pause&Exit