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Saturday, December 27, 2008

Command-line automation – set

This is the fourth in a series of posts containing information on what I consider the building blocks to automate repetitive tasks at the Windows command-line. These components are the for, find, findstr, set, if and echo commands, control files used to control data input, combined with errorlevels, command concatenation, nested loops and if/then/else constructs.

Described in this post is the ‘set’ command, which I find very useful when automating repetitive tasks with ‘for’. I use set in four ways – string manipulation, simple arithmetic operations with ‘set /a’, gathering input with ‘set /p’ or concatenating output in a single for loop with delayed variable expansion.

String Manipulation

My most frequent use of the set command is simple string manipulation at the command prompt. I use this to replace one character with another or extract a substring.

Replace

Replacing one string with another is a core function of string manipulation. Even though this is included under help for ‘set’, this functionality works for other commands, including the echo command.

For example, say you have the fully qualified user DNS name – my.domain.com – and this conveniently is also the distinguished name of your AD domain, the following command replaces periods with ‘,DC=’ and prefixes an additional string to provide the distinguished name:
echo DC=%userdnsdomain:.=,DC=%

This would turn ‘my.domain.com’ into:
DC=my,DC=domain,DC=com

Wildcards can also be used, for example, to remove the first element, you could replace up to the first dot with nothing:
echo %userdnsdomain:*.=%

You could also remove spaces and print the output, for example, if the ‘test’ variable contained ‘A Sentence With Spaces’, running:
echo %test: =%

Would result in:
ASentenceWithSpaces

Substrings

You can extract from existing variables (or inline if using delayed expansion), providing Left/Right/Mid functionality. This can be useful in many ways, but for most of the input I deal with, splitting with ‘for’ and delimiters (such as comma or space) makes for simpler processing with dynamic input. For examples sake, if you had a defined length of padded text, and you wanted the 10 characters starting at the 20th character (0-based), you could run:
echo %input:~19,10%

You could also extract the last 10 characters:
echo %input:~-10%

Or all but the last 10 characters:
echo %input:~0,-10%

Arithmetic operations

‘set /a’ supports a relatively simple but still powerful set of arithmetic, bitwise and shift operators, and while I’ve never found much use for them, upon occasion they have come in handy.

For example, this command will tell you what next year will be and set the NextYear environment variable, based on the current year + 1:
for /f "tokens=3 delims=/" %i in ('echo %date%') do set /a NextYear=%i+1

Similarly, find the current month, increment by one, unless the current month is 12, then wrap around and start at 1 again:
for /f "tokens=2 delims=/" %i in ('echo %date%') do if %i==12 (Set NextMonth=1 & echo !NextMonth!) else (Set /a NextMonth=%i+1)

Note that the example above uses delayed environment variable expansion, see below for a description.

Incidentally, if you were going to use this output for something and if the month were a single digit you would lose the padding, which would then be incorrectly sorted (as compared to other padded output). The example below pads the month, then extracts the last two characters, effectively padding single-digit months (see string manipulation above):
set NextMonth=0%NextMonth%
echo %nextmonth:~-2%


An actual use I have found for the arithmetic operators is to calculate subnets from the command-line. I have made use of this when parsing netsh dhcp output (there were thousands of leases across several DHCP servers serving dozens of subnets, and I wanted to know from the leases which subnets were in use).

This can be done with a single command-line, but it’s a little complicated, the batch file below may be easier to demonstrate the ‘set /a’ functionality. Running this batch file with an IP and mask, eg ‘findsubnet.bat 192.168.11.40 255.255.254.0’ will return the subnet (192.168.10.0 in this example) using bitwise AND of the two sets of octets:



@Echo Off
Set Input=%1
Set Mask=%2
If /i "%Input%"=="" Goto End

for /f "tokens=1-8 delims=.- " %%i in ('echo %Input% %Mask%') do Call :FindOctet %%i %%j %%k %%l %%m %%n %%o %%p
Goto End

:FindOctet
::Echo %1 %2 %3 %4 %5 %6 %7 %8

set /a Octet1="%1 & %5" >nul
set /a Octet2="%2 & %6" >nul
set /a Octet3="%3 & %7" >nul
set /a Octet4="%4 & %8" >nul
Echo %1.%2.%3.%4,%Octet1%.%Octet2%.%Octet3%.%Octet4%,%5.%6.%7.%8
Set Octet1=
Set Octet2=
Set Octet3=
Set Octet4=
Goto End

:InvalidArgs
Echo Invalid arguments, please provide an IP and longhand mask, eg 192.168.11.40 255.255.254.0
Goto End

:End



Input

Set can be used to gather input, making your commands or batch files interactive. For example, the following command assigns the input to the ‘test’ environment variable, after asking the question:
set /p test=Tell me something?

I don’t often use this, as it somewhat defeats the purpose of automation – I prefer to have all the required input in a control file, which separates the intended action from the actual operator.

Set with Delayed expansion

Delayed environment variable expansion provides a method of executing commands and storing the output for use by other commands within a loop – either in different iterations or nested commands. This is required when looping through ‘for’ commands or nesting commands, otherwise state between each command is not maintained.

For example, without delayed expansion, the following would not work:
set test=test variable & echo %test%

However, using delayed environment variable expansion, this problem can be easily overcome, using the !variable! syntax, instead of the normal %variable%. Note that you may need to enable delayed environment variable expansion, which can be done using ‘cmd /v:on’ when starting the command prompt, or running ‘setlocal ENABLEDELAYEDEXPANSION’ within an existing shell.

With delayed expansion, the first command would set the variable and the second echo the newly set variable:
set test=test variable & echo !test!

Or to re-use an example above, calculate next year and then do something with that information (echo in this case):
for /f "tokens=3 delims=/" %i in ('echo %date%') do set /a NextYear=%i+1 1>nul & echo Next year is !NextYear!

Unless there is already a variable of the same name, running the commands above using the percent signed variables will result in the variable name being printed, as the environment hasn’t yet been changed by the first command.

Note that while testing, it seems the Vista command shell has been updated and as long as delayed expansion is enabled, this command would work:
set test=test variable & echo %test%

The following command is a one-line version of the batch above, using delayed variable expansion to calculate the variables as each command is executed:
for /f "tokens=1-8 delims=.- " %i in ('echo 192.168.11.40 255.255.254.0') do set /a Octet1="%i & %m" >nul & set /a Octet2="%j & %n" >nul & set /a Octet3="%k & %o" >nul & set /a Octet4="%l & %p" >nul & Echo %i.%j.%k.%l,!Octet1!.!Octet2!.!Octet3!.!Octet4!,%m.%n.%o.%p

An easier example to follow, set the environment variable called ‘number’ to a random number (using %random% - very useful), echo the newly set variable, and then use ‘set /a’ to increment the number by one:
set number=%random% & echo !number! & set /a number=!number!+1

If you are using nested ‘for’ commands, you can just reference variables from an earlier command, but in some circumstances delayed expansion is the only possibility. I have had to use this syntax when using ‘set /a’ and using the replace/substring functionality, as this requires a full variable (eg. %path%, not a temporary %i type variable.

This was a basic overview of ‘set’ and some of the ways in which I use this command when automating tasks at the command-line.

For more real-world examples of how I use ‘set’, see my command-line operations:
http://waynes-world-it.blogspot.com/2008/09/useful-ntfs-and-security-command-line.html
http://waynes-world-it.blogspot.com/2008/09/useful-windows-printer-command-line.html
http://waynes-world-it.blogspot.com/2008/09/useful-windows-mscs-cluster-command.html
http://waynes-world-it.blogspot.com/2008/09/useful-vmware-esx-and-vc-command-line.html
http://waynes-world-it.blogspot.com/2008/09/useful-general-command-line-operations.html
http://waynes-world-it.blogspot.com/2008/09/useful-dns-dhcp-and-wins-command-line.html
http://waynes-world-it.blogspot.com/2008/09/useful-active-directory-command-line.html


Wayne's World of IT (WWoIT), Copyright 2008 Wayne Martin.

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About Me

I’ve worked in IT for over 13 years, and I know just about enough to realise that I don’t know very much.