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System User's Guide: Operating System and Devices
Before the C shell executes a
command, it scans the command line for redirection characters. These
special notations direct the shell to redirect input and output.
You can redirect the standard
input and output of a command with the following syntax statements:
| < File
| Opens the specified File (which is first variable, command,
and file name expanded) as the standard input.
|
| <<Word
| Reads the shell input up to the line that matches the value of the
Word variable. The Word variable is not subjected
to variable, file name, or command substitution. Each input line is
compared to the Word variable before any substitutions are done on
the line. Unless a quoting character (\, ", ' or
`.) appears in the Word variable, the shell
performs variable and command substitution on the intervening lines, allowing
the \ character to quote the $, \, and ` characters.
Commands that are substituted have all blanks, tabs, and new-line characters
preserved, except for the final new-line character, which is dropped.
The resultant text is placed in an anonymous temporary file, which is given to
the command as standard input.
|
> File
>!File
>&
File
| >&! File
| Uses the specified File as standard output. If
File does not exist, it is created. If File
exists, it is truncated, and its previous contents are lost. If the
noclobber shell variable is set, File must not exist or
be a character special file, or an error results. This helps prevent
accidental destruction of files. In this case, use the forms including
an ! to suppress this check. File is expanded in the same
way as < input file names. The form >& redirects both
standard output and standard error to the specified File.
The following example shows how to separately redirect standard output to
/dev/tty and standard error to /dev/null. The
parentheses are required to allow standard output and standard error to be
separate.
% (find / -name vi -print > /dev/tty) >& /dev/null
|
> >File
> >!
File
> >&
File
| > >&!
File
| Uses the specified File as standard output like >, but
appends output to the end of File. If the
noclobber shell variable is set, an error results if
File does not exist, unless one of the forms including an ! is
given. Otherwise, it is similar to >.
|
A command receives the environment
in which the shell was invoked, as changed by the input/output parameters and
the presence of the command as a pipeline. Thus, unlike some previous
shells, commands that run from a shell script do not have access to the text
of the commands by default. Rather, they receive the original standard
input of the shell. Use the << mechanism to present inline
data. This lets shell command files function as components of pipelines
and also lets the shell block read its input. Note that the default
standard input for a command run detached is not changed to the empty
/dev/null file. Rather, the standard input remains the
original standard input of the shell.
To redirect the standard error
through a pipe with the standard output, use the form |& rather than just
the |.
The shell contains commands that
can be used to regulate the flow of control in command files (shell scripts)
and (in limited but useful ways) from shell command-line input. These
commands all operate by forcing the shell to repeat, or skip, in its
input.
The foreach, switch,
and while statements, and the if-then-else form of
the if statement, require that the major keywords appear in a
single simple command on an input line.
If the shell input is not
searchable, the shell buffers input whenever a loop is being read and searches
the internal buffer to do the rereading implied by the loop. To the
extent that this is allowed, backward gotos succeed on inputs that
you cannot search.
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