Ölbaum’s Delirium

A quick reference to pipes and redirections in sh.

Notes

In sh, the standard output is identified as file descriptor 1 and the diagnostic output as file descriptor 2.

Pipes

• Use the standard output of program_1 as the standard input of program_2:

  program_1 | program_2
  

• Use the standard and diagnostic outputs of program_1 as the standard input of program_2:

  program_1 2>&1 | program_2
  

Redirections

• Open file name as the standard input:

  program < name
  

• Use file name as the standard output:

  program > name
  

• Append the output to the end of file name:

  program >> name
  

• Route both the standard and diagnostic outputs to file name:

  ( program 2>&1 ) > name
  

• Append both the standard and diagnostic outputs to file name:

  ( program 2>&1 ) >> name
  

Tips

• To route the standard and diagnostic outputs to separate files, use the following syntax:

  program > outfile 2> errfile
  

• To route the diagnostic output to a file and the standard output to the terminal, use:

  program 2> errfile
  

• To retain only the diagnostic output, redirect the standard output to /dev/null:

  program > /dev/null
  program > /dev/null 2> errfile
  

• In order to test pipes and redirections, use the following Perl program:

  #!/usr/bin/perl
  
  
  print STDOUT "STDOUT: Standard output\n";
  print STDERR "STDERR: Diagnostic output\n";