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<div class="dw-content-page "><!-- content --><div class="dw-content"><p>
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<input type="hidden" name="sectok" value=""/><input type="hidden" name="id" value="syntax:redirection"/><input type="hidden" name="rev" value="0"/><input type="hidden" name="date" value="0"/><input type="hidden" name="prefix" value="."/><input type="hidden" name="suffix" value=""/><input type="hidden" name="changecheck" value="b20711f1986cd343aaad44f36e6db417"/><input type="hidden" name="target" value="section"/><textarea name="wikitext" id="wiki__text" dir="auto" class="edit form-control" cols="80" rows="10" tabindex="1" readonly="readonly">
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====== Redirection ======
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<wrap left todo>Fix me: To be continued</wrap>
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\\
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Redirection makes it possible to control where the output of a command goes to, and where the input of a command comes from. It's a mighty tool that, together with pipelines, makes the shell powerful. The redirection operators are checked whenever a [[syntax:grammar:parser_exec | simple command is about to be executed]].
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Under normal circumstances, there are 3 files open, accessible by the file descriptors 0, 1 and 2, all connected to your terminal:
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^Name^FD^Description^
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|''stdin''|0|standard input stream (e.g. keyboard)|
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|''stdout''|1|standard output stream (e.g. monitor)|
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|''stderr''|2|standard error output stream (usually also on monitor)|
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<wrap center info>The terms "monitor" and "keyboard" refer to the same device, the **terminal** here. Check your preferred UNIX(r)-FAQ for details, I'm too lazy to explain what a terminal is ;-) </wrap>
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Both, ''stdout'' and ''stderr'' are output file descriptors. Their difference is the **convention** that a program outputs payload on ''stdout'' and diagnostic- and error-messages on ''stderr''. If you write a script that outputs error messages, please make sure you follow this convention!
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Whenever you **name** such a filedescriptor, i.e. you want to redirect this descriptor, you just use the number:
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<code>
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# this executes the cat-command and redirects its error messages (stderr) to the bit bucket
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cat some_file.txt 2>/dev/null
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</code>
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Whenever you **reference** a descriptor, to point to its current target file, then you use a "''&''" followed by a the descriptor number:
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<code>
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# this executes the echo-command and redirects its normal output (stdout) to the standard error target
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echo "There was an error" 1>&2
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</code>
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The redirection operation can be **anywhere** in a simple command, so these examples are equivalent:
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<code>
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cat foo.txt bar.txt >new.txt
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cat >new.txt foo.txt bar.txt
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>new.txt cat foo.txt bar.txt
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</code>
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<wrap center important>Every redirection operator takes one or two words as operands. If you have to use operands (e.g. filenames to redirect to) that contain spaces you **must** quote them!</wrap>
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===== Valid redirection targets and sources =====
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This syntax is recognized whenever a ''TARGET'' or a ''SOURCE'' specification (like below in the details descriptions) is used.
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^Syntax^Description^
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|''FILENAME''|references a normal, ordinary filename from the filesystem (which can of course be a FIFO, too. Simply everything you can reference in the filesystem)|
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|''&N''|references the current target/source of the filedescriptor ''N'' ("duplicates" the filedescriptor)|
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|''&-''|closes the redirected filedescriptor, useful instead of ''> /dev/null'' constructs (''> &-'')|
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|''/dev/fd/N''|duplicates the filedescriptor ''N'', if ''N'' is a valid integer|
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|''/dev/stdin''|duplicates filedescriptor 0 (''stdin'')|
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|''/dev/stdout''|duplicates filedescriptor 1 (''stdout'')|
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|''/dev/stderr''|duplicates filedescriptor 2 (''stderr'')|
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|''/dev/tcp/HOST/PORT''|assuming ''HOST'' is a valid hostname or IP address, and ''PORT'' is a valid port number or service name: redirect from/to the corresponding TCP socket|
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|''/dev/udp/HOST/PORT''|assuming ''HOST'' is a valid hostname or IP address, and ''PORT'' is a valid port number or service name: redirect from/to the corresponding UDP socket|
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If a target/source specification fails to open, the whole redirection operation fails. Avoid referencing file descriptors above 9, since you may collide with file descriptors Bash uses internally.
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===== Redirecting output =====
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<code>
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N > TARGET
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</code>
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This redirects the file descriptor number ''N'' to the target ''TARGET''. If ''N'' is omitted, ''stdout'' is assumed (FD 1). The ''TARGET'' is **truncated** before writing starts.
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If the option ''noclobber'' is set with [[commands:builtin:set | the set builtin]], with cause the redirection to fail, when ''TARGET'' names a regular file that already exists. You can manually override that behaviour by forcing overwrite with the redirection operator ''>|'' instead of ''>''.
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===== Appending redirected output =====
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<code>
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N >> TARGET
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</code>
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This redirects the file descriptor number ''N'' to the target ''TARGET''. If ''N'' is omitted, ''stdout'' is assumed (FD 1). The ''TARGET'' is **not truncated** before writing starts.
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===== Redirecting output and error output =====
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<code>
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&> TARGET
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</code>
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<code>
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>& TARGET
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</code>
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This special syntax redirects both, ''stdout'' and ''stderr'' to the specified target. It's **equivalent** to
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<code>
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> TARGET 2>&1
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</code>
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Since Bash4, there's ''&<nowiki>>>TARGET</nowiki>'', which is equivalent to ''<nowiki>>></nowiki> TARGET 2>&1''.
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<wrap center important>This syntax is deprecated and should not be used. See the page about [[scripting:obsolete | obsolete and deprecated syntax]].</wrap>
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===== Appending redirected output and error output =====
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To append the cumulative redirection of ''stdout'' and ''stderr'' to a file you simply do
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<code>
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>> FILE 2>&1
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</code>
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<code>
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&>> FILE
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</code>
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===== Transporting stdout and stderr through a pipe =====
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<code>
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COMMAND1 2>&1 | COMMAND2
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</code>
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<code>
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COMMAND1 |& COMMAND2
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</code>
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===== Redirecting input =====
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<code>
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N < SOURCE
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</code>
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The input descriptor ''N'' uses ''SOURCE'' as its data source. If ''N'' is omitted, filedescriptor 0 (''stdin'') is assumed.
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===== Here documents =====
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<BOOKMARK:tag_heredoc>
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<code>
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<<TAG
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...
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TAG
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</code>
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<code>
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<<-TAG
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...
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TAG
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</code>
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A here-document is an input redirection using source data specified directly at the command line (or in the script), no "external" source. The redirection-operator ''<nowiki><<</nowiki>'' is used together with a tag ''TAG'' that's used to mark the end of input later:
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<code>
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# display help
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cat <<EOF
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Sorry...
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No help available yet for $PROGRAM.
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Hehe...
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EOF
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</code>
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As you see, substitutions are possible. To be precise, the following substitutions and expansions are performed in the here-document data:
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* [[syntax:pe | Parameter expansion]]
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* [[syntax:expansion:cmdsubst | Command substitution]]
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* [[syntax:expansion:arith | Arithmetic expansion]]
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You can avoid that by quoting the tag:
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<code>
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cat <<"EOF"
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This won't be expanded: $PATH
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EOF
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</code>
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Last but not least, if the redirection operator ''<nowiki><<</nowiki>'' is followed by a ''-'' (dash), all **leading TAB** from the document data will be ignored. This might be useful to have optical nice code also when using here-documents.
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The tag you use **must** be the only word in the line, to be recognized as end-of-here-document marker.
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<wrap center info>It seems that here-documents (tested on versions ''1.14.7'', ''2.05b'' and ''3.1.17'') are correctly terminated when there is an EOF before the end-of-here-document tag. The reason is unknown, but it seems to be done on purpose. Bash 4 introduced a warning message when end-of-file is seen before the tag is reached.</wrap>
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===== Here strings =====
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<code>
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<<< WORD
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</code>
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The here-strings are a variation of the here-documents. The word ''WORD'' is taken for the input redirection:
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<code>
|
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cat <<< "Hello world... $NAME is here..."
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</code>
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Just beware to quote the ''WORD'' if it contains spaces. Otherwise the rest will be given as normal parameters.
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The here-string will append a newline (''\n'') to the data.
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===== Multiple redirections =====
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More redirection operations can occur in a line of course. The order is **important**! They're evaluated from **left to right**.
|
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If you want to redirect both, ''stderr'' and ''stdout'' to the same file (like ''/dev/null'', to hide it), this is **the wrong way**:
|
|
<code bash>
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|
# { echo OUTPUT; echo ERRORS >&2; } is to simulate something that outputs to STDOUT and STDERR
|
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# you can test with it
|
|
{ echo OUTPUT; echo ERRORS >&2; } 2>&1 1>/dev/null
|
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</code>
|
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Why? Relatively easy:
|
|
* initially, ''stdout'' points to your terminal (you read it)
|
|
* same applies to ''stderr'', it's connected to your terminal
|
|
* ''2>&1'' redirects ''stderr'' away from the terminal to the target for ''stdout'': **the terminal** (again...)
|
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* ''1>/dev/null'' redirects ''stdout'' away from your terminal to the file ''/dev/null''
|
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What remains? ''stdout'' goes to ''/dev/null'', ''stderr'' still (or better: "again") goes to the terminal. You have to swap the order to make it do what you want:
|
|
<code bash>
|
|
{ echo OUTPUT; echo ERRORS >&2; } 1>/dev/null 2>&1
|
|
</code>
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|
===== Examples =====
|
|
|
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How to make a program quiet (assuming all output goes to ''STDOUT'' and ''STDERR''?
|
|
<code>
|
|
command >/dev/null 2>&1
|
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</code>
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|
|
===== See also =====
|
|
* Internal: [[howto:redirection_tutorial | Illustrated Redirection Tutorial]]
|
|
* Internal: [[commands:builtin:set#tag_noclobber | The noclobber option]]
|
|
* Internal: [[commands:builtin:exec | The exec builtin command]]
|
|
* Internal: [[syntax:grammar:parser_exec | Simple commands parsing and execution]]
|
|
* Internal: [[syntax:expansion:proc_subst | Process substitution syntax]]
|
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* Internal: [[scripting:obsolete | Obsolete and deprecated syntax]]
|
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* Internal: [[scripting:nonportable | Nonportable syntax and command uses]]</textarea>
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