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>A Detailed Introduction to I/O and I/O Redirection</TITLE
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>Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH
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><H1
><A
NAME="IOREDIRINTRO"
></A
>Appendix F. A Detailed Introduction to I/O and I/O Redirection</H1
><P
><SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>written by Stphane Chazelas, and revised
        by the document author</I
></SPAN
></P
><P
><A
NAME="STDINOUTDEF"
></A
></P
><P
>A command expects the first three <A
HREF="io-redirection.html#FDREF"
>file
        descriptors</A
> to be available. The first, <I
CLASS="FIRSTTERM"
>fd
        0</I
> (standard input, <TT
CLASS="FILENAME"
>stdin</TT
>),
        is for reading. The other two (<I
CLASS="FIRSTTERM"
>fd 1</I
>,
        <TT
CLASS="FILENAME"
>stdout</TT
> and <I
CLASS="FIRSTTERM"
>fd 2</I
>,
        <TT
CLASS="FILENAME"
>stderr</TT
>) are for writing.</P
><P
>There is a <TT
CLASS="FILENAME"
>stdin</TT
>, <TT
CLASS="FILENAME"
>stdout</TT
>,
        and a <TT
CLASS="FILENAME"
>stderr</TT
> associated with each command.
        <TT
CLASS="USERINPUT"
><B
>ls 2&#62;&#38;1</B
></TT
> means temporarily connecting the
        <TT
CLASS="FILENAME"
>stderr</TT
> of the <B
CLASS="COMMAND"
>ls</B
> command to the
        same <SPAN
CLASS="QUOTE"
>"resource"</SPAN
> as the shell's
        <TT
CLASS="FILENAME"
>stdout</TT
>.</P
><P
>By convention, a command reads its input from fd 0
        (<TT
CLASS="FILENAME"
>stdin</TT
>), prints normal output to fd
        1 (<TT
CLASS="FILENAME"
>stdout</TT
>), and error ouput to fd 2
        (<TT
CLASS="FILENAME"
>stderr</TT
>). If one of those three fd's is
        not open, you may encounter problems:</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>cat /etc/passwd &#62;&#38;-</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>cat: standard output: Bad file descriptor</TT
>
       </PRE
></TD
></TR
></TABLE
><P
>For example, when <B
CLASS="COMMAND"
>xterm</B
> runs, it first
        initializes itself.  Before running the user's shell,
        <B
CLASS="COMMAND"
>xterm</B
> opens the terminal device
        (/dev/pts/&#60;n&#62; or something similar) three times.</P
><P
>At this point, Bash inherits these three file descriptors,
        and each command (child process) run by Bash inherits
        them in turn, except when you redirect the command.  <A
HREF="io-redirection.html#IOREDIRREF"
>Redirection</A
> means reassigning
        one of the file descriptors to another file (or a pipe, or
        anything permissible). File descriptors may be reassigned
        locally (for a command, a command group, a <A
HREF="subshells.html#SUBSHELLSREF"
>subshell</A
>, a <A
HREF="redircb.html#REDIRREF"
>while or if or case or for loop</A
>...),
        or globally, for the remainder of the shell (using <A
HREF="internal.html#EXECREF"
>exec</A
>).</P
><P
><TT
CLASS="USERINPUT"
><B
>ls &#62; /dev/null</B
></TT
> means
        running <B
CLASS="COMMAND"
>ls</B
> with its fd 1 connected to
        <TT
CLASS="FILENAME"
>/dev/null</TT
>.</P
><P
>      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>lsof -a -p $$ -d0,1,2</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>COMMAND PID     USER   FD   TYPE DEVICE SIZE NODE NAME
 bash    363 bozo        0u   CHR  136,1         3 /dev/pts/1
 bash    363 bozo        1u   CHR  136,1         3 /dev/pts/1
 bash    363 bozo        2u   CHR  136,1         3 /dev/pts/1</TT
>
 
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>exec 2&#62; /dev/null</B
></TT
>
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>lsof -a -p $$ -d0,1,2</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>COMMAND PID     USER   FD   TYPE DEVICE SIZE NODE NAME
 bash    371 bozo        0u   CHR  136,1         3 /dev/pts/1
 bash    371 bozo        1u   CHR  136,1         3 /dev/pts/1
 bash    371 bozo        2w   CHR    1,3       120 /dev/null</TT
>
 
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>bash -c 'lsof -a -p $$ -d0,1,2' | cat</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>COMMAND PID USER   FD   TYPE DEVICE SIZE NODE NAME
 lsof    379 root    0u   CHR  136,1         3 /dev/pts/1
 lsof    379 root    1w  FIFO    0,0      7118 pipe
 lsof    379 root    2u   CHR  136,1         3 /dev/pts/1</TT
>
 
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo "$(bash -c 'lsof -a -p $$ -d0,1,2' 2&#62;&#38;1)"</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>COMMAND PID USER   FD   TYPE DEVICE SIZE NODE NAME
 lsof    426 root    0u   CHR  136,1         3 /dev/pts/1
 lsof    426 root    1w  FIFO    0,0      7520 pipe
 lsof    426 root    2w  FIFO    0,0      7520 pipe</TT
></PRE
></TD
></TR
></TABLE
></P
><P
>This works for different types of redirection.</P
><P
><TT
CLASS="USERINPUT"
><B
>Exercise:</B
></TT
> Analyze the following script.

<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#! /usr/bin/env bash
   2&nbsp;
   3&nbsp;mkfifo /tmp/fifo1 /tmp/fifo2
   4&nbsp;while read a; do echo "FIFO1: $a"; done &#60; /tmp/fifo1 &#38; exec 7&#62; /tmp/fifo1
   5&nbsp;exec 8&#62; &#62;(while read a; do echo "FD8: $a, to fd7"; done &#62;&#38;7)
   6&nbsp;
   7&nbsp;exec 3&#62;&#38;1
   8&nbsp;(
   9&nbsp; (
  10&nbsp;  (
  11&nbsp;   while read a; do echo "FIFO2: $a"; done &#60; /tmp/fifo2 | tee /dev/stderr \
  12&nbsp;   | tee /dev/fd/4 | tee /dev/fd/5 | tee /dev/fd/6 &#62;&#38;7 &#38; exec 3&#62; /tmp/fifo2
  13&nbsp;
  14&nbsp;   echo 1st, to stdout
  15&nbsp;   sleep 1
  16&nbsp;   echo 2nd, to stderr &#62;&#38;2
  17&nbsp;   sleep 1
  18&nbsp;   echo 3rd, to fd 3 &#62;&#38;3
  19&nbsp;   sleep 1
  20&nbsp;   echo 4th, to fd 4 &#62;&#38;4
  21&nbsp;   sleep 1
  22&nbsp;   echo 5th, to fd 5 &#62;&#38;5
  23&nbsp;   sleep 1
  24&nbsp;   echo 6th, through a pipe | sed 's/.*/PIPE: &#38;, to fd 5/' &#62;&#38;5
  25&nbsp;   sleep 1
  26&nbsp;   echo 7th, to fd 6 &#62;&#38;6
  27&nbsp;   sleep 1
  28&nbsp;   echo 8th, to fd 7 &#62;&#38;7
  29&nbsp;   sleep 1
  30&nbsp;   echo 9th, to fd 8 &#62;&#38;8
  31&nbsp;
  32&nbsp;  ) 4&#62;&#38;1 &#62;&#38;3 3&#62;&#38;- | while read a; do echo "FD4: $a"; done 1&#62;&#38;3 5&#62;&#38;- 6&#62;&#38;-
  33&nbsp; ) 5&#62;&#38;1 &#62;&#38;3 | while read a; do echo "FD5: $a"; done 1&#62;&#38;3 6&#62;&#38;-
  34&nbsp;) 6&#62;&#38;1 &#62;&#38;3 | while read a; do echo "FD6: $a"; done 3&#62;&#38;-
  35&nbsp;
  36&nbsp;rm -f /tmp/fifo1 /tmp/fifo2
  37&nbsp;
  38&nbsp;
  39&nbsp;# For each command and subshell, figure out which fd points to what.
  40&nbsp;# Good luck!
  41&nbsp;
  42&nbsp;exit 0</PRE
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