Understanding standard input, standard output, and standard error

Once a command begins running, it has access to three files:

  1. It reads from its standard input file. By default, standard input is the keyboard.
  2. It writes to its standard output file.
    • If you invoke a shell command from the shell, a C program, or a REXX program invoked from TSO READY, standard output is directed to your terminal screen by default.
    • If you invoke a shell command, REXX program, or C program from the ISPF shell, standard output cannot be directed to your terminal screen. You can specify a z/OS UNIX file or use the default, a temporary file.
  3. It writes error messages to its standard error file.
    • If you invoke a shell command from the shell or from a C program or from a REXX program invoked from TSO READY, standard error is directed to your terminal screen by default.
    • If you invoke a shell command, REXX program, or C program from the ISPF shell, standard error cannot be directed to your terminal screen. You can specify a z/OS UNIX file or use the default, a temporary file.

      If the standard output or standard error file contains any data when the command completes, the file is displayed for you to browse.

Using the shell: In the shell, the names for these files are:
  • stdin for the standard input file.
  • stdout for the standard output file.
  • stderr for the standard error file.
The shell sometimes refers to these files by their file descriptors, or identifiers:
  • 0 for stdin
  • 1 for stdout
  • 2 for stderr
For more information about the file descriptors that the shell supports, see the sh command description in z/OS UNIX System Services Command Reference.
Using TSO/E: When you are invoking the BPXBATCH utility, you can specify these standard files in MVS™ DD statements, TSO/E ALLOCATE commands, or DYNALLOC macros using the ddnames:
  • STDIN for standard input
  • STDOUT for standard output
  • STDERR for standard error

For more information about BPXBATCH, see The BPXBATCH utility.

Using ISPF: When you run shell commands, REXX programs, and C programs from the ISPF shell, stdout, and stderr cannot be directed to your terminal. You can specify a z/OS UNIX file, or use the default—a temporary file. If it has any contents, the file is displayed for you to browse when the command or program completes.

Parent topic: Working with z/OS shell commands