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>Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH
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><A
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></A
>Chapter 36. Miscellany</H1
><TABLE
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><TD
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>&nbsp;</TD
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><I
><P
><I
>Nobody really knows what the Bourne shell's grammar is. Even
        examination of the source code is little help.</I
></P
><P
><I
>--Tom Duff</I
></P
></I
></TD
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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="INTANDNONINT"
></A
>36.1. Interactive and non-interactive shells and scripts</H1
><P
>An <I
CLASS="FIRSTTERM"
>interactive</I
> shell reads
	   commands from user input on a <TT
CLASS="FILENAME"
>tty</TT
>. Among
	   other things, such a shell reads startup files on activation,
	   displays a prompt, and enables job control by default. The
	   user can <I
CLASS="FIRSTTERM"
>interact</I
> with the shell.</P
><P
>A shell running a script is always a non-interactive
	   shell. All the same, the script can still access its
	   <TT
CLASS="FILENAME"
>tty</TT
>. It is even possible to emulate an
	   interactive shell in a script.

	   <TABLE
BORDER="0"
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WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;MY_PROMPT='$ '
   3&nbsp;while :
   4&nbsp;do
   5&nbsp;  echo -n "$MY_PROMPT"
   6&nbsp;  read line
   7&nbsp;  eval "$line"
   8&nbsp;  done
   9&nbsp;
  10&nbsp;exit 0
  11&nbsp;
  12&nbsp;# This example script, and much of the above explanation supplied by
  13&nbsp;# Stphane Chazelas (thanks again).</PRE
></TD
></TR
></TABLE
></P
><P
>Let us consider an <I
CLASS="FIRSTTERM"
>interactive</I
>
	   script to be one that requires input from the user, usually
	   with <A
HREF="internal.html#READREF"
>read</A
> statements (see <A
HREF="internal.html#EX36"
>Example 15-3</A
>). <SPAN
CLASS="QUOTE"
>"Real life"</SPAN
> is actually a
	   bit messier than that. For now, assume an interactive script
	   is bound to a tty, a script that a user has invoked from the
	   console or an <I
CLASS="FIRSTTERM"
>xterm</I
>.</P
><P
>Init and startup scripts are necessarily non-interactive,
	   since they must run without human intervention. Many
	   administrative and system maintenance scripts are likewise
	   non-interactive.  Unvarying repetitive tasks cry out for
	   automation by non-interactive scripts.</P
><P
>Non-interactive scripts can run in the background, but
	   interactive ones hang, waiting for input that never comes.
	   Handle that difficulty by having an <B
CLASS="COMMAND"
>expect</B
>
	   script or embedded <A
HREF="here-docs.html#HEREDOCREF"
>here
	   document</A
> feed input to an interactive script running
	   as a background job.  In the simplest case, redirect a
	   file to supply input to a <B
CLASS="COMMAND"
>read</B
> statement
	   (<B
CLASS="COMMAND"
>read variable &#60;file</B
>).  These particular
	   workarounds make possible general purpose scripts that run
	   in either interactive or non-interactive modes.</P
><P
>If a script needs to test whether it is running in an
	   interactive shell, it is simply a matter of finding
	   whether the <I
CLASS="FIRSTTERM"
>prompt</I
> variable, <A
HREF="variables2.html#PS1REF"
>$PS1</A
> is set. (If the user is being
	   prompted for input, then the script needs to display a
	   prompt.)</P
><P
><TABLE
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><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;if [ -z $PS1 ] # no prompt?
   2&nbsp;### if [ -v PS1 ]   # On Bash 4.2+ ...
   3&nbsp;then
   4&nbsp;  # non-interactive
   5&nbsp;  ...
   6&nbsp;else
   7&nbsp;  # interactive
   8&nbsp;  ...
   9&nbsp;fi</PRE
></TD
></TR
></TABLE
></P
><P
><A
NAME="IITEST"
></A
>Alternatively, the script can test
	   for the presence of option <SPAN
CLASS="QUOTE"
>"i"</SPAN
> in the <A
HREF="variables2.html#FLPREF"
>$-</A
> flag.</P
><P
><TABLE
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><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;case $- in
   2&nbsp;*i*)    # interactive shell
   3&nbsp;;;
   4&nbsp;*)      # non-interactive shell
   5&nbsp;;;
   6&nbsp;# (Courtesy of "UNIX F.A.Q.," 1993)</PRE
></TD
></TR
></TABLE
></P
><P
><A
NAME="II2TEST"
></A
>However, John Lange describes
             an alternative method, using the <A
HREF="fto.html#TERMTEST"
><SPAN
CLASS="TOKEN"
>-t</SPAN
>
	     <I
CLASS="FIRSTTERM"
>test</I
> operator</A
>.</P
><P
><TABLE
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><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;# Test for a terminal!
   2&nbsp;
   3&nbsp;fd=0   # stdin
   4&nbsp;
   5&nbsp;#  As we recall, the -t test option checks whether the stdin, [ -t 0 ],
   6&nbsp;#+ or stdout, [ -t 1 ], in a given script is running in a terminal.
   7&nbsp;if [ -t "$fd" ]
   8&nbsp;then
   9&nbsp;  echo interactive
  10&nbsp;else
  11&nbsp;  echo non-interactive
  12&nbsp;fi
  13&nbsp;
  14&nbsp;
  15&nbsp;#  But, as John points out:
  16&nbsp;#    if [ -t 0 ] works ... when you're logged in locally
  17&nbsp;#    but fails when you invoke the command remotely via ssh.
  18&nbsp;#    So for a true test you also have to test for a socket.
  19&nbsp;
  20&nbsp;if [[ -t "$fd" || -p /dev/stdin ]]
  21&nbsp;then
  22&nbsp;  echo interactive
  23&nbsp;else
  24&nbsp;  echo non-interactive
  25&nbsp;fi</PRE
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><P
>Scripts may be forced to run in interactive
	   mode with the <SPAN
CLASS="TOKEN"
>-i</SPAN
> option or with a
	   <TT
CLASS="USERINPUT"
><B
>#!/bin/bash -i</B
></TT
> header. Be aware that
	   this can cause erratic script behavior or show error messages
	   even when no error is present.</P
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