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Redirection and Process Substitution
Both the input and output of a command may be redirected:
command > filename Redirect command output into a file
command >> filename Redirect command output and APPEND into a file
command < filename Redirect a file into a command
command | filename Redirect output from one command into another
command | tee filename | commandB Redirect command into filename AND CommandB
Builtin commands are executed within the shell. If any component of a
pipeline except the last is a builtin command, the pipeline is executed
in a subshell.
Parenthesized commands are always executed in a subshell.
(cd; pwd); pwd
thus prints the home directory, leaving you where you were (printing
this after the home directory), while
cd; pwd
leaves you in the home directory. Parenthesized commands are most
often used to prevent cd from affecting the current shell.
The NOCLOBBER option can prevent you from overwriting an existing file.
The standard input and standard output of a command may be redirected
with the following syntax:
< name Open file name (which is first variable, command and filename
expanded) as the standard input.
<< word Read the shell input up to a line which is identical to word.
word is not subjected to variable, filename or command substi-
tution, and each input line is compared to word before any sub-
stitutions are done on this input line. Unless a quoting `\',
`"', `' or ``' appears in word variable and command substitu-
tion is performed on the intervening lines, allowing `\' to
quote `$', `\' and ``'. Commands which are substituted have
all blanks, tabs, and newlines preserved, except for the final
newline which is dropped. The resultant text is placed in an
anonymous temporary file which is given to the command as stan-
dard input.
> name
>! name
>& name
>&! name
The file name is used as standard output. If the file does not
exist then it is created; if the file exists, it is truncated,
its previous contents being lost.
If the shell variable noclobber is set, then the file must not
exist or be a character special file (e.g., a terminal or
`/dev/null') or an error results. This helps prevent acciden-
tal destruction of files. In this case the `!' forms can be
used to suppress this check.
The forms involving `&' route the diagnostic output into the
specified file as well as the standard output. name is
expanded in the same way as `<' input filenames are.
>> name
>>& name
>>! name
>>&! name
Like `>', but appends output to the end of name. If the shell
variable noclobber is set, then it is an error for the file not
to exist, unless one of the `!' forms is given.
A command receives the environment in which the shell was invoked as
modified by the input-output parameters and the presence of the command
in a pipeline. Thus, unlike some previous shells, commands run from a
file of shell commands have no access to the text of the commands by
default; rather they receive the original standard input of the shell.
The `<<' mechanism should be used to present inline data. This permits
shell command scripts to function as components of pipelines and allows
the shell to block read its input. Note that the default standard
input for a command run detached is not the empty file /dev/null, but
the original standard input of the shell. If this is a terminal and if
the process attempts to read from the terminal, then the process will
block and the user will be notified .
Diagnostic output may be directed through a pipe with the standard out-
put. Simply use the form `|&' rather than just `|'.
The shell cannot presently redirect diagnostic output without also
redirecting standard output, but
`(command > output-file) >& error-file' is often an acceptable
workaround. Either output-file or error-file may be `/dev/tty' to
send output to the terminal.
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 ``'.
