zsh: 7. Redirection

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# 7. Redirection

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.

`<>` 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, even if `CLOBBER` is unset.

`>>` word

  Open file word for writing in append mode as standard output. If the file does not exist, and the `CLOBBER` option is 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, even if `CLOBBER` is unset.

`<<`[`-`] 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.

`<&` number
`>&` number

  The standard input/output is duplicated from file descriptor number (see man page 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 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.

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## 7.1 Opening file descriptors using parameters

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 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:

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

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.

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## 7.2 Multios

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:

    date >foo >bar

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

    date >foo | cat

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

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

    : > *

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

    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 cat, provided the `MULTIOS` option is set. Thus

    sort <foo <fubar

or even

    sort <f{oo,ubar}

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

    cat bar | sort <foo

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

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

    echo foo > bar > baz

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

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

    cat file >file1 >file2
    cat file1 file2

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:

    { cat file } >file >file2

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

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## 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.

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.

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.

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

    < file

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

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This document was generated by Peter Stephenson on December 21, 2012 using texi2html 1.82.