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**Table of Contents**  *generated with [DocToc](https://github.com/thlorenz/doctoc)*

- [7 Redirection](#7-redirection)
  - [7.1 Opening file descriptors using parameters](#71-opening-file-descriptors-using-parameters)
  - [7.2 Multios](#72-multios)
  - [7.3 Redirections with no command](#73-redirections-with-no-command)

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<span id="Redirection"></span> <span id="Redirection-1"></span>

# 7 Redirection

<span id="index-redirection"></span> <span
id="index-file-descriptors"></span> <span
id="index-descriptors_002c-file"></span>

If a command is followed by & and job control is not active, then the
default standard input for the command is the empty file /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 `word` or `digit` is
used except as noted below. If the result of substitution on `word`
produces more than one filename, redirection occurs for each separate
filename in turn.

\< `word`  
Open file `word` for reading as standard input. It is an error to open a
file in this fashion if it does not exist.

\<\> `word`  
Open file `word` for reading and writing as standard input. If the file
does not exist then it is created.

\> `word`  
Open file `word` for writing as standard output. If the file does not
exist then it is created. If the file exists, and the CLOBBER option is
unset, this causes an error; otherwise, it is truncated to zero length.

\>\| `word`  
\>! `word`  
Same as >, except that the file is truncated to zero length if it
exists, regardless of CLOBBER.

\>\> `word`  
Open file `word` for writing in append mode as standard output. If the
file does not exist, and the CLOBBER and APPEND_CREATE options are both
unset, this causes an error; otherwise, the file is created.

\>\>\| `word`  
\>\>! `word`  
Same as >\>, except that the file is created if it does not exist,
regardless of CLOBBER and APPEND_CREATE.

\<\<\[-\] `word`  
The shell input is read up to a line that is the same as `word`, or to
an end-of-file. No parameter expansion, command substitution or filename
generation is performed on `word`. The resulting document, called a
*here-document*, becomes the standard input.

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

Note that `word` itself does not undergo shell expansion. Backquotes in
`word` do not have their usual effect; instead they behave similarly to
double quotes, except that the backquotes themselves are passed through
unchanged. (This information is given for completeness and it is not
recommended that backquotes be used.) Quotes in the form $’`...`’ have
their standard effect of expanding backslashed references to special
characters.

If \<\<- is used, then all leading tabs are stripped from `word` and
from the document.

\<\<\< `word`  
Perform shell expansion on `word` and pass the result to standard input.
This is known as a *here-string*. Compare the use of `word` in
here-documents above, where `word` does not undergo shell expansion. The
result will have a trailing newline after it.

\<& `number`  
\>& `number`  
The standard input/output is duplicated from file descriptor `number`
(see dup2(2)).

\<& -  
\>& -  
Close the standard input/output.

\<& p  
\>& p  
The input/output from/to the coprocess is moved to the standard
input/output.

\>& `word`  
&> `word`  
(Except where ‘>& `word`’ matches one of the above syntaxes; ‘&>’ can
always be used to avoid this ambiguity.) Redirects both standard output
and standard error (file descriptor 2) in the manner of ‘> `word`’. Note
that this does *not* have the same effect as ‘> `word` 2>&1’ in the
presence of multios (see the section below).

\>&\| `word`  
\>&! `word`  
&>\| `word`  
&>! `word`  
Redirects both standard output and standard error (file descriptor 2) in
the manner of ‘>\| `word`’.

\>\>& `word`  
&>\> `word`  
Redirects both standard output and standard error (file descriptor 2) in
the manner of ‘>\> `word`’.

\>\>&\| `word`  
\>\>&! `word`  
&>\>\| `word`  
&>\>! `word`  
Redirects both standard output and standard error (file descriptor 2) in
the manner of ‘>\>\| `word`’.

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

> ... 1>`fname` 2>&1

first associates file descriptor 1 with file `fname`. It then associates
file descriptor 2 with the file associated with file descriptor 1 (that
is, `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 `fname`.

The ‘\|&’ command separator described in [Simple Commands &
Pipelines](Shell-Grammar.html#Simple-Commands-_0026-Pipelines) is a
shorthand for ‘2>&1 \|’.

The various forms of process substitution, ‘\<(`list`)’, and ‘=(`list`)’
for input and ‘>(`list`)’ for output, are often used together with
redirection. For example, if `word` in an output redirection is of the
form ‘>(`list`)’ then the output is piped to the command represented by
`list`. See [Process Substitution](Expansion.html#Process-Substitution).

------------------------------------------------------------------------

<span id="Opening-file-descriptors-using-parameters"></span>

## 7.1 Opening file descriptors using parameters

<span id="index-file-descriptors_002c-use-with-parameters"></span> <span
id="index-parameters_002c-for-using-file-descriptors"></span>

When the shell is parsing arguments to a command, and the shell option
IGNORE_BRACES is not set, a different form of redirection is allowed:
instead of a digit before the operator there is a valid shell identifier
enclosed in braces. The shell will open a new file descriptor that is
guaranteed to be at least 10 and set the parameter named by the
identifier to the file descriptor opened. No whitespace is allowed
between the closing brace and the redirection character. For example:

> ... {myfd}>&1

This opens a new file descriptor that is a duplicate of file descriptor
1 and sets the parameter myfd to the number of the file descriptor,
which will be at least 10. The new file descriptor can be written to
using the syntax \>&$myfd. The file descriptor remains open in subshells

The syntax {`varid`}>&-, for example {myfd}>&-, may be used to close a
file descriptor opened in this fashion. Note that the parameter given by
`varid` must previously be set to a file descriptor in this case.

It is an error to open or close a file descriptor in this fashion when
the parameter is readonly. However, it is not an error to read or write
a file descriptor using \<&$`param` or \>&$`param` if `param` is
readonly.

If the option CLOBBER is unset, it is an error to open a file descriptor
using a parameter that is already set to an open file descriptor
previously allocated by this mechanism. Unsetting the parameter before
using it for allocating a file descriptor avoids the error.

Note that this mechanism merely allocates or closes a file descriptor;
it does not perform any redirections from or to it. It is usually
convenient to allocate a file descriptor prior to use as an argument to
exec. The syntax does not in any case work when used around complex
commands such as parenthesised subshells or loops, where the opening
brace is interpreted as part of a command list to be executed in the
current shell.

The following shows a typical sequence of allocation, use, and closing
of a file descriptor:

<div class="example">

```zsh
integer myfd
exec {myfd}>~/logs/mylogfile.txt
print This is a log message. >&$myfd
exec {myfd}>&-
```

</div>

Note that the expansion of the variable in the expression \>&$myfd
occurs at the point the redirection is opened. This is after the
expansion of command arguments and after any redirections to the left on
the command line have been processed.

------------------------------------------------------------------------

<span id="Multios"></span>

## 7.2 Multios

<span id="index-multios"></span> <span
id="index-MULTIOS_002c-use-of"></span>

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 tee, provided the
MULTIOS option is set, as it is by default. Thus:

<div class="example">

```zsh
date >foo >bar
```

</div>

writes the date to two files, named ‘foo’ and ‘bar’. Note that a pipe is
an implicit redirection; thus

<div class="example">

```zsh
date >foo | cat
```

</div>

writes the date to the file ‘foo’, and also pipes it to cat.

Note that the shell opens all the files to be used in the multio process
immediately, not at the point they are about to be written.

Note also that redirections are always expanded in order. This happens
regardless of the setting of the MULTIOS option, but with the option in
effect there are additional consequences. For example, the meaning of
the expression \>&1 will change after a previous redirection:

<div class="example">

```zsh
date >&1 >output
```

</div>

In the case above, the \>&1 refers to the standard output at the start
of the line; the result is similar to the tee command. However,
consider:

<div class="example">

```zsh
date >output >&1
```

</div>

As redirections are evaluated in order, when the \>&1 is encountered the
standard output is set to the file output and another copy of the output
is therefore sent to that file. This is unlikely to be what is intended.

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

<div class="example">

```zsh
: > *
```

</div>

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

<div class="example">

```zsh
echo exit 0 >> *.sh
```

</div>

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, provided
the MULTIOS option is set. It should be noted that each file is opened
immediately, not at the point where it is about to be read: this
behaviour differs from cat, so if strictly standard behaviour is needed,
cat should be used instead.

Thus

<div class="example">

```zsh
sort <foo <fubar
```

</div>

or even

<div class="example">

```zsh
sort <f{oo,ubar}
```

</div>

is equivalent to ‘cat foo fubar \| sort’.

Expansion of the redirection argument occurs at the point the
redirection is opened, at the point described above for the expansion of
the variable in \>&$myfd.

Note that a pipe is an implicit redirection; thus

<div class="example">

```zsh
cat bar | sort <foo
```

</div>

is equivalent to ‘cat bar foo \| sort’ (note the order of the inputs).

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

<div class="example">

```zsh
echo Hello > bar > baz
```

</div>

when MULTIOS is unset will truncate ‘bar’, and write ‘Hello’ into ‘baz’.

There is a problem when an output multio is attached to an external
program. A simple example shows this:

<div class="example">

```zsh
cat file >file1 >file2
cat file1 file2
```

</div>

Here, it is possible that the second ‘cat’ will not display the full
contents of file1 and file2 (i.e. the original contents of file repeated
twice).

The reason for this is that the multios are spawned after the cat
process is forked from the parent shell, so the parent shell does not
wait for the multios to finish writing data. This means the command as
shown can exit before file1 and file2 are completely written. As a
workaround, it is possible to run the cat process as part of a job in
the current shell:

<div class="example">

```zsh
{ cat file } >file >file2
```

</div>

Here, the {`...`} job will pause to wait for both files to be written.

------------------------------------------------------------------------

<span id="Redirections-with-no-command"></span>

## 7.3 Redirections with no command

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.

<span id="index-NULLCMD_002c-use-of"></span> <span
id="index-CSH_005fNULLCMD_002c-use-of"></span>

If the parameter NULLCMD is not set or the option CSH_NULLCMD is set, an
error is caused. This is the csh behavior and CSH_NULLCMD is set by
default when emulating csh.

<span id="index-SH_005fNULLCMD_002c-use-of"></span>

If the option SH_NULLCMD is set, the builtin ‘:’ is inserted as a
command with the given redirections. This is the default when emulating
sh or ksh.

<span id="index-READNULLCMD_002c-use-of"></span>

Otherwise, if the parameter NULLCMD is set, its value will be used as a
command with the given redirections. If both NULLCMD and 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 NULLCMD is
‘cat’ and for READNULLCMD is ‘more’. Thus

<div class="example">

```zsh
< file
```

</div>

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

------------------------------------------------------------------------

This document was generated on *May 14, 2022* using [*texi2html
5.0*](http://www.nongnu.org/texi2html/).  
Zsh version 5.9, released on May 14, 2022.
