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<H1> 6. Redirection </H1>
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<A NAME="IDX70"></A>
<A NAME="IDX71"></A>
<A NAME="IDX72"></A>
If a command is followed by <TT>&</TT>
and job control is not active,
then the default standard input
for the command is the empty file <TT>/dev/null</TT>.
Otherwise, the environment for the execution of a command contains the
file descriptors of the invoking shell as modified by
input/output specifications.
<P>
The following may appear anywhere in a simple command
or may precede or follow a complex command.
Expansion occurs before <VAR>word</VAR> or <VAR>digit</VAR>
is used except as noted below.
If the result of substitution on <VAR>word</VAR>
produces more than one filename,
redirection occurs for each
separate filename in turn.
</P>
<P>
</P>
<DL COMPACT>
<DT><TT><</TT> <VAR>word</VAR>
<DD>Open file <VAR>word</VAR> for reading as standard input.
<P>
</P>
<DT><TT><></TT> <VAR>word</VAR>
<DD>Open file <VAR>word</VAR> for reading and writing as standard input.
If the file does not exist then it is created.
<P>
</P>
<DT><TT>></TT> <VAR>word</VAR>
<DD>Open file <VAR>word</VAR> for writing as standard output.
If the file does not exist then it is created.
If the file exists, and the <TT>CLOBBER</TT> option is unset,
this causes an error;
otherwise, it is truncated to zero length.
<P>
</P>
<DT><TT>>|</TT> <VAR>word</VAR>
<DD><DT><TT>>!</TT> <VAR>word</VAR>
<DD>Same as <TT>></TT>, except that the file is truncated to zero length
if it exists, even if <TT>CLOBBER</TT> is unset.
<P>
</P>
<DT><TT>>></TT> <VAR>word</VAR>
<DD>Open file <VAR>word</VAR> for writing in append mode as standard output.
If the file does not exist, and the <TT>CLOBBER</TT>
option is unset, this causes an error;
otherwise, the file is created.
<P>
</P>
<DT><TT>>>|</TT> <VAR>word</VAR>
<DD><DT><TT>>>!</TT> <VAR>word</VAR>
<DD>Same as <TT>>></TT>, except that the file is created if it does not
exist, even if <TT>CLOBBER</TT> is unset.
<P>
</P>
<DT><TT><<</TT>[<TT>-</TT>] <VAR>word</VAR>
<DD>The shell input is read up to a line that is the same as
<VAR>word</VAR>, or to an end-of-file.
No parameter expansion, command substitution or
filename generation is performed on <VAR>word</VAR>.
The resulting document, called a
<EM>here-document</EM>, becomes the standard input.
<P>
If any character of <VAR>word</VAR> is quoted with
single or double quotes or a `<TT>\</TT>',
no interpretation is placed upon the characters of the document.
Otherwise, parameter and command substitution
occurs, `<TT>\</TT>' followed by a newline is removed,
and `<TT>\</TT>' must be used to quote the characters
`<TT>\</TT>', `<TT>$</TT>', `<TT>`</TT>' and the first character of <VAR>word</VAR>.
</P>
<P>
If <TT><<-</TT> is used, then all leading
tabs are stripped from <VAR>word</VAR> and from the document.
</P>
<P>
</P>
<DT><TT><<<</TT> <VAR>word</VAR>
<DD>Perform shell expansion on <VAR>word</VAR> and pass the result
to standard input. This is known as a <EM>here-string</EM>.
<P>
</P>
<DT><TT><&</TT> <VAR>number</VAR>
<DD><DT><TT>>&</TT> <VAR>number</VAR>
<DD>The standard input/output is duplicated from file descriptor
<VAR>number</VAR> (see man page dup2(2)).
<P>
</P>
<DT><TT><& -</TT>
<DD><DT><TT>>& -</TT>
<DD>Close the standard input/output.
<P>
</P>
<DT><TT><& p</TT>
<DD><DT><TT>>& p</TT>
<DD>The input/output from/to the coprocess is moved to the standard input/output.
<P>
</P>
<DT><TT>>&</TT> <VAR>word</VAR>
<DD><DT><TT>&></TT> <VAR>word</VAR>
<DD>(Except where `<TT>>&</TT> <VAR>word</VAR>' matches one of the above syntaxes;
`<TT>&></TT>' can always be used to avoid this ambiguity.)
Redirects both standard output and standard error (file descriptor 2)
in the manner of `<TT>></TT> <VAR>word</VAR>'.
Note that this does <EM>not</EM> have the same effect as `<TT>></TT> <VAR>word</VAR> <TT>2>&1</TT>'
in the presence of multios (see the section below).
<P>
</P>
<DT><TT>>&|</TT> <VAR>word</VAR>
<DD><DT><TT>>&!</TT> <VAR>word</VAR>
<DD><DT><TT>&>|</TT> <VAR>word</VAR>
<DD><DT><TT>&>!</TT> <VAR>word</VAR>
<DD>Redirects both standard output and standard error (file descriptor 2)
in the manner of `<TT>>|</TT> <VAR>word</VAR>'.
<P>
</P>
<DT><TT>>>&</TT> <VAR>word</VAR>
<DD><DT><TT>&>></TT> <VAR>word</VAR>
<DD>Redirects both standard output and standard error (file descriptor 2)
in the manner of `<TT>>></TT> <VAR>word</VAR>'.
<P>
</P>
<DT><TT>>>&|</TT> <VAR>word</VAR>
<DD><DT><TT>>>&!</TT> <VAR>word</VAR>
<DD><DT><TT>&>>|</TT> <VAR>word</VAR>
<DD><DT><TT>&>>!</TT> <VAR>word</VAR>
<DD>Redirects both standard output and standard error (file descriptor 2)
in the manner of `<TT>>>|</TT> <VAR>word</VAR>'.
<P>
</DL>
<P>
If one of the above is preceded by a digit, then the file
descriptor referred to is that specified by the digit
instead of the default 0 or 1.
The order in which redirections are specified is significant.
The shell evaluates each redirection in terms of the
(<EM>file descriptor</EM>, <EM>file</EM>)
association at the time of evaluation.
For example:
</P>
<P>
<BLOCKQUOTE>
... <TT>1></TT><VAR>fname</VAR> <TT>2>&1</TT>
</BLOCKQUOTE>
<P>
first associates file descriptor 1 with file <VAR>fname</VAR>.
It then associates file descriptor 2 with the file associated with file
descriptor 1 (that is, <VAR>fname</VAR>).
If the order of redirections were reversed,
file descriptor 2 would be associated
with the terminal (assuming file descriptor 1 had been)
and then file descriptor 1 would be associated with file <VAR>fname</VAR>.
</P>
<P>
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<H2> 6.1 Multios </H2>
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If the user tries to open a file descriptor for writing more than once,
the shell opens the file descriptor as a pipe to a process that copies
its input to all the specified outputs, similar to <CITE>tee</CITE>,
provided the <TT>MULTIOS</TT> option is set, as it is by default. Thus:
<P>
<TABLE><tr><td> </td><td class=example><pre>date >foo >bar
</pre></td></tr></table><P>
writes the date to two files, named `<TT>foo</TT>' and `<TT>bar</TT>'.
Note that a pipe is an implicit redirection; thus
</P>
<P>
<TABLE><tr><td> </td><td class=example><pre>date >foo | cat
</pre></td></tr></table><P>
writes the date to the file `<TT>foo</TT>', and also pipes it to cat.
</P>
<P>
If the <TT>MULTIOS</TT>
option is set, the word after a redirection operator is also subjected
to filename generation (globbing). Thus
</P>
<P>
<TABLE><tr><td> </td><td class=example><pre>: > *
</pre></td></tr></table><P>
will truncate all files in the current directory,
assuming there's at least one. (Without the <TT>MULTIOS</TT>
option, it would create an empty file called `<TT>*</TT>'.)
Similarly, you can do
</P>
<P>
<TABLE><tr><td> </td><td class=example><pre>echo exit 0 >> *.sh
</pre></td></tr></table><P>
If the user tries to open a file descriptor for reading more than once,
the shell opens the file descriptor as a pipe to a process that copies
all the specified inputs to its output in the order
specified, similar to <CITE>cat</CITE>,
provided the <TT>MULTIOS</TT> option is set. Thus
</P>
<P>
<TABLE><tr><td> </td><td class=example><pre>sort <foo <fubar
</pre></td></tr></table><P>
or even
</P>
<P>
<TABLE><tr><td> </td><td class=example><pre>sort <f{oo,ubar}
</pre></td></tr></table><P>
is equivalent to `<TT>cat foo fubar | sort</TT>'.
</P>
<P>
Note that a pipe is an implicit redirection; thus
</P>
<P>
<TABLE><tr><td> </td><td class=example><pre>cat bar | sort <foo
</pre></td></tr></table><P>
is equivalent to `<TT>cat bar foo | sort</TT>' (note the order of the inputs).
</P>
<P>
If the <TT>MULTIOS</TT> option is <EM>un</EM>set,
each redirection replaces the previous redirection for that file descriptor.
However, all files redirected to are actually opened, so
</P>
<P>
<TABLE><tr><td> </td><td class=example><pre>echo foo > bar > baz
</pre></td></tr></table><P>
when <TT>MULTIOS</TT> is unset will truncate bar, and write `<TT>foo</TT>' into baz.
</P>
<P>
</P>
<P>
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<H2> 6.2 Redirections with no command </H2>
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<A NAME="IDX75"></A>
<A NAME="IDX76"></A>
<A NAME="IDX77"></A>
When a simple command consists of one or more redirection operators
and zero or more parameter assignments, but no command name, zsh can
behave in several ways.
<P>
If the parameter <TT>NULLCMD</TT> is not set or the option <TT>CSH_NULLCMD</TT> is
set, an error is caused. This is the <CITE>csh</CITE> behavior and <TT>CSH_NULLCMD</TT>
is set by default when emulating <CITE>csh</CITE>.
</P>
<P>
If the option <TT>SH_NULLCMD</TT> is set, the builtin `<TT>:</TT>' is inserted as a
command with the given redirections. This is the default when emulating
<CITE>sh</CITE> or <CITE>ksh</CITE>.
</P>
<P>
Otherwise, if the parameter <TT>NULLCMD</TT> is set, its value will be used as a
command with the given redirections. If both <TT>NULLCMD</TT> and
<TT>READNULLCMD</TT> are set, then the value of the latter will be used instead
of that of the former when the redirection is an input. The default for
<TT>NULLCMD</TT> is `<TT>cat</TT>' and for <TT>READNULLCMD</TT> is `<TT>more</TT>'. Thus
</P>
<P>
<TABLE><tr><td> </td><td class=example><pre>< file
</pre></td></tr></table><P>
shows the contents of <TT>file</TT> on standard output, with paging if that is a
terminal. <TT>NULLCMD</TT> and <TT>READNULLCMD</TT> may refer to shell functions.
</P>
<P>
<A NAME="Command Execution"></A>
</P>
<P>
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