ch45_22.htm

the unix power tools

34      esac
35  done <"$template" 2>$errors 3>"$completed"
36  
37  if [ -s $errors ]; then
38      /bin/cat $errors 1>&2
39      echo "$myname: should you remove '$completed' file?" 1>&2
40  fi</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
><B
CLASS="emphasis.bold"
>Line&nbsp;10</B
> uses the
<SPAN
CLASS="link"
><CODE
CLASS="literal"
>4&lt;&amp;0</CODE
> operator (<A
CLASS="linkend"
HREF="ch45_21.htm"
TITLE="n&gt;&amp;m: Swap Standard Output and Standard Error "
>45.21</A
>)</SPAN
>
to save the location of the original standard input&nbsp;- usually your
terminal, but not always&nbsp;- as file descriptor 4.
[2]
(We'll need to read that original <EM
CLASS="emphasis"
>stdin</EM
> in line 24.)</P
><BLOCKQUOTE
CLASS="footnote"
><P
CLASS="para"
>[2] We can't assume that standard input is coming from a terminal.
If we do, it prevents you from running <EM
CLASS="emphasis"
>formprog</EM
> this way:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>% <CODE
CLASS="replaceable"
><I
>command-generator-program</I
></CODE
> <CODE
CLASS="userinput"
><B
>| formprog</B
></CODE
>
% <CODE
CLASS="userinput"
><B
>formprog &lt; </B
></CODE
><CODE
CLASS="replaceable"
><I
>command-file</I
></CODE
></PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
></P
></BLOCKQUOTE
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-53944"
></A
><A
CLASS="indexterm"
NAME="AUTOID-53947"
></A
><A
CLASS="indexterm"
NAME="AUTOID-53950"
></A
><A
CLASS="indexterm"
NAME="AUTOID-53953"
></A
>During <B
CLASS="emphasis.bold"
>lines&nbsp;11-35</B
> of the redirected-I/O <EM
CLASS="emphasis"
>while</EM
> loop:
all commands' standard input comes from the file named in <CODE
CLASS="literal"
>$template</CODE
>,
all standard error goes to the <CODE
CLASS="literal"
>$errors</CODE
> file,
and anything written to file descriptor 3 is added to the <CODE
CLASS="literal"
>$completed</CODE
>
file.
UNIX keeps file pointers for all those open files&nbsp;- so each read and
write is done just past the end of the previous one.</P
><P
CLASS="para"
>Here's what happens each time the loop is executed:</P
><OL
CLASS="orderedlist"
><LI
CLASS="listitem"
><P
CLASS="para"
>The
<SPAN
CLASS="link"
><EM
CLASS="emphasis"
>read</EM
> command (<A
CLASS="linkend"
HREF="ch44_13.htm"
TITLE="read: Reading from the Keyboard "
>44.13</A
>)</SPAN
>
in <B
CLASS="emphasis.bold"
>line 11</B
> reads the next line from its standard input&nbsp;- that's the open
<CODE
CLASS="literal"
>$template</CODE
> file.</P
></LI
><LI
CLASS="listitem"
><P
CLASS="para"
>The
<SPAN
CLASS="link"
><EM
CLASS="emphasis"
>case</EM
> (<A
CLASS="linkend"
HREF="ch44_05.htm"
TITLE="Test String Values with Bourne Shell case "
>44.5</A
>)</SPAN
>
in lines&nbsp;15-31 checks the text from the
<CODE
CLASS="literal"
>$template</CODE
> file:</P
><UL
CLASS="itemizedlist"
><LI
CLASS="listitem"
><P
CLASS="para"
>If the text has both a label -ding with a colon (<CODE
CLASS="literal"
>:</CODE
>)) and some
other text (stored in <CODE
CLASS="literal"
>$text</CODE
>), the complete line is written two places.
<B
CLASS="emphasis.bold"
>Line&nbsp;17</B
> writes the line to the standard output&nbsp;- which is probably
your screen (it's not redirected by the script, anyway).
<B
CLASS="emphasis.bold"
>Line&nbsp;18</B
> writes the line to file descriptor 3, the open
<CODE
CLASS="literal"
>$completed</CODE
> file.</P
></LI
><LI
CLASS="listitem"
><P
CLASS="para"
>If the text has just a label, <B
CLASS="emphasis.bold"
>line&nbsp;21</B
> writes the label to
standard output (usually your terminal) without a newline.
We want to read the answer, at <B
CLASS="emphasis.bold"
>line&nbsp;24</B
>, but there's a problem:
on some Bourne shells, the <EM
CLASS="emphasis"
>read</EM
> command can only read from file
descriptor 0 and won't let you use operators like <CODE
CLASS="literal"
>&lt;&amp;4</CODE
> on its
command line.</P
><P
CLASS="para"
>So, in <B
CLASS="emphasis.bold"
>line&nbsp;22</B
>, we save a copy of the open <CODE
CLASS="literal"
>$template</CODE
>
file descriptor <EM
CLASS="emphasis"
>and the location of the open file pointer</EM
> in file
descriptor 5.
<B
CLASS="emphasis.bold"
>Line&nbsp;23</B
> changes standard input so the <EM
CLASS="emphasis"
>read</EM
> in line&nbsp;24 will
read from the right place (usually the terminal).
<B
CLASS="emphasis.bold"
>Line&nbsp;25</B
> adjusts standard input so the next <EM
CLASS="emphasis"
>read</EM
> at the top of
the loop (line&nbsp;11) will come from the <CODE
CLASS="literal"
>$template</CODE
> file.</P
><P
CLASS="para"
>If line&nbsp;24 doesn't read an answer, <B
CLASS="emphasis.bold"
>line&nbsp;27</B
> does not write a line.
Otherwise, <B
CLASS="emphasis.bold"
>line&nbsp;28</B
> writes the line to file descriptor 3, the open
<CODE
CLASS="literal"
>$completed</CODE
> file.</P
></LI
><LI
CLASS="listitem"
><P
CLASS="para"
>If the template label doesn't end with a colon, <B
CLASS="emphasis.bold"
>line&nbsp;33</B
> writes
a message to <EM
CLASS="emphasis"
>stderr</EM
> (file descriptor 2).
These messages, together with messages to <EM
CLASS="emphasis"
>stderr</EM
> from any other
command in the loop, are redirected into the <CODE
CLASS="literal"
>$errors</CODE
> file.
After the loop, if the
<SPAN
CLASS="link"
>test (<A
CLASS="linkend"
HREF="ch44_20.htm"
TITLE="test: Testing Files and Strings "
>44.20</A
>)</SPAN
>
in <B
CLASS="emphasis.bold"
>line&nbsp;37</B
> sees any text in the file,
the text is displayed in <B
CLASS="emphasis.bold"
>line&nbsp;38</B
> and the script prints a
warning.</P
></LI
></UL
></LI
></OL
><P
CLASS="para"
>The loop keeps reading and writing line by line until the <EM
CLASS="emphasis"
>read</EM
>
at the top of the loop reaches the end-of-file of <CODE
CLASS="literal"
>$template</CODE
>.<A
CLASS="indexterm"
NAME="AUTOID-54013"
></A
><A
CLASS="indexterm"
NAME="AUTOID-54014"
></A
><A
CLASS="indexterm"
NAME="AUTOID-54015"
></A
></P
><DIV
CLASS="sect1info"
><P
CLASS="SECT1INFO"
>- <SPAN
CLASS="authorinitials"
>JP</SPAN
></P
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