CLI I/O Redirection

Introduction

The Command Line Interface (CLI) provides three types of input/output (I/O) flow.

1. stdin = "Standard In;" input passed to a command
2. stdout = "Standard Out;" output from a command
3. stderr = "Standard Error;" error output from a command

This means that there are two types of output from any command stdout and stderr. It also means that you can separate them.

Abbreviations

stdio

<   = Send to

stdout

1>  = Redirect stdout, overwrite specified file
1>> = Redirect stdout, append specified file

stderr

2>  = Redirect stderr, overwrite specified file
2>> = Redirect stderr, append specified file

Examples

In this example the user is attempting to list the contents of two directories, '/' and '/dirdoesnotexist/'. The '/' directory is valid and the command will succeed to list its contents. The '/dirdoesnotexist/' directory does not exist, as its name implies, and the command will fail to list its contents. Notice in the output there is both the text for the success and the failure.

diskless241@gimpy:~$ ls / /dirdoesnotexist
ls: /dirdoesnotexist: No such file or directory
/:
bin   cdrom  etc   initrd      initrd.img.old  lib64       media  opt   root  selinux  sys  usr  vmlinuz
boot  dev    home  initrd.img  lib             lost+found  mnt    proc  sbin  srv      tmp  var  vmlinuz.old

If we redirect the stdio to /dev/null we will no longer see it on the screen. We will be left with only the stderr on the console.

diskless241@gimpy:~$ ls / /dirdoesnotexist 1> /dev/null
ls: /dirdoesnotexist: No such file or directory

Inversely, if we redirect the stderr to /dev/null we will no longer see it on the screen. We will be left with only the stdio on the console.

diskless241@gimpy:~$ ls / /dirdoesnotexist 2> /dev/null
/:
bin   cdrom  etc   initrd      initrd.img.old  lib64       media  opt   root  selinux  sys  usr  vmlinuz
boot  dev    home  initrd.img  lib             lost+found  mnt    proc  sbin  srv      tmp  var  vmlinuz.old

Instructor Notes

This class needs some quiz questions.