How To Use Standard Redirection to Input or Output Commands

In Linux and other Unix-like systems, you can use standard redirection to control the input and output of commands. Here’s a brief overview of how to use these redirections:

  1. Standard Output (stdout) Redirection: The standard output (usually the terminal) can be redirected to a file. This means that instead of seeing the output on your screen, it’ll be saved to a file.
  • Overwrite a file: command > filename This will overwrite the filename with the output of the command.
  • Append to a file:
    bash command >> filename
    This will append the output of command to filename without overwriting its current content.
  1. Standard Error (stderr) Redirection: Sometimes commands generate errors, and these errors are sent to stderr.
  • Overwrite a file: command 2> errorfile This will overwrite errorfile with the error messages from command.
  • Append to a file:
    bash command 2>> errorfile
    This will append the error messages from command to errorfile.
  1. Redirect Both stdout and stderr: If you want to redirect both standard output and standard error to the same file:
  • Overwrite a file: command > filename 2>&1
  • Append to a file: command >> filename 2>&1 The 2>&1 syntax tells the shell to redirect the standard error (file descriptor 2) to the same location as the standard output (file descriptor 1).
  1. Pipes (|): The pipe operator allows you to use the output of one command as the input of another. This is very useful for chaining commands together.
   command1 | command2

The output of command1 is used as the input to command2.

  1. Input Redirection: You can also redirect the input of a command:
   command < inputfile

This will use inputfile as the input to command.

  1. Discarding Output: If you want to discard the output (or errors) of a command, you can redirect it to the special file /dev/null:
   command > /dev/null 2>&1

This discards both standard output and standard error.

NOTE: the order of redirections matters. For example, command > filename 2>&1 is not the same as command 2>&1 > filename. The first command redirects both the standard output (stdout) and standard error (stderr) to a file named “filename”. The second command redirects the standard output to the “filename” file, while keeping the standard error in its original location, typically the terminal.

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