1. stdin – 0 – Standard Input (usually keyboard or file)
2. stdout – 1 – Standard Output (usually screen)
3. stderr – 2 – Standard Error (usually screen)

What is a null (/dev/null) file in a Linux or Unix-like systems?

/dev/null is nothing but a special file that discards all data written to it. The length of the null device is always zero. In this example, first, send output of date command to the screen and later to the /dev/null i.e. discards date command output:

### Show on screen ###
date
 
### Discards date command output ###
date > /dev/null

Syntax: Standard Error (stderr -2 no) to a file or /dev/null

The syntax is as follows:

command 2>/dev/null
command arg1 arg2 2>/dev/null
date bar 2>/dev/null
ls -foo 2>/dev/null

In this example, send output of find command to /dev/null:
$ find /etc -type f -name '*' 2>/dev/null
The following example will cause the stderr ouput of a program to be written to a file called errors.txt:
$ find /etc/ -type f -name "*" 2> errors.txt

Linux and Unix redirect all output and error to file

The syntax is:

## send command output to output.txt and error message to error.txt ##
command > output.txt 2> error.txt
command  -arg1 -arg2 > output.txt 2> error.txt

If you want both stderr and stdout in same file, try:

command > log.txt 2>&1

Use cat command to display log.txt on screen:
cat log.txt

See man pages for more information – bash(1),ksh(1).