ch13_01.htm

the unix power tools

> 2&gt;&amp;1</TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>Take <EM
CLASS="emphasis"
>stdin</EM
> from <EM
CLASS="emphasis"
>file</EM
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>prog &lt; <CODE
CLASS="replaceable"
><I
>file</I
></CODE
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>prog &lt; <CODE
CLASS="replaceable"
><I
>file</I
></CODE
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>Send <EM
CLASS="emphasis"
>stdout</EM
> to end of <EM
CLASS="emphasis"
>file</EM
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>prog &gt;&gt; <CODE
CLASS="replaceable"
><I
>file</I
></CODE
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>prog &gt;&gt; <CODE
CLASS="replaceable"
><I
>file</I
></CODE
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>Send <EM
CLASS="emphasis"
>stderr</EM
> to end of <EM
CLASS="emphasis"
>file</EM
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>prog 2&gt;&gt; <CODE
CLASS="replaceable"
><I
>file</I
></CODE
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>Send <EM
CLASS="emphasis"
>stdout</EM
> and <EM
CLASS="emphasis"
>stderr</EM
> to end of <EM
CLASS="emphasis"
>file</EM
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>prog &gt;&gt;&amp; <CODE
CLASS="replaceable"
><I
>file</I
></CODE
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>prog &gt;&gt; <CODE
CLASS="replaceable"
><I
>file</I
></CODE
> 2&gt;&amp;1</TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>Read <EM
CLASS="emphasis"
>stdin</EM
> from keyboard until <EM
CLASS="emphasis"
>c</EM
> (see
article <A
CLASS="xref"
HREF="ch08_18.htm"
TITLE="Here Documents "
>8.18</A
>)</TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>prog &lt;&lt;<CODE
CLASS="replaceable"
><I
>c</I
></CODE
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>prog &lt;&lt;<CODE
CLASS="replaceable"
><I
>c</I
></CODE
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>Pipe <EM
CLASS="emphasis"
>stdout</EM
> to <EM
CLASS="emphasis"
>prog2</EM
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>prog | prog2</TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>prog | prog2</TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>Pipe <EM
CLASS="emphasis"
>stdout</EM
> and <EM
CLASS="emphasis"
>stderr</EM
> to <EM
CLASS="emphasis"
>prog2</EM
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>prog |&amp; prog2</TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>prog 2&gt;&amp;1 | prog2</TD
></TR
></TBODY
></TABLE
><P
CLASS="para"
>Be aware that:</P
><UL
CLASS="itemizedlist"
><LI
CLASS="listitem"
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-14086"
></A
>While standard I/O is a basic feature of UNIX, the syntax used to
redirect standard I/O depends on the shell you are using. Bourne
shell syntax and C shell syntax differ, particularly when you get into
the less commonly used features. The Korn shell and <EM
CLASS="emphasis"
>bash</EM
> are
the same as the Bourne shell, but with a few twists of their own.&#13;</P
></LI
><LI
CLASS="listitem"
><P
CLASS="para"
>You can redirect standard input and standard output in the same command line.
For example, to read from the file <EM
CLASS="emphasis"
>input</EM
> and write to the file
<EM
CLASS="emphasis"
>output</EM
>, give the command:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>% <CODE
CLASS="userinput"
><B
>prog &lt; input &gt; output</B
></CODE
></PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>The Bourne shell will let you go further and write <EM
CLASS="emphasis"
>stderr</EM
> to a third file:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>$ <CODE
CLASS="userinput"
><B
>prog &lt; input &gt; output 2&gt; errors</B
></CODE
></PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
></P
></LI
><LI
CLASS="listitem"
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-14105"
></A
>The C shell doesn't give you an easy way to redirect
standard output without redirecting standard error. A simple trick
will help you do this. To put standard output and standard error in
different files, give a command like:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>% <CODE
CLASS="userinput"
><B
>( prog &gt; output ) &gt;&amp; errors</B
></CODE
></PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>We'll discuss commands like this in articles
<A
CLASS="xref"
HREF="ch13_03.htm"
TITLE="Send (only) Standard Error Down a Pipe "
>13.3</A
>
and
<A
CLASS="xref"
HREF="ch13_05.htm"
TITLE="Redirection in C Shell: Capture Errors, Too? "
>13.5</A
>.</P
></LI
><LI
CLASS="listitem"
><P
CLASS="para"
>Many implementations of both shells don't care what order the
redirections appear in, or even where they appear on the command line.
For example, SunOS lets you type <CODE
CLASS="literal"
>&lt;</CODE
>&nbsp;<CODE
CLASS="literal"
>input</CODE
>&nbsp;<CODE
CLASS="literal"
>&gt;</CODE
>&nbsp;<CODE
CLASS="literal"
>output</CODE
>&nbsp;<CODE
CLASS="literal"
>prog</CODE
>.
However, clarity is always a virtue that computer users have never
appreciated enough. It will be easiest to understand
what you are doing if you type the command name first&nbsp;- then redirect standard
input, followed by standard output, followed by standard error.</P
></LI
></UL
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-14122"
></A
><SPAN
CLASS="link"
> There are some more complex forms of standard I/O redirection,particularly for the Bourne shell . (<A
CLASS="linkend"
HREF="ch45_21.htm"
TITLE="n&gt;&amp;m: Swap Standard Output and Standard Error "
>45.21</A
>, <A
CLASS="linkend"
HREF="ch45_22.htm"
TITLE="Handling Files Line-by-Line "
>45.22</A
>, <A
CLASS="linkend"
HREF="ch45_23.htm"
TITLE="The Ins and Outs of Redirected I/O Loops "
>45.23</A
>)</SPAN
></P
><P
CLASS="para"
>Of course, programs aren't restricted to standard I/O. They can open
other files, define their own special-purpose pipes, and write
directly to the terminal. But standard I/O is the glue that allows
you to make big programs out of smaller ones, and is therefore a
crucial part of the operating system. Most UNIX utilities 
read their data from standard input and write their output to standard
output, allowing you to combine them easily. A program that creates
its own special-purpose pipe may be very useful, but it cannot be
used in combination with standard utilities.</P
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-14128"
></A
>Some UNIX systems, and utilities such as
<SPAN
CLASS="link"
><EM
CLASS="emphasis"
>gawk</EM
> (<A
CLASS="linkend"
HREF="ch33_12.htm"
TITLE="Versions of awk "
>33.12</A
>)</SPAN
>,
support special filenames like <EM
CLASS="emphasis"
>/dev/stdin</EM
>, <EM
CLASS="emphasis"
>/dev/stdout</EM
>, and
<EM
CLASS="emphasis"
>/dev/stderr</EM
>.
You can use these just as you'd use other files.
For instance, to have any ordinary command read from the file <EM
CLASS="emphasis"
>afile</EM
>,
then standard input (from the keyboard, for example), then the file
<EM
CLASS="emphasis"
>bfile</EM
>:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>% <CODE
CLASS="replaceable"
><I
>somecmd</I
></CODE
><CODE
CLASS="userinput"
><B
> afile /dev/stdin bfile</B
></CODE
></PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>In the same way, a process can write to its standard output through
<EM
CLASS="emphasis"
>/dev/stdout</EM
> and the standard error via <EM
CLASS="emphasis"
>/dev/stderr</EM
>.<A
CLASS="indexterm"
NAME="AUTOID-14144"
></A
><A
CLASS="indexterm"
NAME="AUTOID-14145"
></A
></P
><DIV
CLASS="sect1info"
><P
CLASS="SECT1INFO"
>- <SPAN
CLASS="authorinitials"
>ML</SPAN
>, <SPAN
CLASS="authorinitials"
>JP</SPAN
></P
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