Batch script comparing strings inside variables

batchgroup-policy

The What

I'm writing a short script to push out a new line to everyone in my domains hosts file in a GPO – don't ask why we're not doing this via DNS, the reasons are both fun and extensive 🙂

For the most part, this is simple, however I'm hitting upon a somewhat obstinate problem with comparing strings in variables, namely, it doesn't seem to do it. The strings don't seem to match, despite the fact if I echo out the strings they are identical. With the strings not matching, it just keeps adding a line to the end of the hosts file ad infinitum.

For reference, this is the process I want the script to follow:

The How

Set compare value -> Search hosts file for existing entries and save output to variable -> Compare variables if match, do nothing, else

The Code

@echo off
SET compare=10.1.1.1 test.testing.com
FOR /F "usebackq delims==`" %%A IN (`findstr /c:"10.1.1.1 test.testing.com"  %SystemRoot%\system32\drivers\etc\hosts`) do SET search=%%A
if NOT "%search%"=="%compare%" echo Do Something >> output.txt

If I change:

"%search%"=="%compare%"

To:

"sometext"=="sometext"

It works fine. If I echo the output of %search% and %compare% I get:

10.1.1.1 test.testing.com
10.1.1.1 test.testing.com

Best Answer

Yeah,

So I eventually figured this out, whitespace was my issue. A very minor change to the script resolved the problem.

@echo off
SET compare=10.1.1.1 blah.blah.com
FOR /F "usebackq delims==`" %%A IN (`findstr /c:"blah.blah.com" %SystemRoot%\system32\drivers\etc\hosts`) do SET compared=%%A
if NOT "%compare%"=="%compared%" echo 10.1.1.1 blah.blah.com>> %SystemRoot%\system32\drivers\etc\hosts
set compare=
set compared=

Related Solutions

Windows – Variables in batch file not being set when inside IF

Environment variables in batch files are expanded when a line is parsed. In the case of blocks delimited by parentheses (as your if defined) the whole block counts as a "line" or command.

This means that all occurrences of %FOO% are replaces by their values before the block is run. In your case with nothing, as the variable doesn't have a value yet.

To solve this you can enable delayed expansion:

setlocal enabledelayedexpansion

Delayed expansion causes variables delimited by exclamation marks (!) to be evaluated on execution instead of parsing which will ensure the correct behavior in your case:

if not defined BAR (
    set FOO=1
    echo Foo: !FOO!
)

help set details this too:

Finally, support for delayed environment variable expansion has been added. This support is always disabled by default, but may be enabled/disabled via the /V command line switch to CMD.EXE. See CMD /?

Delayed environment variable expansion is useful for getting around the limitations of the current expansion which happens when a line of text is read, not when it is executed. The following example demonstrates the problem with immediate variable expansion:
set VAR=before
if "%VAR%" == "before" (
    set VAR=after
    if "%VAR%" == "after" @echo If you see this, it worked
)
would never display the message, since the %VAR% in both IF statements is substituted when the first IF statement is read, since it logically includes the body of the IF, which is a compound statement. So the IF inside the compound statement is really comparing "before" with "after" which will never be equal. Similarly, the following example will not work as expected:
set LIST=
for %i in (*) do set LIST=%LIST% %i
echo %LIST%
in that it will not build up a list of files in the current directory, but instead will just set the LIST variable to the last file found. Again, this is because the %LIST% is expanded just once when the FOR statement is read, and at that time the LIST variable is empty. So the actual FOR loop we are executing is:
for %i in (*) do set LIST= %i
which just keeps setting LIST to the last file found.

Delayed environment variable expansion allows you to use a different character (the exclamation mark) to expand environment variables at execution time. If delayed variable expansion is enabled, the above examples could be written as follows to work as intended:
set VAR=before
if "%VAR%" == "before" (
    set VAR=after
    if "!VAR!" == "after" @echo If you see this, it worked
)

set LIST=
for %i in (*) do set LIST=!LIST! %i
echo %LIST%

Best Answer

Related Solutions

Windows – Variables in batch file not being set when inside IF

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