Question

I’m writing a program, foo, in C++. It’s typically invoked on the command line like this:

foo *.txt

My main() receives the arguments in the normal way. On many systems, argv[1] is literally *.txt, and I have to call system routines to do the wildcard expansion. On Unix systems, however, the shell expands the wildcard before invoking my program, and all of the matching filenames will be in argv.

Suppose I wanted to add a switch to foo that causes it to recurse into subdirectories.

foo -a *.txt

would process all text files in the current directory and all of its subdirectories.

I don’t see how this is done, since, by the time my program gets a chance to see the -a, then shell has already done the expansion and the user’s *.txt input is lost. Yet there are common Unix programs that work this way. How do they do it?

In Unix land, how can I control the wildcard expansion?

(Recursing through subdirectories is just one example. Ideally, I’m trying to understand the general solution to controlling the wildcard expansion.)

Answer 1

You program has no influence over the shell’s command line expansion. Which program will be called is determined after all the expansion is done, so it’s already too late to change anything about the expansion programmatically.

The user calling your program, on the other hand, has the possibility to create whatever command line he likes. Shells allow you to easily prevent wildcard expansion, usually by putting the argument in single quotes:

program -a '*.txt'

If your program is called like that it will receive two parameters -a and *.txt.

On Unix, you should just leave it to the user to manually prevent wildcard expansion if it is not desired.