UNIX for Dummies Questions & Answers

Hi all and tks in advance

I am working my way through 'Learning the bash Shell'. I cannot be sure if I really understand why "echo *" (my double quotes) returns all the files in the current directory.

I get the * ( wildcard) expansion of "*", and I think I understand that "echo" returns it's argument/s to the standard output. But, somehow, I do not see **why** the wildcard * would return all the current working directory files.


Thanks again.

echo means - display the characters in the argument(s) the arguments in this case are filenames.

what were you expecting?

echo doesn't return them, it prints them. Minor but important difference. Programs return a number that you don't see but can check for to see if the program succeeded or failed... 0=success, anything else means error.

Shell expansion (often called globbing) happens before the program is run, the same way it substitutes variables. This means you can drop a * almost anywhere you like and it will become a list of files -- not just into shell commands but also shell builtins, lists, arrays, and operators... To make * not expand you have to put it in quotes like "*" or '*'

So when you do "echo *", the * becomes a list of files before echo is run at all, and echo doesn't have to do anything special to do what you'd expect it to. Do an echo "*" though and the shell will not substitute for *, and echo just prints it unchanged.

Yes...thanks for the semantics...as you say, a perhaps trivial but important distinction.

What I was missing was the fact that the '*' ( unquoted) expands to a list of files in the current directory. It had been mentioned up to now as a "simple" wild card, without the implication you mention. So ls * would match any string in the current directory. But, using * on it's own is what I was missing.

thank you.

It also means that in places you might think of using ls * or echo * you can rip out ls or echo and use * by itself. You can use it almost anywhere a list of filenames makes sense, and you can also use it in places that make no sense if you're not careful. The shell's flexible enough to understand its own substitutions like that.

For additional detail, check the bash man page sections on special characters and expansions. The technical term is "globbing", and is also referred to in nearly any discussion of "regular expressions".