Why can I not access a file named `-` even when quoting the filename? [duplicate]

Question

I have a file called -. I want to display its contents.

• One way is to do

  cat ./-
  

  since cat - reads from standard input.
• However, why are cat "-" and cat '-' also interpreted by the shell as cat -?

Answer 1

The shell removes any quotes before cat sees them. So cat - and cat "-" and cat '-' all get passed through as the array ['cat', '-'] after whitespace tokenization, wildcard expansion, and quote removal by the shell.

Answer 2

Quotes are use by the shell, and not passed to the command:

e.g.

cat "hello world" #this passes the single token `hello world`, to `cat`, as argument 1. However the quotes are not passed.
cat "-" # this passes the single token `-`, to `cat`, as argument 1. The same as if no quotes had been used.

Answer 3

GNU cat is coded to compare the given filename against the string "-" before trying to open it as a file:

  if (STREQ (infile, "-"))
    {
      have_read_stdin = true;
      input_desc = STDIN_FILENO;
      if (file_open_mode & O_BINARY)
        xset_binary_mode (STDIN_FILENO, O_BINARY);
    }
  else
    {
      input_desc = open (infile, file_open_mode);
      if (input_desc < 0)
        {
          error (0, errno, "%s", quotef (infile));
          ok = false;
          continue;
        }
    }

So, if you have a file named -, you need to defeat this logic by giving cat the path and name.

Quotation marks protect a value from white space splitting and, if single quotes, variable expansion. Quotation marks do not signal that the thing quoted is a file. To explicitly signal a value is a file, prefix it with a relative or absolute path.

All that said, one might suggest that GNU cat should also check if - is a file in the current working directory, but it'd be unusual for filenames to start with a hyphen or to be solely a hyphen, so that may be a performance concern. David Wheeler's essay, Fixing UNIX and Linux Filenames, has a nice discussion of this problem in historical context.