File Descriptors and /dev/null


In Linux everything is a file, including virtual devices like keyboards. Processes (programmes) can request access to or from these devices.

The only difference between these virtual device "files" and real files, is that for a virtual device the OS generates the data that goes into the file, instead of reading the data from storage.

/dev/null is a virtual device that looks like a file and is used to write output into a black hole that is discarded, lost forever and never seen. It isn't written to the terminal.

File Descriptors

File descriptors are integer values assigned to a file.

  • stdin has a file descriptor of 0
  • stdout has a file descriptor of 1
  • stderr has a file descriptor of 2

Two outputs are generated whenever a CLI is run stdout and stderr. By default, both these data streams are associated with the terminal. You can use file descriptors to redirect them.

If a command exits successfully, the exit status is 0.

If it exits unsuccessfully, the exit status will be something else.

If you don't specify which file descriptor you want to use, bash will use stdout by default.

The following redirects stdout away from the terminal and into /dev/null.

$ echo “Hello World” > log.txt

This will redirect stderr into a file:

$ asdfadsa 2> error.txt

If you run a command that generates lots of error messages along with "good" messages, you can redirect all the error messages (stderr) into /dev/null so that you can only see the useful stdout messages. e.g.:

$ grep -r hello /sys/ 2> /dev/null

If you want to run a command and only see the errors, (stderr) then you can filter out all the stdout by redirecting the stdout messages to /dev/null:

$ ping 1> /dev/null

Redirect all output into /dev/null if you want a command to run quietly,

Redirect all the output. The command below redirects stdout to /dev/null (the default file descriptor is 1 if it isn't specified) and then redirects file descriptor 2 into file descriptor 1.

$ grep -r hello /sys/ > /dev/null 2>&1

Read input from a file instead of the terminal


Direct stderr to append to a particular file


Combining file descriptors

2>&1 means send stderr wherever stdout is going. This means that you've combined stdout and stderr into one data stream and you can't separate them anymore. It also means you can pipe stderr the same as you can stdout.


You can redirect stdin similarly. If you run </dev/null then if the program attempt to read from stdin then it will get end-of-file.

The merge (or redirect) syntax (for example <&2) won't work, because you can only redirect in the same direction.