It is very common for command line applications to just wait for input if no arguments are given.

For example: cat, grep, wc, sed, awk, sed and many others. Try these without a file argument and they will just hang there and wait for input. Use Ctrl-D to give eof

None of these applications warn if there is no standard input as that is the normal way they work and it is what make them easy to use in pipes.

Other command line applications have a flag or use dash - instead of the filename to specify that you want them to read from standard in. For example: xmllint, (I know there are many more but can’t think of one now). This way you can show them a help text indicating that they need to provide a filename or - for stdin if they haven’t provided any arguments.

It is also common to write things to standard out, or at least give an option to write to standard out instead of a file. For example wget allows you to specify and outfile with -O filename but the option to write to standard out with -O -. Funnily enough curl works the other way around where the default is to write to standard out but you can save to a file with -o filename.

As I almost exclusively work with command line applications I think a well behaved command line app should

• Support reading and writing from stdin/stdout (if it makes sense for the application). I don’t mind if I have to provide a flag for this
• Write errors and warnings to stderr
• Return correct error codes 0 for success. non zero for failure.
• Have a man page with good descriptions and examples. This is getting less common
• Have a short hand help through -h or --help. I like this to be short and fit on a page. The rest is for man.
• Have a non-interactive flag if normally interactive (so that things can get automated)
• Likely more which I haven’t thought about

So perhaps your cli app can do this (just an example).

$ ./wordcount
Usage: wordcount OPTION... [FILE]...
Counts words in each FILE and print to standard output

When FILE is -, read standard input.

OPTIONS are:

  -i, --ignore=REGEX  Ignore words matching REGEX

$ ./wordcount --help
Usage: wordcount OPTION... [FILE]...
Counts words in each FILE and print to standard output

When FILE is -, read standard input.

OPTIONS are:

  -i, --ignore=REGEX  Ignore words matching REGEX

$ ./wordcount hello.txt
32

$ cat hello.txt | ./wordcount -
32

$ ./wordcount hello.txt world.txt
132