Standard file descriptors

File Descriptor Abbreviation Description
0 STDIN Standard input, The input is read from what you type in the terminal using your keyboard. When you use the input redirect symbol (<), Linux substitutes the file referenced by the redirection for the standard input file descriptor. It reads the file and retrieves data in the same way that you would type it on a keyboard.
1 STDOUT Standard Output. The terminal monitor is the standard output on a terminal interface. All output from the shell (including programs and scripts run in the shell) is directed to the monitor's standard output. When you use the output redirect symbol (>), Linux replaces the terminal with a file referenced by the redirection symbol.
2 STDERR Standard Error. The STDERR file descriptor, by default, points to the same location as the STDOUT file descriptor (despite being assigned different file descriptor values). This means that by default, all error messages are displayed on the monitor. To redirect the errors you use a special redirection operator 2> .

<aside> 💡 Unless they are closed, file descriptors always point to a file. When bash starts, all three file descriptors, stdin (file descriptor 0), stdout (file descriptor 1), and stderr (file descriptor 2), usually point to your terminal.

You can also open and close additional file descriptors (such as 3, 4, 5,...). File descriptors can also be copied. You can also write to and read from them.

</aside>

Redirections Operators

Redirection Description
command > file Redirect the standard output (stdout) of a command to a specified file. This also the same as command 1> file , as 1 is the default file descriptor (fd) for standard output (stdout).
command 2> file Redirect the standard error (stderr) of a command to a specified file. 2 is the default file descriptor (fd) for standard error (stderr).
command >> file Redirect the standard output (stdout) of a command to a specified file without overriding the existing data.
command 2>> file Redirect the standard error (stderrt) of a command to a specified file without overriding the existing data.
command &> file Redirect both the standard output (stdout) and standard error (stderr) of a command to a file. This is also the same as command > file 2>&1 but command 2>&1 > file (the redirection order matters!)
command > /dev/null Discard standard output (stdout) of a command. The  /dev/null is a special file that discards all data written to it.
command 2> /dev/null Discard standard error (stderr) of a command.
command &> /dev/null Discard both the standard output (stdout) and the standard error (stderr) of a command. This also the same as command >/dev/null 2>&1.
command < file Redirect the contents of the file to the standard input (stdin) of a command. This also the same as command 0< file . 0 is the default file descriptor (fd) for standard input (stdin).
`command << EOL
line1
line2
EOL` Redirect a bunch of lines to the standard input (stdin) of a command using the here-document redirection operator <<MARKER . This operator instructs bash to read the input from stdin until a line containing only MARKER
is found.
`command <<- EOL
<tab>foo
<tab><tab>bar
EOL` Redirect a bunch of lines to the standard input (stdin) and strip the all leading tabs. Notice the difference (<<- )
command <<< 'string'
command <<< $word The here string is used for input redirection from a single line of text or a variable. A here string can be considered as a stripped-down form of here document
`command > file`
exec 2> file Redirect standard error of all commands to a file forever.
exec > file Redirect standard output of all commands to a file forever.
exec 3< file Create a custom file descriptor (3) for reading from a file
exec 3> file Create a custom file descriptor (3) for writing to a file
exec 3<> file Create a custom file descriptor (3) for both writing and reading a file
exec 3>&- Close the created file descriptor.
exec 4<&3 Make file descriptor 4 to be a copy of file descriptor 3. (Copy fd 3 to 4.)
exec 4>&3- Copy file descriptor 3 to 4 and close file descriptor 3.
echo "foo" >&3 Write to a custom file descriptor 3.
cat <&3 Read from a custom file descriptor 3.
exec 3<> /dev/tcp/host/port Open a TCP connection to host:port. (This is a bash feature, not Linux feature).
exec 3<> /dev/udp/host/port Open a UDP connection to host:port. (This is a bash feature, not Linux feature).
(command1; command2) > file Redirect stdout from multiple commands to a file (using a sub-shell). The parenthesis instructs bash to create a subshell and run commands from there.
{ command1; command2 } > file Redirect stdout from multiple commands to a file (This is faster as there is no need setting up a subshell environment to execute the commands). Notice the trailing and leading spaces between the curly braces and the commands.
command <(command1) Redirect stdout of command1 to an anonymous pipe fifo, then pass the fifo to command as an argument. Useful when command doesn’t read from stdin directly.
command < < (command1) Redirect stdout of command to an anonymous pipe fifo, then redirect the fifo to stdin of command. Best example: `diff <(find /path1
command <(command1) <(command2) Redirect stdout of command1 and command2 to two anonymous fifos, then pass both fifos as arguments to command.
command1 >(command2) Run command2 with its stdin connected to an anonymous fifo, and pass the filename of the pipe as an argument to command1.
command1 > > (command2) Run command2 with its stdin connected to an anonymous fifo, then redirect stdout of command1 to this anonymous pipe.
`command1 command2`
`command1 & command2`
`commnad tee file`
exec {filew}> file Open a file for writing using a named file descriptor called {filew} (bash 4.1+).
command 3>&1 1>&2 2>&3 Swap stdout and stderr of command .
cmd > >(cmd1) 2> >(cmd2) Send stdout of cmd to cmd1 and stderr of cmd to cmd2.
`cmd1 cmd2
echo ${PIPESTATUS[@]}` Find out the exit codes of all piped commands.

<aside> 💡 Read this article for an explanation of each of this redirections

The original cheatsheet for redirection operators can be found here:

</aside>