Learning to write Batch Scripts

jclast

So I'm trying to learn to write batch scripts for work, and this for loop is screwing me up.
First, here are the contents of "file_test.txt":

1 2 3
4 5 6

Second, here are the contents of "test.bat":

@echo off
SetLocal EnableDelayedExpansion
FOR /F "delims=" %%a IN (test_file.txt) DO (
   echo a: %%a
   set b = %a%
   echo b: !b!
)

And this is the results I get:

a: 1 2 3
b: 
a: 4 5 6
b:

What on Earth am I doing wrong?

EDIT: I suppose I should say, this is the results I want:

a: 1 2 3
b: 1 2 3
a: 4 5 6
b: 4 5 6

ArbitraryDescriptor

It's been a while, but I don't think SET works in a DO.

You can call a subroutine to similar effect:

FOR /F "delims="  %%a IN (test_file.txt) DO ( call :foo %%a)
GOTO :eof

:foo
 echo a: %1, %2, %3
 set b=%1, %2, %3
 echo b: %b%

Drawback there is the subroutine interprets %%a as 3 items (delimited by " "), so you need to know how many args are coming in. Probably a way around that, but like I said: I'm rusty.

However. If you're just starting, could I suggest you check out Python instead? The syntax isn't much more complicated, and it's a lot more flexible. I used to be all about a nice bat file, but I've moved on and haven't looked back.

full text of a .py file that does the same thing you're trying to do:

file = open("test_file.txt","r").read()
for line in file.split("\n"):
    print("a: "+line)
    b = "b: "+line
    print(b)

the above is Python 3.2

2.x is a bit different (incompatible), but still widely used. Whichever you choose, if you google up some code and aren't sure which version it is (not usually a problem), check if the print functions use parenthesis:

print("3.x")
print "2.x"

ArbitraryDescriptor

Tested it. Variables are SET after the FOR/DO is complete. That sounds familiar.

@echo off

set b=Nostalgia!
echo Before Loop 1, b = %b%
FOR %%a IN (one) DO (
	set b=You haven't changed at all!  That's... cool
	echo In Loop 1, b = %b%
)
echo After Loop 1, b = %b%
FOR %%a IN (one) DO ( call :foo %%a)
echo After Loop 2, b = %b%

echo (Spoiler: It was you.)
GOTO :eof

:foo
 set b=Look batch, it's not you, it's me.  I'm going back to python
 echo In Loop 2, b = %b%

Output:

Before Loop 1, b = Nostalgia!
In Loop 1, b = Nostalgia!
After Loop 1, b = You haven't changed at all! That's... cool
In Loop 2, b = Look batch, it's not you, it's me. I'm going back to python
After Loop 2, b = Look batch, it's not you, it's me. I'm going back to python
(Spoiler: It was you.)

Chake99

I'm going to be entirely unhelpful and say if you have any option you might want to consider using perl, python or bash instead of batch.

Of course if it's for work you quite possibly can't. It's not like I use tcsh everyday because it's what I would freely choose.

jclast

I may well have the option and already know perl and have done a little python in the past. I think I'll present that today. Thanks, guys!

Aioua

Also, if you want to stick strictly vanilla windows you should look into powershell. Most of the command line stuff seems to be getting deprecated anyway. Powershell's the new hotness. Also is pretty good at simple tasks like this:

Get-Content .\test.txt | ForEach-Object {
    Write-Output "a: $_"
    Write-Output "b: $_"
}

And condensed!

gc .\test.txt|%{echo "a: $_";echo "b: $_"}

Sir Headless VII

fyi what is not working is you don't have the set command properly formatted. It needs to be

set b=%%a

with no spaces and with %%a instead of %a%.

@echo off
SetLocal EnableDelayedExpansion
FOR /F "delims=" %%a IN (test_file.txt) DO (
   echo a: %%a
   set b=%%a
   echo b: !b!
)

works for me.

jclast

Weird. What determines when you need %a% or %%a?

ArbitraryDescriptor

For loops. (though there may be other cases I'm not aware of)

@echo off
SETLOCAL ENABLEDELAYEDEXPANSION

set a=MainA

FOR %%a IN (LoopA) DO (
	set b=%a%
	echo !b!
	set b=%%a
	echo !b!
)
pause

The 'a' you're using in your loop is not the same variable as %a%.

Sir Headless VII wrote: »

works for me.

Oops. I've never used (or seen) '!b!' before, or used delayed expansion. So I guess that's what it does!

jclast

Thanks very much for the help. What I'm really coming away from this with, though, is that I should convince my boss that perl would be better. I know that already and it'snot reliant on whitespace (or lack thereof).

ArbitraryDescriptor

True.

But it's worth mentioning that the %%a variables in FOR loops actually hold more information than you might think.

@echo off

for /r %%a in (*.*) do (
	echo FQ Path: %%~fa
	echo Filename: %%~na
	echo Extension: %%~xa
	echo Path: %%~pa
	echo Size: %%~za bytes
	echo -------
        set b=%%a
)
echo b = This is just a simple string: %b%

run 'for /?' for a full list if you get shut down on using an alternative to batch.

Rend

jclast wrote: »

Thanks very much for the help. What I'm really coming away from this with, though, is that I should convince my boss that perl python would be better. I know that already and it'snot reliant on whitespace (or lack thereof).

Go for python over perl if you can manage it. It's like a billion times more readable!

jclast

Perl is perfectly readable if you comment and write it like a sane person who learned C first (i.e., me).

Rend

A professor of mine (who is quite proficient in perl) once joked that perl is commonly known as a "write-only" language.

Not that you should make your decision based on a programming joke, but if you haven't checked out python you definitely should at least take a look at it! Readable as your perl may be, python is quite pretty.

jclast

I've done a little python, and it is super-readable. White space (or lack of it) being important bothers me though. I'm sure I just cling to perl because I already know it well. It baffles me that it forces indentation so hard that it got rid of braces altogether.

Rend

jclast wrote: »

I've done a little python, and it is super-readable. White space (or lack of it) being important bothers me though. I'm sure I just cling to perl because I already know it well. It baffles me that it forces indentation so hard that it got rid of braces altogether.

The reason it does that is to actively fight against obfuscated code. It does feel weird not to use braces, especially for a person who primarily uses c-like languages. We share that quality. You just have to think of it this way:

In every single language, whether you can write a whole app on one line or not, white space is the most important factor there is when readability is concerned. It is one of the only things that just about every programmer will do exactly the same (length of indentation notwithstanding, of course), and the reason is because everyone expects everyone else to have identical indentation.

In this way, python is not enforcing whitespace, style, or convention, it is enforcing readability.