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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 fileword
for reading as standard input. It is an error to open a file in this fashion if it does not exist. -
<>
word
Open fileword
for reading and writing as standard input. If the file does not exist then it is created. -
>
word
Open fileword
for writing as standard output. If the file does not exist then it is created. If the file exists, and theCLOBBER
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 ofCLOBBER
. -
>>
word
Open fileword
for writing in append mode as standard output. If the file does not exist, and theCLOBBER
andAPPEND_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 ofCLOBBER
andAPPEND_CREATE
. -
<<
[-
]word
The shell input is read up to a line that is the same asword
, or to an end-of-file. No parameter expansion, command substitution or filename generation is performed onword
. 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 ofword
.Note that
word
itself does not undergo shell expansion. Backquotes inword
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 fromword
and from the document. -
<<<
word
Perform shell expansion onword
and pass the result to standard input. This is known as a here-string. Compare the use ofword
in here-documents above, whereword
does not undergo shell expansion. -
<&
number
>&
number
The standard input/output is duplicated from file descriptornumber
(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- in the manner of ‘
>|
word
’.
- in the manner of ‘
-
>>&
word
&>>
word
Redirects both standard output and standard error (file descriptor- in the manner of ‘
>>
word
’.
- in the manner of ‘
-
>>&|
word
>>&!
word
&>>|
word
&>>!
word
Redirects both standard output and standard error (file descriptor- in the manner of ‘
>>|
word
’.
- in the manner of ‘
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
- 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.
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 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:
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.
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.
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:
date >&1 >output
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:
date >output >&1
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
: > *
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, 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
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 Hello > bar > baz
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:
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.
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.
This document was generated on February 15, 2020 using
texi2html 5.0.
Zsh version 5.8, released on February 14, 2020.