Question

I see the - operator behaving in different ways with different commands.

For example,

cd - 

cds to the previous directory, whereas,

vim -

reads from stdin

So I want to know why the - operator is behaving in 2 different ways here. Can someone point me to some detailed documentation of the - operator?

Answer 1

It is not an operator, it is an argument. When you write a program in C or C++ it comes as argv[1] (when it is the first argument) and you can do whatever you like with it.

By convention, many programs use - as a placeholder for stdin where an input file name is normally required, and stdout where an output file name is expected. But cd does not require reading a file stream, why should it need stdin or stdout?

Extra: here below is the excerpt from vim’s main.c that parses arguments that begin with -: if there is no additional character it activates STDIN input.

    else if (argv[0][0] == '-' && !had_minmin)
    {
        want_argument = FALSE;
        c = argv[0][argv_idx++];
#ifdef VMS
        ...
#endif
        switch (c)
        {
        case NUL:       /* "vim -"  read from stdin */
                /* "ex -" silent mode */
        if (exmode_active)
            silent_mode = TRUE;
        else
        {
            if (parmp->edit_type != EDIT_NONE)
            mainerr(ME_TOO_MANY_ARGS, (char_u *)argv[0]);
            parmp->edit_type = EDIT_STDIN;
            read_cmd_fd = 2;    /* read from stderr instead of stdin */
        }