Redirection - Revision history

Jimbo: Reverted edits by 193.28.153.27 (Talk); changed back to last version by 24.20.109.51

2007-12-31T22:32:47Z

Reverted edits by 193.28.153.27 (Talk); changed back to last version by 24.20.109.51

24.20.109.51: /* A little more about ''fd n'' in `sh` */

2006-09-13T20:13:52Z

A little more about ''fd n'' in <code>sh</code>

Ninereasons: /* Shell pipelines and redirection */

2006-07-12T21:45:56Z

Shell pipelines and redirection

Ninereasons: change bullets to sub-heads

2006-06-30T01:28:34Z

change bullets to sub-heads

Ninereasons: operand (noun) => operator(verb)

2006-06-28T17:08:44Z

operand (noun) => operator(verb)

Ninereasons: /* A little more about ''fd n'' in `sh` */ - perhaps even more clear

2006-06-28T03:24:49Z

A little more about ''fd n'' in <code>sh</code>: - perhaps even more clear

Ninereasons: /* Redirection in `sh` compared to `tcsh` */

2006-06-28T03:17:13Z

Redirection in <code>sh</code> compared to <code>tcsh</code>

Ninereasons: consistent terminology

2006-06-28T03:15:21Z

consistent terminology

Ninereasons: /* A little more about ''fd n'' in `sh` */ - clarify stream swapping example

2006-06-28T02:41:52Z

A little more about ''fd n'' in <code>sh</code>: - clarify stream swapping example

Ninereasons: filename incorrect

2006-06-28T01:40:40Z

filename incorrect

@@ Line 141: / Line 141: @@
 So, what you'll see on your screen is the output of ''myprogram'' on <code>stderr</code> (not passed to ''command''), followed by errors (if any) from ''command''.  In short, what's been accomplished is, a swap of errors for output, from ''myprogram'' to ''command'', the output of which is redirected to ''errors.txt''.  There is no reliable way to do the same thing in <code>tcsh</code>.
-Also, see the [http://www.freebsd.org/cgi/man.cgi?query=mkfifo&apropos=0&sektion=1&manpath=FreeBSD+6.1-RELEASE&format=html man page] for [[mkfifo]], a utility for creating arbitrary file descriptor files.
+Also, see the [http://www.freebsd.org/cgi/man.cgi?query=mkfifo&apropos=0&sektion=1&manpath=FreeBSD+6.1-RELEASE&format=html man page] for [[mkfifo]], a utility for creating arbitrary [[FIFO]] files.
 [[Category:FreeBSD Terminology]] [[Category:Common Tasks]]

@@ Line 34: / Line 34: @@
 Redirection means that, the file descriptor is temporarily reassigned to somewhere other than the terminal device (a file, a pipe, another file descriptor).  When the next process inherits the open ''fd'' from the shell the stream of data is passed along, for example, by writing to <code>stdout</code>, which has been temporarily reassigned to the <code>stdin</code> for the next process by means of a ''pipe'' ('''|''').  This handing off of open file descriptors by the shell, from one sub-process to another, is called a "shell pipeline".
-There are ''fd n'' (3-9) descriptors potentially available, but only 0-2 are preconnected to the shell; the others must be created from the shell.  In some shells, a standard file descriptor can be detached and reassigned to another file descriptor for as long as the terminal device is open,  using the [[exec]] command (examples appear below).  This capability is just one case among many of why <code>sh</code> is useful for scripting, and <code>tcsh</code> (which lacks it) is not (''see the [http://www.faqs.org/faqs/unix-faq/shell/csh-whynot/ FAQ]).
+There are ''fd n'' (0-9) descriptors potentially available, but only 0-2 are preconnected to the shell; the others must be created from the shell.  In some shells, a standard file descriptor can be detached and reassigned to another file descriptor for as long as the terminal device is open,  using the [[exec]] command (examples appear below).  This capability is just one case among many of why <code>sh</code> is useful for scripting, and <code>tcsh</code> (which lacks it) is not (''see the [http://www.faqs.org/faqs/unix-faq/shell/csh-whynot/ FAQ]).
 == Redirection in <code>sh</code> compared to <code>tcsh</code> ==

@@ Line 39: / Line 39: @@
 The c-shells (<code>tcsh</code> or <code>csh</code>) and the Bourne shells (<code>sh</code> or <code>bash</code>) do not handle redirection or piping in quite the same way.
-* '''tcsh and sh'''
+=== ''tcsh'' and ''sh'' ===
-*# Write output to a file
+# Write output to a file
-*#: % mycmd > out.txt
+#: % mycmd > out.txt
-*#:: <span style=color:blue >stderr stuff</span>
+#:: <span style=color:blue >stderr stuff</span>
-*# Append output to a file
+# Append output to a file
-*#: % mycmd >> out.txt
+#: % mycmd >> out.txt
-*#:: <span style=color:blue >stderr stuff</span>
+#:: <span style=color:blue >stderr stuff</span>
-*# Redirect the output of a remote command to ''local.txt''.
+# Redirect the output of a remote command to ''local.txt''.
-*#: % localcmd "remotecmd" > local.txt
+#: % localcmd "remotecmd" > local.txt
-*#:: <span style=color:blue >stderr stuff</span>
+#:: <span style=color:blue >stderr stuff</span>
-*# Same command as above, showing only the changes compared to ''local.txt''.<br />''Note: Many programs recognize '-' as a shortcut for '/dev/stdin'. These two commands are equivalent. ''
+# Same command as above, showing only the changes compared to ''local.txt''.<br />''Note: Many programs recognize '-' as a shortcut for '/dev/stdin'. These two commands are equivalent. ''
-*#: % localcmd "remotecmd" | diff /dev/stdin local.txt
+#: % localcmd "remotecmd" | diff /dev/stdin local.txt
-*#: % localcmd "remotecmd" | diff - local.txt
+#: % localcmd "remotecmd" | diff - local.txt
-*#:: <span style=color:blue >stderr stuff</span>
+#:: <span style=color:blue >stderr stuff</span>
-*# Direct <code>stdout</code>+<code>stderr</code> to file
+# Direct <code>stdout</code>+<code>stderr</code> to file
-*#: % mycmd >& out.txt
+#: % mycmd >& out.txt
-*# Sort lines of jumble.txt into sorted.txt<br />''Note: the sequence in which redirection appears is not important.  All of the following are exactly equivalent.''
+# Sort lines of jumble.txt into sorted.txt<br />''Note: the sequence in which redirection appears is not important.  All of the following are exactly equivalent.''
-*#: % <jumble.txt sort >sorted.txt
+#: % <jumble.txt sort >sorted.txt
-*#: % >sorted.txt sort <jumble.txt
+#: % >sorted.txt sort <jumble.txt
-*#: % sort < jumble.txt >sorted.txt
+#: % sort < jumble.txt >sorted.txt
-*#: % <jumble.txt>sorted.txt sort
+#: % <jumble.txt>sorted.txt sort
-*#:: <span style=color:blue>stderr stuff</span>
+#:: <span style=color:blue>stderr stuff</span>
-*# Sort unique lines of jumble.txt into sorted.txt
+# Sort unique lines of jumble.txt into sorted.txt
-*#: % <jumble.txt sort | uniq >sorted.txt
+#: % <jumble.txt sort | uniq >sorted.txt
-*#:: <span style=color:blue>stderr stuff</span>
+#:: <span style=color:blue>stderr stuff</span>
-*# Sort ''HERE doc'' delimited by "lines"
+# Sort ''HERE doc'' delimited by "lines"
-*#: % <<lines sort<br />
+#: % <<lines sort<br />
-*#::? a second line<br />
+#::? a second line<br />
-*#::? a first line<br />
+#::? a first line<br />
-*#::lines<br /><span style=color:green >a first line</span><br /><span style=color:green >a second line</span><br /><span style=color:blue>stderr stuff</span>
+#::lines<br /><span style=color:green >a first line</span><br /><span style=color:green >a second line</span><br /><span style=color:blue>stderr stuff</span>
-* '''tcsh''' only
+=== ''tcsh'' only ===
-*# Discard errors, watch output (probably evil)<br />''Note: There is no reliable way to do this in tcsh. Here we exploit the fact that terminal reads from <code>stdin</code>.  ''
+# Discard errors, watch output (probably evil)<br />''Note: There is no reliable way to do this in tcsh. Here we exploit the fact that terminal reads from <code>stdin</code>.  ''
-*#: % ( mycmd > /dev/stdin ) > & /dev/null
+#: % ( mycmd > /dev/stdin ) > & /dev/null
-*#:: <span style=color:green  > stdout stuff</span>
+#:: <span style=color:green  > stdout stuff</span>
-*#: % ( mycmd > /dev/tty ) > & /dev/null
+#: % ( mycmd > /dev/tty ) > & /dev/null
-*#:: <span style=color:green > stdout stuff</span>
+#:: <span style=color:green > stdout stuff</span>
-*# Append output to out.txt; discard messages
+# Append output to out.txt; discard messages
-*#: % (mycmd >> out.txt) >& /dev/null
+#: % (mycmd >> out.txt) >& /dev/null
-*# Write output to out.txt; store and watch errors<br />''Note: Compare the same task in ''sh''. ''
+# Write output to out.txt; store and watch errors<br />''Note: Compare the same task in ''sh''. ''
-*#: % ( mycmd > out.txt ) | & tee err.txt
+#: % ( mycmd > out.txt ) | & tee err.txt
-*#:: <span style=color:blue >stderr stuff</span>
+#:: <span style=color:blue >stderr stuff</span>
-* '''sh''' only
+=== ''sh'' only ===
-*# Discard errors, watch output
+# Discard errors, watch output
-*#: $  mycmd 2> /dev/null
+#: $  mycmd 2> /dev/null
-*#:: <span style=color:green  > stdout stuff</span>
+#:: <span style=color:green  > stdout stuff</span>
-*# Append output to out.txt; discard messages
+# Append output to out.txt; discard messages
-*#: $ mycmd 2> /dev/null >> out.txt
+#: $ mycmd 2> /dev/null >> out.txt
-*# Write output to out.txt; store and watch errors<br />''Note: Compare the same task in ''tcsh''. ''
+# Write output to out.txt; store and watch errors<br />''Note: Compare the same task in ''tcsh''. ''
-*#: $ mycmd 2>&1 > out.txt | tee err.txt
+#: $ mycmd 2>&1 > out.txt | tee err.txt
-*#:: <span style=color:blue >stderr stuff</span>
+#:: <span style=color:blue >stderr stuff</span>
-*# Write messages to err.txt; write output to out.txt and copy output to terminal
+# Write messages to err.txt; write output to out.txt and copy output to terminal
-*#: $ mycmd 2> err.txt | tee out.txt
+#: $ mycmd 2> err.txt | tee out.txt
-*#:: <span style=color:green>stdout stuff</span>
+#:: <span style=color:green>stdout stuff</span>
-*# Assign a variable from stored.txt
+# Assign a variable from stored.txt
-*#: $ <stored.txt read var; mycmd $var
+#: $ <stored.txt read var; mycmd $var
-*#:: <span style=color:blue>stderr stuff</span>
+#:: <span style=color:blue>stderr stuff</span>
-*#:: <span style=color:green>stdout stuff</span>
+#:: <span style=color:green>stdout stuff</span>
-*# Assign first three lines of stored.txt to three different variables
+# Assign first three lines of stored.txt to three different variables
-*#: $ exec 3<&0; exec <stored.txt; read v1; read v2; read v3; exec 0<&3 3<&-; echo $v1 $v2 $v3
+#: $ exec 3<&0; exec <stored.txt; read v1; read v2; read v3; exec 0<&3 3<&-; echo $v1 $v2 $v3
-*#:: <span style=color:blue>stderr stuff</span>
+#:: <span style=color:blue>stderr stuff</span>
-*#:: <span style=color:green>stdout stuff</span>
+#:: <span style=color:green>stdout stuff</span>
-*# Use all unique lines in stored.txt as an argument variable for ''mycmd'', appending to result.txt
+# Use all unique lines in stored.txt as an argument variable for ''mycmd'', appending to result.txt
-*#: $ exec 3<&0; exec <stored.txt; sort | uniq | while read line; do mycmd $line >> result.txt ; done; exec 0<&3 3<&-;
+#: $ exec 3<&0; exec <stored.txt; sort | uniq | while read line; do mycmd $line >> result.txt ; done; exec 0<&3 3<&-;
-*#:: <span style=color:blue>stderr stuff</span>
+#:: <span style=color:blue>stderr stuff</span>
-*: ''Note:'' By default, redirection pointed right represents <code>stdout</code>, so that these two commands are exactly equivalent:
+: ''Note:'' By default, redirection pointed right represents <code>stdout</code>, so that these two commands are exactly equivalent:
-*:: $ mycmd 1> out.txt
+:: $ mycmd 1> out.txt
-*:: $ mycmd > out.txt
+:: $ mycmd > out.txt
-*::: <span style=color:blue >stderr stuff</span>
+::: <span style=color:blue >stderr stuff</span>
-*: ''Note:'' By default, redirection pointed left represents <code>stdin</code>, so that these two commands are exactly equivalent:
+: ''Note:'' By default, redirection pointed left represents <code>stdin</code>, so that these two commands are exactly equivalent:
-*:: $ mycmd 0< in.txt
+:: $ mycmd 0< in.txt
-*:: $ mycmd < in.txt
+:: $ mycmd < in.txt
-*::: <span style=color:blue >stderr stuff</span>
+::: <span style=color:blue >stderr stuff</span>
-*::: <span style=color:green >stdout stuff</span>
+::: <span style=color:green >stdout stuff</span>
-*: ''Note:'' To close a file descriptor, say <code>n<&-</code>
+: ''Note:'' To close a file descriptor, say <code>n<&-</code>
-*:: $ mycmd >out.txt 1<&-;
+:: $ mycmd >out.txt 1<&-;
-*:: ''-sh: fails without messages''
+:: ''-sh: fails without messages''
-*:: <span style=color:blue >-bash: mycmd: write error: Bad file descriptor</span>
+:: <span style=color:blue >-bash: mycmd: write error: Bad file descriptor</span>
 === A little more about ''fd n'' in <code>sh</code> ===

← Older revision		Revision as of 17:08, 28 June 2006
Line 1:		Line 1:
−	'''Redirection''' is sending the output of a program to somewhere other than where it would otherwise go - for example you can '''redirect''' the output of an [[ls]] command to a text file for later processing or to the [[grep]] command for filtering. ~~Common operands~~ include: '''>''', '''>>''', '''<''', '''<<''', and the ever popular '''\|'''.	+	'''Redirection''' is sending the output of a program to somewhere other than where it would otherwise go - for example you can '''redirect''' the output of an [[ls]] command to a text file for later processing or to the [[grep]] command for filtering. Shell redirection operators include: '''>''', '''>>''', '''<''', '''<<''', and the ever popular '''\|'''.

	> sends output to a file (may include [[special files]] such as [[/dev/null]])		> sends output to a file (may include [[special files]] such as [[/dev/null]])

@@ Line 124: / Line 124: @@
   samizdata# myprogram 1>&2 2>&1 | command > errors.txt
 because when you switched output to the messages file descriptor, it's done ''right away''.  When you tried to switch errors to output there's nothing left to pipe. It's exactly as though you had said
-  samizdata# myprogram 1>&2 | command > errors.txt
+  samizdata# myprogram 1>/dev/stderr | command > errors.txt
 ''command'' receives no data, because it's all been sent to your screen.  ''errors.txt'' will be empty.  If you had tried it the other way, it's just as though you would have said:
-  samizdata# myprogram 2>&1 | command > errors.txt
+  samizdata# myprogram 2>/dev/stdout | command > errors.txt
 In which case, errors and output are both piped to ''command'', which was not what you wanted.