texinode(Redirection)(Command Execution)(Shell Grammar)(Top)
chapter(Redirection)
cindex(redirection)
ifzman(\
sect(Redirection)
)\
cindex(file descriptors)
cindex(descriptors, file)
If a command is followed by tt(&)
and job control is not active,
then the default standard input
for the command is the empty file tt(/dev/null).
Otherwise, the environment for the execution of a command contains the
file descriptors of the invoking shell as modified by
input/output specifications.

The following may appear anywhere in a simple command
or may precede or follow a complex command.
Expansion occurs before var(word) or var(digit)
is used except as noted below.
If the result of substitution on var(word)
produces more than one filename,
redirection occurs for each
separate filename in turn.

startitem()
item(tt(<) var(word))(
Open file var(word) for reading as standard input.
)
item(tt(<>) var(word))(
Open file var(word) for reading and writing as standard input.
If the file does not exist then it is created.
)
item(tt(>) var(word))(
Open file var(word) for writing as standard output.
If the file does not exist then it is created.
If the file exists, and the tt(CLOBBER) option is unset,
this causes an error;
otherwise, it is truncated to zero length.
)
xitem(tt(>|) var(word))
item(tt(>!) var(word))(
Same as tt(>), except that the file is truncated to zero length
if it exists, even if tt(CLOBBER) is unset.
)
item(tt(>>) var(word))(
Open file var(word) for writing in append mode as standard output.
If the file does not exist, and the tt(CLOBBER)
option is unset, this causes an error;
otherwise, the file is created.
)
xitem(tt(>>|) var(word))
item(tt(>>!) var(word))(
Same as tt(>>), except that the file is created if it does not
exist, even if tt(CLOBBER) is unset.
)
item(tt(<<)[tt(-)] var(word))(
The shell input is read up to a line that is the same as
var(word), or to an end-of-file.
No parameter expansion, command substitution or
filename generation is performed on var(word).
The resulting document, called a
em(here-document), becomes the standard input.

If any character of var(word) is quoted with
single or double quotes or a `tt(\)',
no interpretation is placed upon the characters of the document.
Otherwise, parameter and command substitution
occurs, `tt(\)' followed by a newline is removed,
and `tt(\)' must be used to quote the characters
`tt(\)', `tt($)', `tt(`)' and the first character of var(word).

If tt(<<-) is used, then all leading
tabs are stripped from var(word) and from the document.
)
item(tt(<<<) var(word))(
Perform shell expansion on var(word) and pass the result
to standard input.  This is known as a em(here-string).
)
xitem(tt(<&) var(number))
item(tt(>&) var(number))(
The standard input/output is duplicated from file descriptor
var(number) (see manref(dup2)(2)).
)
xitem(tt(<& -))
item(tt(>& -))(
Close the standard input/output.
)
xitem(tt(<& p))
item(tt(>& p))(
The input/output from/to the coprocess is moved to the standard input/output.
)
xitem(tt(>&) var(word))
item(tt(&>) var(word))(
(Except where `tt(>&) var(word)' matches one of the above syntaxes;
`tt(&>)' can always be used to avoid this ambiguity.)
Redirects both standard output and standard error (file descriptor 2)
in the manner of `tt(>) var(word)'.
Note that this does em(not) have the same effect as `tt(>) var(word) tt(2>&1)'
in the presence of multios (see the section below).
)
xitem(tt(>&|) var(word))
xitem(tt(>&!) var(word))
xitem(tt(&>|) var(word))
item(tt(&>!) var(word))(
Redirects both standard output and standard error (file descriptor 2)
in the manner of `tt(>|) var(word)'.
)
xitem(tt(>>&) var(word))
item(tt(&>>) var(word))(
Redirects both standard output and standard error (file descriptor 2)
in the manner of `tt(>>) var(word)'.
)
xitem(tt(>>&|) var(word))
xitem(tt(>>&!) var(word))
xitem(tt(&>>|) var(word))
item(tt(&>>!) var(word))(
Redirects both standard output and standard error (file descriptor 2)
in the manner of `tt(>>|) var(word)'.
)
enditem()

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(file))
association at the time of evaluation.
For example:

indent(... tt(1>)var(fname) tt(2>&1))

first associates file descriptor 1 with file var(fname).
It then associates file descriptor 2 with the file associated with file
descriptor 1 (that is, var(fname)).
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).
sect(Multios)
pindex(MULTIOS, use of)
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 bf(tee),
provided the tt(MULTIOS) option is set.  Thus:

example(date >foo >bar)

writes the date to two files, named `tt(foo)' and `tt(bar)'.
Note that a pipe is an implicit redirection; thus

example(date >foo | cat)

writes the date to the file `tt(foo)', and also pipes it to cat.

If the tt(MULTIOS)
option is set, the word after a redirection operator is also subjected
to filename generation (globbing).  Thus

example(: > *)

will truncate all files in the current directory,
assuming there's at least one.  (Without the tt(MULTIOS)
option, it would create an empty file called `tt(*)'.)
Similarly, you can do

example(echo exit 0 >> *.sh)

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 bf(cat),
provided the tt(MULTIOS) option is set.  Thus

example(sort <foo <fubar)

or even

example(sort <f{oo,ubar})

is equivalent to `tt(cat foo fubar | sort)'.

Note that a pipe is an implicit redirection; thus

example(cat bar | sort <foo)

is equivalent to `tt(cat bar foo | sort)' (note the order of the inputs).

If the tt(MULTIOS) option is em(un)set,
each redirection replaces the previous redirection for that file descriptor.
However, all files redirected to are actually opened, so

example(echo foo > bar > baz)

when tt(MULTIOS) is unset will truncate bar, and write `tt(foo)' into baz.

sect(Redirections with no command)
vindex(NULLCMD, use of)
vindex(READNULLCMD, use of)
If a simple command consists of one or more redirection operators
and zero or more parameter assignments, but no command name, and the
parameter tt(NULLCMD) is not set, an error is caused.  If the parameter
tt(NULLCMD) is set, its value will be inserted as a command with the
given redirections.  If both tt(NULLCMD) and tt(READNULLCMD) 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) is `tt(cat)' and for
tt(READNULLCMD) is `tt(more)'. Thus

example(< file)

shows the contents of tt(file) on standard output, with paging if that is a
terminal.  tt(NULLCMD) and tt(READNULLCMD) may refer to shell functions.

The standard Bourne shell behaviour is obtained by setting tt(NULLCMD) and
tt(READNULLCMD) to `tt(:)'.  This is the default when zsh is emulating bf(sh)
or bf(ksh).
