clap/examples/pacman.md

pacman defines subcommands via flags.

Here, -S is a short flag subcommand:

$ pacman -S package
Installing package...

Here --sync is a long flag subcommand:

$ pacman --sync package
Installing package...

Now the short flag subcommand (-S) with a long flag:

$ pacman -S --search name
Searching for name...

And the various forms of short flags that work:

$ pacman -S -s name
Searching for name...

$ pacman -Ss name
Searching for name...

(users can "stack" short subcommands with short flags or with other short flag subcommands)

In the help, this looks like:

$ pacman -h
pacman 5.2.1
Pacman Development Team
package manager utility

USAGE:
    pacman[EXE] <SUBCOMMAND>

OPTIONS:
    -h, --help       Print help information
    -V, --version    Print version information

SUBCOMMANDS:
    help                Print this message or the help of the given subcommand(s)
    query -Q --query    Query the package database.
    sync -S --sync      Synchronize packages.

$ pacman -S -h
pacman-sync 
Synchronize packages.

USAGE:
    pacman[EXE] {sync|--sync|-S} [OPTIONS] [--] [package]...

ARGS:
    <package>...    packages

OPTIONS:
    -h, --help                  Print help information
    -i, --info                  view package information
    -s, --search <search>...    search remote repositories for matching strings

And errors:

$ pacman -S -s foo -i bar
? failed
error: The argument '--search <search>...' cannot be used with '--info'

USAGE:
    pacman[EXE] {sync|--sync|-S} --search <search>... <package>...

For more information try --help

NOTE: Keep in mind that subcommands, flags, and long flags are case sensitive: -Q and -q are different flags/subcommands. For example, you can have both -Q subcommand and -q flag, and they will be properly disambiguated. Let's make a quick program to illustrate.