Redirection and Process Substitution

Both the input and output of a command can be redirected:

 command  >  filename    Redirect command output (stdout) into a file

 command  >  /dev/null   Discard stdout of command

 command  2> filename    Redirect error output (stderr) to a file

 command  2>&1 filename  Redirect stderr to stdout 

 command  1>&2 filename  Redirect stdout to stderr

 command  >> filename    Redirect command output and APPEND into a file

 command  <  filename    Redirect a file into a command

 commandA  < (commandB)  Redirect the output of commandB as file input to commandA

 commandA | tee filename | commandB    Redirect commandA into filename AND commandB

 commandA | commandB     Redirect stdout of commandA to commandB

 commandA |& commandB    Redirect stdERR of commandA to commandB

 commandA & commandB     Run commandA and then run commandB (asynchronous).

 commandA ; commandB     Run commandA and afterwards run commandB (synchronous)

 commandA && commandB    Run commandB only if commandA is successful  (synchronous AND)

 commandA || commandB    Run commandB only if commandA is NOT successful

 command &               Run command in a subshell.
                         If a command is terminated by the control operator &, the shell
                         executes the command in the background in a subshell.

 command &> filename     Redirect every output of command to filename

The NOCLOBBER option can prevent overwriting an existing file:
$ set -o noclobber turns ON noclobber
$ set +o noclobber turns OFF noclobber

Redirecting Input

Redirection of input causes the file whose name results from the expansion of word to be opened for reading on file descriptor n, or the standard input (file descriptor 0) if n is not specified.

The general format for redirecting input is:
[n]<word

Redirecting Output

Redirection of output causes the file whose name results from the expansion of word to be opened for writing on file descriptor n, or the standard output (file descriptor 1) if n is not specified. If the file does not exist it is created; if it does exist it is truncated to zero size.

The general format for redirecting output is:
[n]>[|]word
If the redirection operator is '>', and the noclobber option to the set builtin has been enabled, the redirection will fail if the file whose name results from the expansion of word exists and is a regular file. If the redirection operator is '>|', or the redirection operator is '>' and the noclobber option is not enabled, the redirection is attempted even if the file named by word exists.

Appending Redirected Output

Redirection of output in this fashion causes the file whose name results from the expansion of word to be opened for appending on file descriptor n, or the standard output (file descriptor 1) if n is not specified. If the file does not exist it is created.

The general format for appending output is:
[n]>>word

Redirecting Standard Output and Standard Error

Bash allows both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to be redirected to the file whose name is the expansion of word with this construct.

There are two formats for redirecting standard output and standard error:
&>word
and
>&word
Of the two forms, the first is preferred. This is semantically equivalent to
>word 2>&1

Redirection can also be used to open and close files for the current shell execution environment. The following redirection operators can precede or appear anywhere within a simple command or can follow a command.

Redirections are processed in the order they appear, from left to right.

Note that the order of redirections is significant. For example, the command

ls > dirlist 2>&1

directs both standard output (file descriptor 1) and standard error (file descriptor 2) to the file dirlist, while the command

ls 2>&1 > dirlist

directs only the standard output to file dirlist, because the standard error was duplicated as standard output before the standard output was redirected to dirlist.

Bash handles several filenames specially when they are used in redirections, as described in the following table:

/dev/fd/fd: If fd is a valid integer, file descriptor fd is duplicated.
/dev/stdin: File descriptor 0 is duplicated.
/dev/stdout: File descriptor 1 is duplicated.
/dev/stderr: File descriptor 2 is duplicated.
/dev/tcp/host/port: If host is a valid hostname or Internet address, and port is an integer port number, Bash attempts to open a TCP connection to the corresponding socket.
/dev/udp/host/port: If host is a valid hostname or Internet address, and port is an integer port number, Bash attempts to open a UDP connection to the corresponding socket.

A failure to open or create a file causes the redirection to fail.

Here Documents

This type of redirection instructs the shell to read input from the current source until a line containing only word (with no trailing blanks) is seen. All of the lines read up to that point are then used as the standard input for a command.

The format of here-documents is as follows:
<<[-]word
        here-document
delimiter
No parameter expansion, command substitution, arithmetic expansion, or filename expansion is performed on word. If any characters in word are quoted, the delimiter is the result of quote removal on word, and the lines in the here-document are not expanded. If word is unquoted, all lines of the here-document are subjected to parameter expansion, command substitution, and arithmetic expansion. In the latter case, the character sequence \newline is ignored, and '\' must be used to quote the characters '\', '$', and '''.

If the redirection operator is '<<-', then all leading tab characters are stripped from input lines and the line containing delimiter. This allows here-documents within shell scripts to be indented in a natural fashion.

Duplicating File Descriptors

The redirection operator
[n]<&word
is used to duplicate input file descriptors. If word expands to one or more digits, the file descriptor denoted by n is made to be a copy of that file descriptor. If the digits in word do not specify a file descriptor open for input, a redirection error occurs. If word evaluates to '-', file descriptor n is closed. If n is not specified, the standard input (file descriptor 0) is used.

The operator
[n]>&word
is used similarly to duplicate output file descriptors. If n is not specified, the standard output (file descriptor 1) is used. If the digits in word do not specify a file descriptor open for output, a redirection error occurs. As a special case, if n is omitted, and word does not expand to one or more digits, the standard output and standard error are redirected as described previously.

Opening File Descriptors for Reading and Writing

The redirection operator
[n]<>word
causes the file whose name is the expansion of word to be opened for both reading and writing on file descriptor n, or on file descriptor 0 if n is not specified. If the file does not exist, it is created.

Process Substitution

Piping the stdout of a command into the stdin of another is a powerful technique. But, what if you need to pipe the stdout of multiple commands? This is where process substitution comes in.

Process substitution is supported on systems that support named pipes (FIFOs) or the /dev/fd method of naming open files. It takes the form of

Template - command list enclosed within parentheses:

>(command_list)

<(command_list)

The process list is run with its input or output connected to a FIFO or some file in /dev/fd. The name of this file is passed as an argument to the current command as the result of the expansion. If the >(list) form is used, writing to the file will provide input for list. If the <(list) form is used, the file passed as an argument should be read to obtain the output of list. Note that no space can appear between the < or > and the left parenthesis, otherwise the construct would be interpreted as a redirection.

When available, process substitution is performed simultaneously with parameter and variable expansion, command substitution, and arithmetic expansion.

In all the descriptions above, if the file descriptor number is omitted, and the first character of the redirection operator is '<', the redirection refers to the standard input (file descriptor 0). If the first character of the redirection operator is '>', the redirection refers to the standard output (file descriptor 1).

The word following the redirection operator in all the descriptions above, unless otherwise noted, is subjected to brace expansion, tilde expansion, parameter expansion, command substitution, arithmetic expansion, quote removal, filename expansion, and word splitting. If it expands to more than one word, Bash reports an error.

Related linux commands:

BASH Syntax
Windows equivalent: Redirection / PowerShell redirection