Walking Randomly

From time to time I find myself having to write or modify windows batch files.  Sometimes it might be better to use PowerShell, VBScript or Python but other times a simple batch script will do fine.  As I’ve been writing these scripts, I’ve kept notes on how to do things for future reference.  Here’s a summary of these notes.  They were written for my own personal use and I put them here for my own convenience but if you find them useful, or have any comments or corrections, that’s great.

These notes were made for Windows 7 and may contain mistakes, please let me know if you spot any.  If you use any of the information here, we agree that its not my fault if you break your Windows installation.  No warranty and all that.

These notes are not meant to be a tutorial.

Comments

Comments in windows batch files start with REM. Some people use :: which is technically a label. Apparently using :: can result in faster script execution (See here and here). I’ve never checked.

REM This is a comment
:: This is a comment too...but different. Might be faster.

If statements

If "%foo%"=="bar" (
REM Do stuff
REM Do more stuff
)
else (
REM Do different stuff
)

Check for existence of file

if exist {insert file name here} (
    rem file exists
) else (
    rem file doesn't exist
)

Or on a single line (if only a single action needs to occur):

if exist {insert file name here} {action}

for example, the following opens notepad for autoexec.bat, if the file exists:

if exist c:\autoexec.bat notepad c:\autoexec.bat

Echoing and piping output
To get a newline in echoed output, chain commands together with &&

echo hello && echo world

gives

hello
world

To pipe output to a file use > or >> The construct 2>&1 ensures that you get both standard output and standard error in your file

REM > Clobbers log.txt, overwriting anything you already have in it
"C:\SomeProgram.exe" > C:\log.txt 2>&1

REM >> concatenates output of SomeProgram.exe to log.txt
"C:\SomeProgram.exe" >> C:\log.txt 2>&1

Environment Variables

set and setx

• set – sets variable immediately in the current context only.  So, variable will be lost when you close cmd.exe.
• setx – sets variable permanently but won’t be valid until you start a new context (i.e. open a new cmd.exe)

List all environment variables defined in current session using the set command

set

To check if the environment variable FOO is defined

if defined FOO (
 echo "FOO is defined and is set to %FOO%"
)

To permanently set the system windows environment variable FOO, use setx /m

setx FOO /m "someValue"

To permanently unset the windows environment variable FOO, set it to an empty value

setx FOO ""

A reboot may be necessary. Strictly speaking this does not remove the variable since it will still be in the registry and will still be visible from Control Panel->System->Advanced System Settings->Environment variables. However, the variable will not be listed when you perform a set command and defined FOO will return false.  To remove all trace of the variable, delete it from the registry.

Environment variables in the registry

On Windows 7:

•  System environment variables are at HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Environment
•  User environment variables are at HKEY_CURRENT_USER\Environment

If you change environment variables using the registry, you will need to reboot for them to take effect.

Pausing
This command will pause for 10 seconds

TIMEOUT /T 10

Killing an application

This command will kill the notepad.exe window with the title Readme.txt

taskkill /f /im notepad.exe /fi "WINDOWTITLE eq Readme.txt"

Time stamping

The variable %time% expands to the current time which leads you to do something like the following to create time stamps between the execution of commands.

echo %time%
timeout /t 1
echo %time%

This works fine unless your code is in a block (i.e. surrounded by brackets), as it might be if it is part of an if-statement for example:

(
echo %time%
timeout /t 1
echo %time%
)

If you do this, the echoed time will be identical in both cases because the %time% entries get parsed at the beginning of the code block. This is almost certainly not what you want.

Setlocal EnableDelayedExpansion
(
echo !time!
timeout /t 1
echo !time!
)

Now we get the type of behaviour we expect.

Where is this script?

Sometimes your script will need to know where it is.  Say test.bat is at C:\Users\mike\Desktop\test.bat and contains the following

set whereAmI=%~dp0

When you run test.bat, the variable whereAmI will contain C:\Users\mike\Desktop\

Details on %dp0 can be found at StackOverflow.

Variable substrings
This came from StackOverflow’s Hidden features of Windows batch files which is a great resource.  They’ve tightened up on what constitutes a ‘valid question’ these days and so great Q+A such as this won’t be appearing there in future.

> set str=0123456789
> echo %str:~0,5%
01234
> echo %str:~-5,5%
56789
> echo %str:~3,-3%
3456