/* steve jansen */

// another day in paradise hacking code and more

Windows Batch Scripting: Functions

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Functions are de facto way to reuse code in just about any procedural coding language. While DOS lacks a bona fide function keyword, you can fake it till you make it thanks to labels and the CALL keyword.

There are a few gotchas to pay attention to:

  1. Your quasi functions need to be defined as labels at the bottom of your script.
  2. The main logic of your script must have a EXIT /B [errorcode] statement. This keeps your main logic from falling through into your functions.

Defining a function

In this example, we’ll implement a poor man’s version of the *nix tee utility to write a message to both a file and the stdout stream. We’ll use a variable global to the entire script, %log% in the function.


:: script global variables
SET me=%~n0
SET log=%TEMP%\%me%.txt

:: The "main" logic of the script
IF EXIST "%log%" DELETE /Q %log% >NUL

:: do something cool, then log it
CALL :tee "%me%: Hello, world!"

:: force execution to quit at the end of the "main" logic

:: a function to write to a log file and write to stdout
ECHO %* >> "%log%"

Calling a function

We use the CALL keyword to invoke the quasi function labelled :tee. We can pass command line arguments just like we’re calling another batch file.

We have to remember to EXIT /B keyword at the end our function. Sadly, there is no way to return anything other than an exit code.

Return values

The return value of CALL will always be the exit code of the function. Like any other invokation of an executable, the caller reads %ERRORLEVEL% to get the exit code.

You have to get creative to pass anything other than integer return codes. The function can ECHO to stdout, letting the caller decide to handle the output by pipeling the output as the input to another executable, redirecting to a file, or parsing via the FOR command.

The caller could also pass data by modifying a global variable, however, I try to avoid this approach.

<< Part 6 – Loops Part 8 – Parsing Input >>