Usage of dash (-) in place of a filename

Question

For a command, if using - as an argument in place of a file name will mean STDIN or STDOUT.

1. But in this example, it creates a file with the name -:

   echo hello > -
   

   How can I make - in this example mean STDOUT?

2. Conversely, how can I make - mean a file named - in examples such as:

   cat -
   

Answer 1

Using - as a filename to mean stdin/stdout is a convention that a lot of programs use. It is not a special property of the filename. The kernel does not recognise - as special so any system calls referring to - as a filename will use - literally as the filename.

With bash redirection, - is not recognised as a special filename¹, so bash will use that as the literal filename.

When cat sees the string - as a filename, it treats it as a synonym for stdin. To get around this, you need to alter the string that cat sees in such a way that it still refers to a file called -. The usual way of doing this is to prefix the filename with a path - ./-, or /home/Tim/-. This technique is also used to get around similar issues where command line options clash with filenames, so a file referred to as ./-e does not appear as the -e command line option to a program, for example.

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^{¹ Except for the >& filename operator that bash eventually copied from csh to redirect both stdout and stderr, where >&- is still about closing file descriptors like in the Bourne shell, even when - is quoted or the result of an expansion which means that >& operator can't be used for arbitrary file names. Use the standard/Bourne > "$file" 2>&1 syntax or the bash-specific &> "$file" instead.}

Answer 2

1. Instead of echo hello > -, you can use echo hello > /dev/stdout.

   While '-' is a convention that has to be implemented by each program wanting to support it, /dev/stdin, /dev/stdout and /dev/stderr are, when supported by the OS (at least Solaris, Linux and BSDs do), independent of the application and then will work as you intend.

Answer 3

As camh mentioned, - is just a naming convention used by some programs. If you want to refer to these streams with a file descriptor the shell will recognize, jiliagre was correct in having you use the name /dev/stdin or /dev/stdout instead. Those file names should work any place a normal file name would work.

1. That being said, your first example is kind of silly. Any output that would be caught by the redirect operator to write to a file is already ON standard-output, so redirecting it and writing it back to where it came from is useless. The behavior you use there is the pipe, not a redirect:

   echo hello |
   

2. In your second example you simply need to give can some indication that you want a litteral file of that name, not the internal alias it has. You can do this easiest by specifying a path to the file like this:

   cat ./-
   

Answer 4

As for 1, the program has to support it. You can't just arbitrarily use it. As for 2, redirect input from (e.g., cat < -).

Answer 5

The '-' approach has a lot of problems. First of all it requires an interpretation of the '-' character and many programs don't perform such interpretation. And furthermore, there are some programs that interpret an hyphen as a delimiter marking the end of command line options. The programs are written to work with filename arguments, the '-' approach is an hack, nice but weak.

The best way is:

$ echo hello > /dev/fd/1

/dev/stdout is a symbolik link of /dev/fd/1

Answer 6

Special characters have mostly two meanings:

ASCII numeric chart.

Scripting or symbolic.

It's possible that a single character represents a string, or act as a string. as my understanding.

in C language fopen() function takes two arguments first file stream and the second mode in which file will be open. the mode is a string. even if it's single character.

cat > "-" works.