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6. Redirection

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.

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.

<& 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.

6.1 Multios

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'.

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.

6.2 Redirections with no command

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.