How to securely echo literal string “%I”?

问题:

How can I echo the string %I in a secure way, independent on whether or not the echo command line is placed within a for %I loop?

For example, in command prompt (cmd):

>>> rem // `echo` is outside of `for` scope, so this works:

>>> echo %I
%I

>>> rem // `echo` is inside of `for` scope, so `%I` is resolved:

>>> for %I in (.) do @echo %I
.

>>> rem // escaping does not help, `%I` is still resolved:

>>> for %I in (.) do @(echo ^%I & echo %^I & echo ^%^I & echo %%I)
.
.
.
%.

And in a batch file...:

@echo off & rem // Here you need to double the `%` signs!
rem // `echo` is outside of `for` scope, so this works:
echo %%I & echo %%%%I
echo/
rem // `echo` is inside of `for` scope, so `%%I` is resolved:
for %%I in (.) do (echo %%I & echo %%%%I)
echo/
rem // escaping does not help, `%%I` is still resolved:
for %%I in (.) do (echo ^%%I & echo %%^I & echo ^%%^I & echo ^%^%I & echo ^%^%^I & echo ^%%^%%^I)

...the result is:

%I
%%I

.
%.

.
.
.
I
^I
%.

So how do I have to change the above approaches (both cmd and batch file) to get %I echoed?

回答1:

There are two ways to work around the unintended expansion of the for reference %I:

Delayed Expansion

In cmd:

>>> rem // Ensure that delayed expansion is enabled!

>>> set "STR=%I"

>>> echo !STR!
%I

>>> for %I in (.) do @echo !STR!
%I

In a batch file...:

@echo off & rem // Here you need to double the `%` signs!
setlocal EnableDelayedExpansion
set "STR=%%I"
echo !STR!
for %%I in (.) do echo !STR!

set "STR=%%%%I"
echo !STR!
for %%I in (.) do echo !STR!
endlocal

...with the result:

%I
%I
%%I
%%I

However, delayed expansion could harm under certain circumstances; so could the environment localisation done by setlocal/endlocal.

But there is still another way:

Wrap Around Another for Loop

In cmd:

>>> for %J in (%I) do @echo %J
%I

>>> for %J in (%I) do @for %I in (.) do @echo %J
%I

In a batch-file...:

@echo off & rem // Here you need to double the `%` signs!
for %%J in (%%I) do (
    echo %%J
    for %%I in (.) do echo %%J
)
for %%J in (%%%%I) do (
    echo %%J
    for %%I in (.) do echo %%J
)

...with the result:

%I
%I
%%I
%%I

Although you cannot echo %%J now any more, of course.

Wrap Around Another for Loop (edited according to dbenham's suggestion)

In cmd:

>>> for %J in (%) do @echo %JI
%I

>>> for %J in (%) do @for %I in (.) do @echo %JI
%I

In a batch-file...:

@echo off & rem // Here you need to double the `%` signs!
for %%J in (%%) do (
    echo %%JI
    for %%I in (.) do echo %%JI
)
for %%J in (%%%%) do (
    echo %%JI
    for %%I in (.) do echo %%JI
)

...with the result:

%I
%I
%%I
%%I

This time even %%J could be echoed.