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
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
nis not specified.
The general format for redirecting input is:[n]<word
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
nis 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
noclobberoption to the
setbuiltin 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
noclobberoption is not enabled, the redirection is attempted even if the file named by word exists.
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
nis not specified. If the file does not exist it is created.
The general format for appending output is:[n]>>word
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
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:
A failure to open or create a file causes the redirection to fail.
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
\newlineis 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.
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
nis 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
nis closed. If
nis not specified, the standard input (file descriptor 0) is used.
is used similarly to duplicate output file descriptors. If
nis 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
nis omitted, and word does not expand to one or more digits, the standard output and standard error are redirected as described previously.
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
nis not specified. If the file does not exist, it is created.
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:
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
>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:
Windows equivalent: Redirection - Spooling output to a file, piping input