Windows – Batch File, PING, errorlevel and > null what do they do

batchbatch filepingwindows 7

This seems like a really easy question (running Windows 7). But I have looked for the answer and couldn't find anything at Microsoft's sites (XP PING & Everything else PING) or from a bunch of Google searches.

I am learning DOS/Batch file. My beginning code is to PING a server.

If I enter this in the command prompt it runs fine, and gives me some results

PING -n 1 Server04

Results:

Pinging Server04.aaaa.xxxx.net [x.x.x.x] with 32 bytes of data:     
Reply from x.x.x.x: bytes=32 time=1ms TTL=125

If I run this in a batch file, it never stops, but the -n (or /n) should only send the PING once, and the PAUSE should also stop and hold the cmd.exe window open.

PING -n 1 winsvmnsdev04
PAUSE

Results: 

C:\Users\Me\Desktop>PING -n 1 Server04
C:\Users\Me\Desktop>PING -n 1 Server04
C:\Users\Me\Desktop>PING -n 1 Server04
Continues until I use Ctrl + C

I have looked at a bunch of Q&A where many include 'errorlevel' to echo the response and some that have a trailing '> null'. In trying to understand, I have looked and neither of these are listed as attributes of 'PING' in the Microsoft Libraries.

When I run 

PING -n 1 winsvmnsdev04 > null
PAUSE

Or run

PING -n 1 winsvmnsdev04 > null

I get one line 

C:\Users\Me\Desktop>PING -n 1 Server04  1>null

I have no idea why. Can someone explain these two attributes of PING, or point towards the official documents? /n and -n don't not limit to one PING, the >null and the PAUSE don't work when the PING is running wild. None of these line up with what the Microsoft Documentation is describing.

Note: errorlevel is not listed as Command-Line Reference by Microsoft

Best Answer

I suspect that you've named your batch file ping.bat, which as explained here would be the source of your problem.

If this indeed is the case, what's happening is that ping isn't actually running the ping command, but ping.bat with those arguments - sending you into an infinite recursion that doesn't end until you break it.

That's why you see the command constantly iterating in Command Prompt without any output when you run the batch file, because it's not actually executing the ping command, but calling on itself with those very arguments, which then calls itself, which - well, you get the picture.

The > null part is a redirection; the > operator means that the program's output will be written to the following file rather than the console. The special file name nul (not null) is handled by Windows internally; anything you write there is simply discarded. This way a script can run a program and just check if it succeeded, without cluttering the console. (Similarly, < would redirect input from file.)

1> is the full form of > and it means "redirect the first file descriptor".

  • On Unix-like operating systems, a file descriptor is a number used by programs to reference files that the program currently holds open. By convention, file descriptors 0, 1, 2 correspond to "standard input" (stdin), "standard output" (stdout), and "standard error output" (stderr); all higher numbers correspond to files that the program itself has opened.

  • Windows differs from this in many ways – it uses "file handles" and doesn't number them 0-1-2-3... are just a few small differences. However, the cmd.exe script interpreter still understands 1> and 2> to mean redirection of regular output and error output specifically, to make things just a little bit easier for those coming from Linux or other Unix-likes.

As for the errorlevel, it is not a command, but a mode in the if command. Each program, upon exiting, returns a number called an "exit status" to the system (and to the parent program); by convention, 0 means success, and ≥1 means some sort of failure.

  • In cmd.exe, you can access the last command's exit status through the special %errorlevel% variable, for example:

    if %errorlevel% NEQ 0 echo The previous command failed!

  • There is another syntax, coming from old Windows and MS-DOS versions that did not have the special variable; it checks if the exit status is equal to or greather than a given number:

    if errorlevel 1 echo The previous command failed!
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