Linux I/O Redirection

Redirection with Files

Let's get started with a few simple examples of redirection:

command > filename # Redirect command's output to a new or overwritten file
command < filename # Use filename's contents as standard input for command

The first command would overwrite an existing file. So what if we want to only append command's output to a file?

command >> filename # Append command's output to filename

If you're a C++ programmer, you can remember the difference between this and the first output redirection command by recalling that C++ uses the >> operator to add to a stream, not replace it.

Speaking of C++, it uses three main streams:
stdin: 0
stdout: 1
stderr: 2

If we want to save standard error to a file, we can:

command 2> error_output

We could do the same with standard output, or even redirect standard error to standard output:

command 2>&1

And that's the basics of using I/O redirection with files. What if we want to send the output of one command to another as it's input? Linux allows us to do this using pipes.

Pipes

We can pipe the output of one command into the input of another:

first_command | second_command

We could even create a named pipe that delivers one command's output to another as input. The cool thing about named pipes is that they allow programs in different terminals and among different users to communicate. As Linux treats named pipes as ordinary files, we can even set their permissions!

mkfifo name_of_pipe # create a named pipe
command > name_of_pipe # direct output to named pipe as if it's a file
command < name_of_pipe # accept input from named pipe; can be done as different user

Now you have the basics of redirection with files and pipes! Have a nice day :)