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>Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH
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><DIV
CLASS="CHAPTER"
><H1
><A
NAME="VARIABLES2"
></A
>Chapter 9. Another Look at Variables</H1
><P
>Used properly, variables can add power and flexibility
	  to scripts. This requires learning their subtleties and
	  nuances.</P
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="INTERNALVARIABLES"
></A
>9.1. Internal Variables</H1
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="REPLACEABLE"
><I
><A
HREF="internal.html#BUILTINREF"
>Builtin</A
> variables:</I
></TT
></DT
><DD
><P
>variables affecting bash script behavior</P
></DD
><DT
><A
NAME="BASHVARREF"
></A
><TT
CLASS="VARNAME"
>$BASH</TT
></DT
><DD
><P
>The path to the <I
CLASS="FIRSTTERM"
>Bash</I
>
	    binary itself
	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo $BASH</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>/bin/bash</TT
></PRE
></TD
></TR
></TABLE
>
	    </P
></DD
><DT
><A
NAME="BASHENVREF"
></A
><TT
CLASS="VARNAME"
>$BASH_ENV</TT
></DT
><DD
><P
>An <A
HREF="othertypesv.html#ENVREF"
>environmental
	  variable</A
> pointing to a Bash startup file to be read
	  when a script is invoked</P
></DD
><DT
><A
NAME="BASHSUBSHELLREF"
></A
><TT
CLASS="VARNAME"
>$BASH_SUBSHELL</TT
></DT
><DD
><P
>A variable indicating the <A
HREF="subshells.html#SUBSHELLSREF"
>subshell</A
> level. This is a
	      new addition to Bash, <A
HREF="bashver3.html#BASH3REF"
>version 3</A
>.</P
><P
>See <A
HREF="subshells.html#SUBSHELL"
>Example 21-1</A
> for usage.</P
></DD
><DT
><A
NAME="BASHPIDREF"
></A
><TT
CLASS="VARNAME"
>$BASHPID</TT
></DT
><DD
><P
><I
CLASS="FIRSTTERM"
>Process ID</I
>
            of the current instance of Bash. This is not the same as the
            <A
HREF="variables2.html#PROCCID"
>$$</A
> variable, but it often
            gives the same result.</P
><P
>		  <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash4$ </TT
><TT
CLASS="USERINPUT"
><B
>echo $$</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>11015</TT
>
 
 
 <TT
CLASS="PROMPT"
>bash4$ </TT
><TT
CLASS="USERINPUT"
><B
>echo $BASHPID</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>11015</TT
>
 
 
 <TT
CLASS="PROMPT"
>bash4$ </TT
><TT
CLASS="USERINPUT"
><B
>ps ax | grep bash4</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>11015 pts/2    R      0:00 bash4</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	    </P
><P
><A
NAME="BASHPIDREF2"
></A
>But ...</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash4
   2&nbsp;
   3&nbsp;echo "\$\$ outside of subshell = $$"                              # 9602
   4&nbsp;echo "\$BASH_SUBSHELL  outside of subshell = $BASH_SUBSHELL"      # 0
   5&nbsp;echo "\$BASHPID outside of subshell = $BASHPID"                   # 9602
   6&nbsp;
   7&nbsp;echo
   8&nbsp;
   9&nbsp;( echo "\$\$ inside of subshell = $$"                             # 9602
  10&nbsp;  echo "\$BASH_SUBSHELL inside of subshell = $BASH_SUBSHELL"      # 1
  11&nbsp;  echo "\$BASHPID inside of subshell = $BASHPID" )                # 9603
  12&nbsp;  # Note that $$ returns PID of parent process.</PRE
></TD
></TR
></TABLE
></P
></DD
><DT
><TT
CLASS="VARNAME"
>$BASH_VERSINFO[n]</TT
></DT
><DD
><P
>A 6-element <A
HREF="arrays.html#ARRAYREF"
>array</A
>
	    containing version information about the installed release
	    of Bash. This is similar to <TT
CLASS="VARNAME"
>$BASH_VERSION</TT
>,
	    below, but a bit more detailed.</P
><P
>	  <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;# Bash version info:
   2&nbsp;
   3&nbsp;for n in 0 1 2 3 4 5
   4&nbsp;do
   5&nbsp;  echo "BASH_VERSINFO[$n] = ${BASH_VERSINFO[$n]}"
   6&nbsp;done  
   7&nbsp;
   8&nbsp;# BASH_VERSINFO[0] = 3                      # Major version no.
   9&nbsp;# BASH_VERSINFO[1] = 00                     # Minor version no.
  10&nbsp;# BASH_VERSINFO[2] = 14                     # Patch level.
  11&nbsp;# BASH_VERSINFO[3] = 1                      # Build version.
  12&nbsp;# BASH_VERSINFO[4] = release                # Release status.
  13&nbsp;# BASH_VERSINFO[5] = i386-redhat-linux-gnu  # Architecture
  14&nbsp;                                            # (same as $MACHTYPE).</PRE
></TD
></TR
></TABLE
>
	  </P
></DD
><DT
><TT
CLASS="VARNAME"
>$BASH_VERSION</TT
></DT
><DD
><P
>The version of Bash installed on the system</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo $BASH_VERSION</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>3.2.25(1)-release</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	    </P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>tcsh% </TT
><TT
CLASS="USERINPUT"
><B
>echo $BASH_VERSION</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>BASH_VERSION: Undefined variable.</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	    </P
><P
>Checking $BASH_VERSION is a good method of determining which
	    shell is running. <A
HREF="variables2.html#SHELLVARREF"
>$SHELL</A
>
	    does not necessarily give the correct answer.</P
></DD
><DT
><A
NAME="CDPATHREF"
></A
><TT
CLASS="VARNAME"
>$CDPATH</TT
></DT
><DD
><P
>A colon-separated list of search paths
	    available to the <A
HREF="internal.html#CDREF"
>cd</A
>
	    command, similar in function to the <A
HREF="variables2.html#PATHREF"
>$PATH</A
> variable for binaries.
            The <TT
CLASS="VARNAME"
>$CDPATH</TT
> variable may be set in the
	    local <A
HREF="sample-bashrc.html#BASHRC"
><TT
CLASS="FILENAME"
>~/.bashrc</TT
></A
>
	    file.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>cd bash-doc</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>bash: cd: bash-doc: No such file or directory</TT
>
 
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>CDPATH=/usr/share/doc</B
></TT
>
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>cd bash-doc</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>/usr/share/doc/bash-doc</TT
>
 
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo $PWD</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>/usr/share/doc/bash-doc</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
           </P
></DD
><DT
><A
NAME="DIRSTACKREF"
></A
><TT
CLASS="VARNAME"
>$DIRSTACK</TT
></DT
><DD
><P
>The top value in the directory stack

              <A
NAME="AEN4671"
HREF="#FTN.AEN4671"
>[1]</A
>

	    (affected by <A
HREF="internal.html#PUSHDREF"
>pushd</A
> and <A
HREF="internal.html#POPDREF"
>popd</A
>)</P
><P
>This builtin
	    variable corresponds to the <A
HREF="internal.html#DIRSD"
>dirs</A
>
	    command, however <B
CLASS="COMMAND"
>dirs</B
> shows the entire
	    contents of the directory stack.</P
></DD
><DT
><TT
CLASS="VARNAME"
>$EDITOR</TT
></DT
><DD
><P
>The default editor invoked by a script, usually
	      <B
CLASS="COMMAND"
>vi</B
> or <B
CLASS="COMMAND"
>emacs</B
>.</P
></DD
><DT
><A
NAME="EUIDREF"
></A
><TT
CLASS="VARNAME"
>$EUID</TT
></DT
><DD
><P
><SPAN
CLASS="QUOTE"
>"effective"</SPAN
> user ID number</P
><P
>Identification number of whatever identity the
	      current user has assumed, perhaps by means of <A
HREF="system.html#SUREF"
>su</A
>.</P
><DIV
CLASS="CAUTION"
><TABLE
CLASS="CAUTION"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/caution.png"
HSPACE="5"
ALT="Caution"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The <TT
CLASS="VARNAME"
>$EUID</TT
> is not necessarily
	      the same as the <A
HREF="variables2.html#UIDREF"
>$UID</A
>.</P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><TT
CLASS="VARNAME"
>$FUNCNAME</TT
></DT
><DD
><P
>Name of the current function</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;xyz23 ()
   2&nbsp;{
   3&nbsp;  echo "$FUNCNAME now executing."  # xyz23 now executing.
   4&nbsp;}
   5&nbsp;
   6&nbsp;xyz23
   7&nbsp;
   8&nbsp;echo "FUNCNAME = $FUNCNAME"        # FUNCNAME =
   9&nbsp;                                   # Null value outside a function.</PRE
></TD
></TR
></TABLE
>
            </P
><P
>See also <A
HREF="contributed-scripts.html#USEGETOPT"
>Example A-50</A
>.</P
></DD
><DT
><TT
CLASS="VARNAME"
>$GLOBIGNORE</TT
></DT
><DD
><P
>A list of filename patterns to be excluded from
	    matching in <A
HREF="globbingref.html"
>globbing</A
>.</P
></DD
><DT
><A
NAME="GROUPSREF"
></A
><TT
CLASS="VARNAME"
>$GROUPS</TT
></DT
><DD
><P
>Groups current user belongs to</P
><P
>This is a listing (array) of the group id numbers for
	      current user, as  recorded in
	      <A
HREF="files.html#DATAFILESREF1"
><TT
CLASS="FILENAME"
>/etc/passwd</TT
></A
>
	      and <TT
CLASS="FILENAME"
>/etc/group</TT
>.
	    </P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>root# </TT
><TT
CLASS="USERINPUT"
><B
>echo $GROUPS</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>0</TT
>
 
 
 <TT
CLASS="PROMPT"
>root# </TT
><TT
CLASS="USERINPUT"
><B
>echo ${GROUPS[1]}</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>1</TT
>
 
 
 <TT
CLASS="PROMPT"
>root# </TT
><TT
CLASS="USERINPUT"
><B
>echo ${GROUPS[5]}</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>6</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	      </P
></DD
><DT
><A
NAME="HOMEDIRREF"
></A
><TT
CLASS="VARNAME"
>$HOME</TT
></DT
><DD
><P
>Home directory of the user, usually <TT
CLASS="FILENAME"
>/home/username</TT
> (see <A
HREF="parameter-substitution.html#EX6"
>Example 10-7</A
>)</P
></DD
><DT
><A
NAME="HOSTNAMEREF"
></A
><TT
CLASS="VARNAME"
>$HOSTNAME</TT
></DT
><DD
><P
>The <A
HREF="system.html#HNAMEREF"
>hostname</A
> command
	    assigns the system host name at bootup in an init script.
	    However, the <TT
CLASS="FUNCTION"
>gethostname()</TT
> function
	    sets the Bash internal variable <TT
CLASS="VARNAME"
>$HOSTNAME</TT
>.
	    See also <A
HREF="parameter-substitution.html#EX6"
>Example 10-7</A
>.</P
></DD
><DT
><TT
CLASS="VARNAME"
>$HOSTTYPE</TT
></DT
><DD
><P
>host type</P
><P
>Like <A
HREF="variables2.html#MACHTYPEREF"
>$MACHTYPE</A
>,
	      identifies the system hardware.</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo $HOSTTYPE</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>i686</TT
></PRE
></TD
></TR
></TABLE
></DD
><DT
><A
NAME="IFSREF"
></A
><TT
CLASS="VARNAME"
>$IFS</TT
></DT
><DD
><P
>internal field separator</P
><P
>This variable determines how Bash recognizes <A
HREF="special-chars.html#FIELDREF"
>fields</A
>, or word boundaries,
	      when it interprets character strings.</P
><P
><A
NAME="IFSWS"
></A
></P
><P
>$IFS defaults to <A
HREF="special-chars.html#WHITESPACEREF"
>whitespace</A
> (space,
	      tab, and newline), but may be changed, for example,
	      to parse a comma-separated data file. Note that
	      <A
HREF="variables2.html#APPREF"
>$*</A
> uses the first
	      character held in <TT
CLASS="VARNAME"
>$IFS</TT
>. See <A
HREF="quoting.html#WEIRDVARS"
>Example 5-1</A
>.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo "$IFS"</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>(With $IFS set to default, a blank line displays.)</TT
>
 	      
 
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo "$IFS" | cat -vte</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
> ^I$
 $</TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>(Show whitespace: here a single space, ^I [horizontal tab],
  and newline, and display "$" at end-of-line.)</TT
>
 
 
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>bash -c 'set w x y z; IFS=":-;"; echo "$*"'</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>w:x:y:z</TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>(Read commands from string and assign any arguments to pos params.)</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	  </P
><P
>Set <TT
CLASS="VARNAME"
>$IFS</TT
> to eliminate whitespace
		  in <A
HREF="special-chars.html#PATHNAMEREF"
>pathnames</A
>.
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;IFS="$(printf '\n\t')"   # Per David Wheeler.</PRE
></TD
></TR
></TABLE
>
	  </P
><DIV
CLASS="CAUTION"
><TABLE
CLASS="CAUTION"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/caution.png"
HSPACE="5"
ALT="Caution"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
><TT
CLASS="VARNAME"
>$IFS</TT
> does not handle whitespace
	    the same as it does other characters.

	  <DIV
CLASS="EXAMPLE"
><HR><A
NAME="IFSH"
></A
><P
><B
>Example 9-1. $IFS and whitespace</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# ifs.sh
   3&nbsp;
   4&nbsp;
   5&nbsp;var1="a+b+c"
   6&nbsp;var2="d-e-f"
   7&nbsp;var3="g,h,i"
   8&nbsp;
   9&nbsp;IFS=+
  10&nbsp;# The plus sign will be interpreted as a separator.
  11&nbsp;echo $var1     # a b c
  12&nbsp;echo $var2     # d-e-f
  13&nbsp;echo $var3     # g,h,i
  14&nbsp;
  15&nbsp;echo
  16&nbsp;
  17&nbsp;IFS="-"
  18&nbsp;# The plus sign reverts to default interpretation.
  19&nbsp;# The minus sign will be interpreted as a separator.
  20&nbsp;echo $var1     # a+b+c
  21&nbsp;echo $var2     # d e f
  22&nbsp;echo $var3     # g,h,i
  23&nbsp;
  24&nbsp;echo
  25&nbsp;
  26&nbsp;IFS=","
  27&nbsp;# The comma will be interpreted as a separator.
  28&nbsp;# The minus sign reverts to default interpretation.
  29&nbsp;echo $var1     # a+b+c
  30&nbsp;echo $var2     # d-e-f
  31&nbsp;echo $var3     # g h i
  32&nbsp;
  33&nbsp;echo
  34&nbsp;
  35&nbsp;IFS=" "
  36&nbsp;# The space character will be interpreted as a separator.
  37&nbsp;# The comma reverts to default interpretation.
  38&nbsp;echo $var1     # a+b+c
  39&nbsp;echo $var2     # d-e-f
  40&nbsp;echo $var3     # g,h,i
  41&nbsp;
  42&nbsp;# ======================================================== #
  43&nbsp;
  44&nbsp;# However ...
  45&nbsp;# $IFS treats whitespace differently than other characters.
  46&nbsp;
  47&nbsp;output_args_one_per_line()
  48&nbsp;{
  49&nbsp;  for arg
  50&nbsp;  do
  51&nbsp;    echo "[$arg]"
  52&nbsp;  done #  ^    ^   Embed within brackets, for your viewing pleasure.
  53&nbsp;}
  54&nbsp;
  55&nbsp;echo; echo "IFS=\" \""
  56&nbsp;echo "-------"
  57&nbsp;
  58&nbsp;IFS=" "
  59&nbsp;var=" a  b c   "
  60&nbsp;#    ^ ^^   ^^^
  61&nbsp;output_args_one_per_line $var  # output_args_one_per_line `echo " a  b c   "`
  62&nbsp;# [a]
  63&nbsp;# [b]
  64&nbsp;# [c]
  65&nbsp;
  66&nbsp;
  67&nbsp;echo; echo "IFS=:"
  68&nbsp;echo "-----"
  69&nbsp;
  70&nbsp;IFS=:
  71&nbsp;var=":a::b:c:::"               # Same pattern as above,
  72&nbsp;#    ^ ^^   ^^^                #+ but substituting ":" for " "  ...
  73&nbsp;output_args_one_per_line $var
  74&nbsp;# []
  75&nbsp;# [a]
  76&nbsp;# []
  77&nbsp;# [b]
  78&nbsp;# [c]
  79&nbsp;# []
  80&nbsp;# []
  81&nbsp;
  82&nbsp;# Note "empty" brackets.
  83&nbsp;# The same thing happens with the "FS" field separator in awk.
  84&nbsp;
  85&nbsp;
  86&nbsp;echo
  87&nbsp;
  88&nbsp;exit</PRE
></TD
></TR
></TABLE
><HR></DIV
>

	  </P
></TD
></TR
></TABLE
></DIV
><P
>(Many thanks, Stphane Chazelas, for clarification
	  and above examples.)</P
><P
>See also <A
HREF="communications.html#ISSPAMMER"
>Example 16-41</A
>, <A
HREF="loops.html#BINGREP"
>Example 11-8</A
>, and <A
HREF="here-docs.html#MAILBOXGREP"
>Example 19-14</A
>
             for instructive examples of using
             <TT
CLASS="VARNAME"
>$IFS</TT
>.</P
></DD
><DT
><TT
CLASS="VARNAME"
>$IGNOREEOF</TT
></DT
><DD
><P
>Ignore EOF: how many end-of-files (control-D)
	    the shell will ignore before logging out.</P
></DD
><DT
><TT
CLASS="VARNAME"
>$LC_COLLATE</TT
></DT
><DD
><P
>Often set in the <A
HREF="sample-bashrc.html"
><TT
CLASS="FILENAME"
>.bashrc</TT
></A
>
	    or <TT
CLASS="FILENAME"
>/etc/profile</TT
> files, this
	    variable controls collation order in filename
	    expansion and pattern matching. If mishandled,
	    <TT
CLASS="VARNAME"
>LC_COLLATE</TT
> can cause unexpected results in
	    <A
HREF="globbingref.html"
>filename globbing</A
>.</P
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>As of version 2.05 of Bash,
	    filename globbing no longer distinguishes between lowercase
	    and uppercase letters in a character range between
	    brackets. For example, <B
CLASS="COMMAND"
>ls [A-M]*</B
>
	    would match both <TT
CLASS="FILENAME"
>File1.txt</TT
>
	    and <TT
CLASS="FILENAME"
>file1.txt</TT
>. To revert to
	    the customary behavior of bracket matching, set
	    <TT
CLASS="VARNAME"
>LC_COLLATE</TT
> to <TT
CLASS="OPTION"
>C</TT
>
	    by an <TT
CLASS="USERINPUT"
><B
>export LC_COLLATE=C</B
></TT
>
	    in <TT
CLASS="FILENAME"
>/etc/profile</TT
> and/or
	    <TT
CLASS="FILENAME"
>~/.bashrc</TT
>.</P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><TT
CLASS="VARNAME"
>$LC_CTYPE</TT
></DT
><DD
><P
>This internal variable controls character interpretation
	    in <A
HREF="globbingref.html"
>globbing</A
> and pattern
	    matching.</P
></DD
><DT
><A
NAME="LINENOREF"
></A
><TT
CLASS="VARNAME"
>$LINENO</TT
></DT
><DD
><P
>This variable is the line number of the shell
	    script in which this variable appears. It has significance only
	    within the script in which it appears, and is chiefly useful for
	    debugging purposes.</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;# *** BEGIN DEBUG BLOCK ***
   2&nbsp;last_cmd_arg=$_  # Save it.
   3&nbsp;
   4&nbsp;echo "At line number $LINENO, variable \"v1\" = $v1"
   5&nbsp;echo "Last command argument processed = $last_cmd_arg"
   6&nbsp;# *** END DEBUG BLOCK ***</PRE
></TD
></TR
></TABLE
></P
></DD
><DT
><A
NAME="MACHTYPEREF"
></A
><TT
CLASS="VARNAME"
>$MACHTYPE</TT
></DT
><DD
><P
>machine type</P
><P
>Identifies the system hardware.</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo $MACHTYPE</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>i686</TT
></PRE
></TD
></TR
></TABLE
></DD
><DT
><A
NAME="OLDPWD"
></A
><TT
CLASS="VARNAME"
>$OLDPWD</TT
></DT
><DD
><P
>Old working directory
	  (<SPAN
CLASS="QUOTE"
>"OLD-Print-Working-Directory"</SPAN
>,
	    previous directory you were in).</P
></DD
><DT
><TT
CLASS="VARNAME"
>$OSTYPE</TT
></DT
><DD
><P
>operating system type</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo $OSTYPE</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>linux</TT
></PRE
></TD
></TR
></TABLE
></DD
><DT
><A
NAME="PATHREF"
></A
><TT
CLASS="VARNAME"
>$PATH</TT
></DT
><DD
><P
>Path to binaries, usually 
	      <TT
CLASS="FILENAME"
>/usr/bin/</TT
>, 
	      <TT
CLASS="FILENAME"
>/usr/X11R6/bin/</TT
>, 
	      <TT
CLASS="FILENAME"
>/usr/local/bin</TT
>, etc.</P
><P
>When given a command, the shell automatically does
	      a hash table search on the directories listed in the
	      <I
CLASS="FIRSTTERM"
>path</I
> for the executable. The path
	      is stored in the <A
HREF="othertypesv.html#ENVREF"
>environmental
	      variable</A
>, <TT
CLASS="VARNAME"
>$PATH</TT
>, a list
	      of directories, separated by colons. Normally,
	      the system stores the <TT
CLASS="VARNAME"
>$PATH</TT
>
	      definition in <TT
CLASS="FILENAME"
>/etc/profile</TT
>
	      and/or <A
HREF="sample-bashrc.html"
><TT
CLASS="FILENAME"
>~/.bashrc</TT
></A
>
	      (see <A
HREF="files.html"
>Appendix H</A
>).</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><B
CLASS="COMMAND"
>echo $PATH</B
>
 <TT
CLASS="COMPUTEROUTPUT"
>/bin:/usr/bin:/usr/local/bin:/usr/X11R6/bin:/sbin:/usr/sbin</TT
></PRE
></TD
></TR
></TABLE
>
              </P
><P
><TT
CLASS="USERINPUT"
><B
>PATH=${PATH}:/opt/bin</B
></TT
> appends
	      the <TT
CLASS="FILENAME"
>/opt/bin</TT
>
	      directory to the current path. In a script, it may be
	      expedient to temporarily add a directory to the path
	      in this way. When the script exits, this restores the
	      original <TT
CLASS="VARNAME"
>$PATH</TT
> (a child process, such
	      as a script, may not change the environment of the parent
	      process, the shell).</P
><P
><A
NAME="CURRENTWDREF"
></A
></P
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The current <SPAN
CLASS="QUOTE"
>"working directory"</SPAN
>, 
	      <TT
CLASS="FILENAME"
>./</TT
>, is usually
	      omitted from the <TT
CLASS="VARNAME"
>$PATH</TT
> as a security
	      measure.</P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><A
NAME="PIPESTATUSREF"
></A
><TT
CLASS="VARNAME"
>$PIPESTATUS</TT
></DT
><DD
><P
><A
HREF="arrays.html#ARRAYREF"
>Array</A
> variable holding
            <A
HREF="exit-status.html#EXITSTATUSREF"
>exit status</A
>(es) of
	    last executed <I
CLASS="FIRSTTERM"
>foreground</I
> <A
HREF="special-chars.html#PIPEREF"
>pipe</A
>.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo $PIPESTATUS</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>0</TT
>
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>ls -al | bogus_command</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>bash: bogus_command: command not found</TT
>
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo ${PIPESTATUS[1]}</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>127</TT
>
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>ls -al | bogus_command</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>bash: bogus_command: command not found</TT
>
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo $?</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>127</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	      </P
><P
>The members of the <TT
CLASS="VARNAME"
>$PIPESTATUS</TT
>
		array hold the exit status of each respective command
		executed in a pipe. <TT
CLASS="VARNAME"
>$PIPESTATUS[0]</TT
>
		holds the exit status of the first command in the pipe,
		<TT
CLASS="VARNAME"
>$PIPESTATUS[1]</TT
> the exit status of
		the second command, and so on.</P
><DIV
CLASS="CAUTION"
><TABLE
CLASS="CAUTION"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/caution.png"
HSPACE="5"
ALT="Caution"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>	      The <TT
CLASS="VARNAME"
>$PIPESTATUS</TT
> variable
              may contain an erroneous <SPAN
CLASS="ERRORCODE"
>0</SPAN
> value
              in a login shell (in releases prior to 3.0 of Bash).
              </P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>tcsh% </TT
><TT
CLASS="USERINPUT"
><B
>bash</B
></TT
>
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>who | grep nobody | sort</B
></TT
>
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo ${PIPESTATUS[*]}</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>0</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	      </P
><P
>	      The above lines contained in a script would produce the expected
	      <TT
CLASS="COMPUTEROUTPUT"
>0 1 0</TT
> output.
              </P
><P
>	      Thank you, Wayne Pollock for pointing this out and supplying the
	      above example.
	      </P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The <TT
CLASS="VARNAME"
>$PIPESTATUS</TT
> variable gives
	        unexpected results in some contexts.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo $BASH_VERSION</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>3.00.14(1)-release</TT
>
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>$ ls | bogus_command | wc</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>bash: bogus_command: command not found
 0       0       0</TT
>
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo ${PIPESTATUS[@]}</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>141 127 0</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	      </P
><P
>Chet Ramey attributes the above output to the behavior of
		<A
HREF="external.html#LSREF"
>ls</A
>. If <I
CLASS="FIRSTTERM"
>ls</I
>
		writes to a <I
CLASS="FIRSTTERM"
>pipe</I
> whose output is not
		read, then <TT
CLASS="REPLACEABLE"
><I
>SIGPIPE</I
></TT
> kills it,
		and its <A
HREF="exit-status.html#EXITSTATUSREF"
>exit status</A
>
		is <SPAN
CLASS="RETURNVALUE"
>141</SPAN
>. Otherwise
		its exit status is <SPAN
CLASS="RETURNVALUE"
>0</SPAN
>,
		as expected. This likewise is the case for <A
HREF="textproc.html#TRREF"
>tr</A
>.</P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
><TT
CLASS="VARNAME"
>$PIPESTATUS</TT
> is a
		    <SPAN
CLASS="QUOTE"
>"volatile"</SPAN
> variable. It needs to be
		    captured immediately after the pipe in question, before
		    any other command intervenes.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>$ ls | bogus_command | wc</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>bash: bogus_command: command not found
 0       0       0</TT
>
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo ${PIPESTATUS[@]}</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>0 127 0</TT
>
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo ${PIPESTATUS[@]}</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>0</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	      </P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The <A
HREF="bashver3.html#PIPEFAILREF"
>pipefail option</A
>
		    may be useful in cases where
		    <TT
CLASS="VARNAME"
>$PIPESTATUS</TT
> does not give the desired
		    information.</P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><A
NAME="PPIDREF"
></A
><TT
CLASS="VARNAME"
>$PPID</TT
></DT
><DD
><P
></P
><P
>The <TT
CLASS="VARNAME"
>$PPID</TT
> of a process is
	    the process ID (<TT
CLASS="VARNAME"
>pid</TT
>) of its parent process.

	      <A
NAME="AEN5154"
HREF="#FTN.AEN5154"
>[2]</A
>
	      </P
><P
>Compare this with the <A
HREF="system.html#PIDOFREF"
>pidof</A
> command.</P
></DD
><DT
><TT
CLASS="VARNAME"
>$PROMPT_COMMAND</TT
></DT
><DD
><P
>A variable holding a command to be executed
	      just before the primary prompt, <TT
CLASS="VARNAME"
>$PS1</TT
>
	      is to be displayed.</P
></DD
><DT
><A
NAME="PS1REF"
></A
><TT
CLASS="VARNAME"
>$PS1</TT
></DT
><DD
><P
>This is the main prompt, seen at the command-line.</P
></DD
><DT
><A
NAME="SECPROMPTREF"
></A
><TT
CLASS="VARNAME"
>$PS2</TT
></DT
><DD
><P
>The secondary prompt, seen when additional input is
	      expected. It displays as <SPAN
CLASS="QUOTE"
>"&#62;"</SPAN
>.</P
></DD
><DT
><TT
CLASS="VARNAME"
>$PS3</TT
></DT
><DD
><P
>The tertiary prompt, displayed in a
	    <A
HREF="testbranch.html#SELECTREF"
>select</A
> loop (see <A
HREF="testbranch.html#EX31"
>Example 11-30</A
>).</P
></DD
><DT
><TT
CLASS="VARNAME"
>$PS4</TT
></DT
><DD
><P
>The quartenary prompt, shown at the beginning of
	    each line of output when invoking a script with the
	    <SPAN
CLASS="TOKEN"
>-x</SPAN
> <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>[verbose trace]</I
></SPAN
>
	    <A
HREF="options.html#OPTIONSREF"
>option</A
>. It displays as
	    <SPAN
CLASS="QUOTE"
>"+"</SPAN
>.</P
><P
>As a debugging aid, it may be useful to embed diagnostic
	    information in <TT
CLASS="VARNAME"
>$PS4</TT
>.
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;P4='$(read time junk &#60; /proc/$$/schedstat; echo "@@@ $time @@@ " )'
   2&nbsp;# Per suggestion by Erik Brandsberg.
   3&nbsp;set -x
   4&nbsp;# Various commands follow ...</PRE
></TD
></TR
></TABLE
></P
></DD
><DT
><A
NAME="PWDREF"
></A
><TT
CLASS="VARNAME"
>$PWD</TT
></DT
><DD
><P
>Working directory (directory you are in at the time)</P
><P
>This is the analog to the <A
HREF="internal.html#PWD2REF"
>pwd</A
>
	    builtin command.</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;E_WRONG_DIRECTORY=85
   4&nbsp;
   5&nbsp;clear # Clear the screen.
   6&nbsp;
   7&nbsp;TargetDirectory=/home/bozo/projects/GreatAmericanNovel
   8&nbsp;
   9&nbsp;cd $TargetDirectory
  10&nbsp;echo "Deleting stale files in $TargetDirectory."
  11&nbsp;
  12&nbsp;if [ "$PWD" != "$TargetDirectory" ]
  13&nbsp;then    # Keep from wiping out wrong directory by accident.
  14&nbsp;  echo "Wrong directory!"
  15&nbsp;  echo "In $PWD, rather than $TargetDirectory!"
  16&nbsp;  echo "Bailing out!"
  17&nbsp;  exit $E_WRONG_DIRECTORY
  18&nbsp;fi  
  19&nbsp;
  20&nbsp;rm -rf *
  21&nbsp;rm .[A-Za-z0-9]*    # Delete dotfiles.
  22&nbsp;# rm -f .[^.]* ..?*   to remove filenames beginning with multiple dots.
  23&nbsp;# (shopt -s dotglob; rm -f *)   will also work.
  24&nbsp;# Thanks, S.C. for pointing this out.
  25&nbsp;
  26&nbsp;#  A filename (`basename`) may contain all characters in the 0 - 255 range,
  27&nbsp;#+ except "/".
  28&nbsp;#  Deleting files beginning with weird characters, such as -
  29&nbsp;#+ is left as an exercise. (Hint: rm ./-weirdname or rm -- -weirdname)
  30&nbsp;result=$?   # Result of delete operations. If successful = 0.
  31&nbsp;
  32&nbsp;echo
  33&nbsp;ls -al              # Any files left?
  34&nbsp;echo "Done."
  35&nbsp;echo "Old files deleted in $TargetDirectory."
  36&nbsp;echo
  37&nbsp;
  38&nbsp;# Various other operations here, as necessary.
  39&nbsp;
  40&nbsp;exit $result</PRE
></TD
></TR
></TABLE
></P
></DD
><DT
><A
NAME="REPLYREF"
></A
><TT
CLASS="VARNAME"
>$REPLY</TT
></DT
><DD
><P
>The default value when a variable is not
	    supplied to <A
HREF="internal.html#READREF"
>read</A
>. Also
	    applicable to <A
HREF="testbranch.html#SELECTREF"
>select</A
> menus,
	    but only supplies the item number of the variable chosen,
	    not the value of the variable itself.</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# reply.sh
   3&nbsp;
   4&nbsp;# REPLY is the default value for a 'read' command.
   5&nbsp;
   6&nbsp;echo
   7&nbsp;echo -n "What is your favorite vegetable? "
   8&nbsp;read
   9&nbsp;
  10&nbsp;echo "Your favorite vegetable is $REPLY."
  11&nbsp;#  REPLY holds the value of last "read" if and only if
  12&nbsp;#+ no variable supplied.
  13&nbsp;
  14&nbsp;echo
  15&nbsp;echo -n "What is your favorite fruit? "
  16&nbsp;read fruit
  17&nbsp;echo "Your favorite fruit is $fruit."
  18&nbsp;echo "but..."
  19&nbsp;echo "Value of \$REPLY is still $REPLY."
  20&nbsp;#  $REPLY is still set to its previous value because
  21&nbsp;#+ the variable $fruit absorbed the new "read" value.
  22&nbsp;
  23&nbsp;echo
  24&nbsp;
  25&nbsp;exit 0</PRE
></TD
></TR
></TABLE
></P
></DD
><DT
><TT
CLASS="VARNAME"
>$SECONDS</TT
></DT
><DD
><P
>The number of seconds the script has been running.</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;TIME_LIMIT=10
   4&nbsp;INTERVAL=1
   5&nbsp;
   6&nbsp;echo
   7&nbsp;echo "Hit Control-C to exit before $TIME_LIMIT seconds."
   8&nbsp;echo
   9&nbsp;
  10&nbsp;while [ "$SECONDS" -le "$TIME_LIMIT" ]
  11&nbsp;do   #   $SECONDS is an internal shell variable.
  12&nbsp;  if [ "$SECONDS" -eq 1 ]
  13&nbsp;  then
  14&nbsp;    units=second
  15&nbsp;  else  
  16&nbsp;    units=seconds
  17&nbsp;  fi
  18&nbsp;
  19&nbsp;  echo "This script has been running $SECONDS $units."
  20&nbsp;  #  On a slow or overburdened machine, the script may skip a count
  21&nbsp;  #+ every once in a while.
  22&nbsp;  sleep $INTERVAL
  23&nbsp;done
  24&nbsp;
  25&nbsp;echo -e "\a"  # Beep!
  26&nbsp;
  27&nbsp;exit 0</PRE
></TD
></TR
></TABLE
></P
></DD
><DT
><TT
CLASS="VARNAME"
>$SHELLOPTS</TT
></DT
><DD
><P
>The list of enabled shell <A
HREF="options.html#OPTIONSREF"
>options</A
>, a readonly variable.
	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo $SHELLOPTS</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>braceexpand:hashall:histexpand:monitor:history:interactive-comments:emacs</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	    </P
></DD
><DT
><A
NAME="SHLVLREF"
></A
><TT
CLASS="VARNAME"
>$SHLVL</TT
></DT
><DD
><P
>Shell level, how deeply Bash is nested.
	      <A
NAME="AEN5320"
HREF="#FTN.AEN5320"
>[3]</A
>
	    If, at the command-line, $SHLVL is 1, then in a script it
	    will increment to 2.</P
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>This variable is <A
HREF="subshells.html#SUBSHNLEVREF"
>	      <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
> affected by
	      subshells</A
>. Use <A
HREF="variables2.html#BASHSUBSHELLREF"
>$BASH_SUBSHELL</A
> when you
	      need an indication of subshell nesting.</P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><A
NAME="TMOUTREF"
></A
><TT
CLASS="VARNAME"
>$TMOUT</TT
></DT
><DD
><P
>If the <TT
CLASS="REPLACEABLE"
><I
>$TMOUT</I
></TT
>
	  environmental variable is set to a non-zero value
	  <TT
CLASS="VARNAME"
>time</TT
>, then the shell prompt will time out
	  after <TT
CLASS="VARNAME"
>$time</TT
> seconds. This will cause a
	  logout.</P
><P
>As of version 2.05b of Bash, it is now possible to use
	    <TT
CLASS="REPLACEABLE"
><I
>$TMOUT</I
></TT
> in a script in combination
	    with <A
HREF="internal.html#READREF"
>read</A
>.</P
><P
>	  <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;# Works in scripts for Bash, versions 2.05b and later.
   2&nbsp;
   3&nbsp;TMOUT=3    # Prompt times out at three seconds.
   4&nbsp;
   5&nbsp;echo "What is your favorite song?"
   6&nbsp;echo "Quickly now, you only have $TMOUT seconds to answer!"
   7&nbsp;read song
   8&nbsp;
   9&nbsp;if [ -z "$song" ]
  10&nbsp;then
  11&nbsp;  song="(no answer)"
  12&nbsp;  # Default response.
  13&nbsp;fi
  14&nbsp;
  15&nbsp;echo "Your favorite song is $song."</PRE
></TD
></TR
></TABLE
>
	  </P
><P
><A
NAME="TIMINGLOOP"
></A
></P
><P
>There are other, more complex, ways of implementing
	    timed input in a script. One alternative is to set up
	    a timing loop to signal the script when it times out.
	    This also requires a signal handling routine to <A
HREF="debugging.html#TRAPREF1"
>trap</A
> (see <A
HREF="debugging.html#EX76"
>Example 32-5</A
>)
	    the interrupt generated by the timing loop (whew!).</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="TMDIN"
></A
><P
><B
>Example 9-2. Timed Input</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# timed-input.sh
   3&nbsp;
   4&nbsp;# TMOUT=3    Also works, as of newer versions of Bash.
   5&nbsp;
   6&nbsp;TIMER_INTERRUPT=14
   7&nbsp;TIMELIMIT=3  # Three seconds in this instance.
   8&nbsp;             # May be set to different value.
   9&nbsp;
  10&nbsp;PrintAnswer()
  11&nbsp;{
  12&nbsp;  if [ "$answer" = TIMEOUT ]
  13&nbsp;  then
  14&nbsp;    echo $answer
  15&nbsp;  else       # Don't want to mix up the two instances. 
  16&nbsp;    echo "Your favorite veggie is $answer"
  17&nbsp;    kill $!  #  Kills no-longer-needed TimerOn function
  18&nbsp;             #+ running in background.
  19&nbsp;             #  $! is PID of last job running in background.
  20&nbsp;  fi
  21&nbsp;
  22&nbsp;}  
  23&nbsp;
  24&nbsp;
  25&nbsp;TimerOn()
  26&nbsp;{
  27&nbsp;  sleep $TIMELIMIT &#38;&#38; kill -s 14 $$ &#38;
  28&nbsp;  # Waits 3 seconds, then sends sigalarm to script.
  29&nbsp;}  
  30&nbsp;
  31&nbsp;
  32&nbsp;Int14Vector()
  33&nbsp;{
  34&nbsp;  answer="TIMEOUT"
  35&nbsp;  PrintAnswer
  36&nbsp;  exit $TIMER_INTERRUPT
  37&nbsp;}  
  38&nbsp;
  39&nbsp;trap Int14Vector $TIMER_INTERRUPT
  40&nbsp;# Timer interrupt (14) subverted for our purposes.
  41&nbsp;
  42&nbsp;echo "What is your favorite vegetable "
  43&nbsp;TimerOn
  44&nbsp;read answer
  45&nbsp;PrintAnswer
  46&nbsp;
  47&nbsp;
  48&nbsp;#  Admittedly, this is a kludgy implementation of timed input.
  49&nbsp;#  However, the "-t" option to "read" simplifies this task.
  50&nbsp;#  See the "t-out.sh" script.
  51&nbsp;#  However, what about timing not just single user input,
  52&nbsp;#+ but an entire script?
  53&nbsp;
  54&nbsp;#  If you need something really elegant ...
  55&nbsp;#+ consider writing the application in C or C++,
  56&nbsp;#+ using appropriate library functions, such as 'alarm' and 'setitimer.'
  57&nbsp;
  58&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
><A
NAME="STTYTO"
></A
></P
><P
>An alternative is using <A
HREF="system.html#STTYREF"
>stty</A
>.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="TIMEOUT"
></A
><P
><B
>Example 9-3. Once more, timed input</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# timeout.sh
   3&nbsp;
   4&nbsp;#  Written by Stephane Chazelas,
   5&nbsp;#+ and modified by the document author.
   6&nbsp;
   7&nbsp;INTERVAL=5                # timeout interval
   8&nbsp;
   9&nbsp;timedout_read() {
  10&nbsp;  timeout=$1
  11&nbsp;  varname=$2
  12&nbsp;  old_tty_settings=`stty -g`
  13&nbsp;  stty -icanon min 0 time ${timeout}0
  14&nbsp;  eval read $varname      # or just  read $varname
  15&nbsp;  stty "$old_tty_settings"
  16&nbsp;  # See man page for "stty."
  17&nbsp;}
  18&nbsp;
  19&nbsp;echo; echo -n "What's your name? Quick! "
  20&nbsp;timedout_read $INTERVAL your_name
  21&nbsp;
  22&nbsp;#  This may not work on every terminal type.
  23&nbsp;#  The maximum timeout depends on the terminal.
  24&nbsp;#+ (it is often 25.5 seconds).
  25&nbsp;
  26&nbsp;echo
  27&nbsp;
  28&nbsp;if [ ! -z "$your_name" ]  # If name input before timeout ...
  29&nbsp;then
  30&nbsp;  echo "Your name is $your_name."
  31&nbsp;else
  32&nbsp;  echo "Timed out."
  33&nbsp;fi
  34&nbsp;
  35&nbsp;echo
  36&nbsp;
  37&nbsp;# The behavior of this script differs somewhat from "timed-input.sh."
  38&nbsp;# At each keystroke, the counter resets.
  39&nbsp;
  40&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>Perhaps the simplest method is using the
	    <TT
CLASS="OPTION"
>-t</TT
> option to <A
HREF="internal.html#READREF"
>read</A
>.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="TOUT"
></A
><P
><B
>Example 9-4. Timed <I
CLASS="FIRSTTERM"
>read</I
></B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# t-out.sh [time-out]
   3&nbsp;# Inspired by a suggestion from "syngin seven" (thanks).
   4&nbsp;
   5&nbsp;
   6&nbsp;TIMELIMIT=4         # 4 seconds
   7&nbsp;
   8&nbsp;read -t $TIMELIMIT variable &#60;&#38;1
   9&nbsp;#                           ^^^
  10&nbsp;#  In this instance, "&#60;&#38;1" is needed for Bash 1.x and 2.x,
  11&nbsp;#  but unnecessary for Bash 3+.
  12&nbsp;
  13&nbsp;echo
  14&nbsp;
  15&nbsp;if [ -z "$variable" ]  # Is null?
  16&nbsp;then
  17&nbsp;  echo "Timed out, variable still unset."
  18&nbsp;else  
  19&nbsp;  echo "variable = $variable"
  20&nbsp;fi  
  21&nbsp;
  22&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
></DD
><DT
><A
NAME="UIDREF"
></A
><TT
CLASS="VARNAME"
>$UID</TT
></DT
><DD
><P
>User ID number</P
><P
>Current user's user identification number, as
	      recorded in <A
HREF="files.html#DATAFILESREF1"
><TT
CLASS="FILENAME"
>/etc/passwd</TT
></A
></P
><P
>This is the current user's real id, even if she has
	      temporarily assumed another identity through <A
HREF="system.html#SUREF"
>su</A
>. <TT
CLASS="VARNAME"
>$UID</TT
> is a
	      readonly variable, not subject to change from the command
	      line or within a script, and is the counterpart to the
	      <A
HREF="system.html#IDREF"
>id</A
> builtin.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="AMIROOT"
></A
><P
><B
>Example 9-5. Am I root?</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# am-i-root.sh:   Am I root or not?
   3&nbsp;
   4&nbsp;ROOT_UID=0   # Root has $UID 0.
   5&nbsp;
   6&nbsp;if [ "$UID" -eq "$ROOT_UID" ]  # Will the real "root" please stand up?
   7&nbsp;then
   8&nbsp;  echo "You are root."
   9&nbsp;else
  10&nbsp;  echo "You are just an ordinary user (but mom loves you just the same)."
  11&nbsp;fi
  12&nbsp;
  13&nbsp;exit 0
  14&nbsp;
  15&nbsp;
  16&nbsp;# ============================================================= #
  17&nbsp;# Code below will not execute, because the script already exited.
  18&nbsp;
  19&nbsp;# An alternate method of getting to the root of matters:
  20&nbsp;
  21&nbsp;ROOTUSER_NAME=root
  22&nbsp;
  23&nbsp;username=`id -nu`              # Or...   username=`whoami`
  24&nbsp;if [ "$username" = "$ROOTUSER_NAME" ]
  25&nbsp;then
  26&nbsp;  echo "Rooty, toot, toot. You are root."
  27&nbsp;else
  28&nbsp;  echo "You are just a regular fella."
  29&nbsp;fi</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>See also <A
HREF="sha-bang.html#EX2"
>Example 2-3</A
>.</P
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The variables <TT
CLASS="VARNAME"
>$ENV</TT
>,
	  <TT
CLASS="VARNAME"
>$LOGNAME</TT
>, <TT
CLASS="VARNAME"
>$MAIL</TT
>,
	  <TT
CLASS="VARNAME"
>$TERM</TT
>, <TT
CLASS="VARNAME"
>$USER</TT
>, and
	  <TT
CLASS="VARNAME"
>$USERNAME</TT
> are <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
>
	  Bash <A
HREF="internal.html#BUILTINREF"
>builtins</A
>. These are,
	  however, often set as <A
HREF="othertypesv.html#ENVREF"
>environmental variables</A
> in
	  one of the <A
HREF="files.html#FILESREF1"
>Bash</A
> or
	  <I
CLASS="FIRSTTERM"
>login</I
> startup files. <A
NAME="SHELLVARREF"
></A
><TT
CLASS="VARNAME"
>$SHELL</TT
>,
	  the name of the user's login shell, may be set from
	  <TT
CLASS="FILENAME"
>/etc/passwd</TT
> or in an <SPAN
CLASS="QUOTE"
>"init"</SPAN
>
	  script, and it is likewise not a Bash builtin.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>tcsh% </TT
><TT
CLASS="USERINPUT"
><B
>echo $LOGNAME</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>bozo</TT
>
 <TT
CLASS="PROMPT"
>tcsh% </TT
><TT
CLASS="USERINPUT"
><B
>echo $SHELL</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>/bin/tcsh</TT
>
 <TT
CLASS="PROMPT"
>tcsh% </TT
><TT
CLASS="USERINPUT"
><B
>echo $TERM</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>rxvt</TT
>
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo $LOGNAME</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>bozo</TT
>
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo $SHELL</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>/bin/tcsh</TT
>
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo $TERM</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>rxvt</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	      </P
></TD
></TR
></TABLE
></DIV
></DD
></DL
></DIV
><DIV
CLASS="VARIABLELIST"
><P
><B
>Positional Parameters</B
></P
><DL
><DT
><A
NAME="POSPARAMREF"
></A
><TT
CLASS="VARNAME"
>$0</TT
>, <TT
CLASS="VARNAME"
>$1</TT
>,
	  <TT
CLASS="VARNAME"
>$2</TT
>, etc.</DT
><DD
><P
>Positional parameters, passed from command
	      line to script, passed to a function, or <A
HREF="internal.html#SETREF"
>set</A
> to a variable (see <A
HREF="othertypesv.html#EX17"
>Example 4-5</A
> and <A
HREF="internal.html#EX34"
>Example 15-16</A
>)</P
></DD
><DT
><A
NAME="CLACOUNTREF"
></A
><TT
CLASS="VARNAME"
>$#</TT
></DT
><DD
><P
>Number of command-line arguments
		<A
NAME="AEN5479"
HREF="#FTN.AEN5479"
>[4]</A
>
	    or positional parameters (see <A
HREF="wrapper.html#EX4"
>Example 36-2</A
>)</P
></DD
><DT
><A
NAME="APPREF"
></A
><TT
CLASS="VARNAME"
>$*</TT
></DT
><DD
><P
>All of the positional parameters, seen as a single word</P
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
><SPAN
CLASS="QUOTE"
>"<TT
CLASS="VARNAME"
>$*</TT
>"</SPAN
> must be
	    quoted.</P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><A
NAME="APPREF2"
></A
><TT
CLASS="VARNAME"
>$@</TT
></DT
><DD
><P
>Same as <SPAN
CLASS="TOKEN"
>$*</SPAN
>, but each parameter is a
	      quoted string, that is, the parameters are passed on
	      intact, without interpretation or expansion. This means,
	      among other things, that each parameter in the argument
	      list is seen as a separate word.</P
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>Of course, <SPAN
CLASS="QUOTE"
>"<TT
CLASS="VARNAME"
>$@</TT
>"</SPAN
>
	    should be quoted.</P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="ARGLIST"
></A
><P
><B
>Example 9-6. <I
CLASS="FIRSTTERM"
>arglist</I
>: Listing arguments
	      with $* and $@</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# arglist.sh
   3&nbsp;# Invoke this script with several arguments, such as "one two three" ...
   4&nbsp;
   5&nbsp;E_BADARGS=85
   6&nbsp;
   7&nbsp;if [ ! -n "$1" ]
   8&nbsp;then
   9&nbsp;  echo "Usage: `basename $0` argument1 argument2 etc."
  10&nbsp;  exit $E_BADARGS
  11&nbsp;fi  
  12&nbsp;
  13&nbsp;echo
  14&nbsp;
  15&nbsp;index=1          # Initialize count.
  16&nbsp;
  17&nbsp;echo "Listing args with \"\$*\":"
  18&nbsp;for arg in "$*"  # Doesn't work properly if "$*" isn't quoted.
  19&nbsp;do
  20&nbsp;  echo "Arg #$index = $arg"
  21&nbsp;  let "index+=1"
  22&nbsp;done             # $* sees all arguments as single word. 
  23&nbsp;echo "Entire arg list seen as single word."
  24&nbsp;
  25&nbsp;echo
  26&nbsp;
  27&nbsp;index=1          # Reset count.
  28&nbsp;                 # What happens if you forget to do this?
  29&nbsp;
  30&nbsp;echo "Listing args with \"\$@\":"
  31&nbsp;for arg in "$@"
  32&nbsp;do
  33&nbsp;  echo "Arg #$index = $arg"
  34&nbsp;  let "index+=1"
  35&nbsp;done             # $@ sees arguments as separate words. 
  36&nbsp;echo "Arg list seen as separate words."
  37&nbsp;
  38&nbsp;echo
  39&nbsp;
  40&nbsp;index=1          # Reset count.
  41&nbsp;
  42&nbsp;echo "Listing args with \$* (unquoted):"
  43&nbsp;for arg in $*
  44&nbsp;do
  45&nbsp;  echo "Arg #$index = $arg"
  46&nbsp;  let "index+=1"
  47&nbsp;done             # Unquoted $* sees arguments as separate words. 
  48&nbsp;echo "Arg list seen as separate words."
  49&nbsp;
  50&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>Following a <B
CLASS="COMMAND"
>shift</B
>, the
	      <TT
CLASS="VARNAME"
>$@</TT
> holds the remaining command-line
	      parameters, lacking the previous <TT
CLASS="VARNAME"
>$1</TT
>,
	      which was lost.
	        <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# Invoke with ./scriptname 1 2 3 4 5
   3&nbsp;
   4&nbsp;echo "$@"    # 1 2 3 4 5
   5&nbsp;shift
   6&nbsp;echo "$@"    # 2 3 4 5
   7&nbsp;shift
   8&nbsp;echo "$@"    # 3 4 5
   9&nbsp;
  10&nbsp;# Each "shift" loses parameter $1.
  11&nbsp;# "$@" then contains the remaining parameters.</PRE
></TD
></TR
></TABLE
>
            </P
><P
>The <TT
CLASS="VARNAME"
>$@</TT
> special parameter finds
	      use as a tool for filtering input into shell scripts. The
	      <B
CLASS="COMMAND"
>cat "$@"</B
> construction accepts input
	      to a script either from <TT
CLASS="FILENAME"
>stdin</TT
> or
	      from files given as parameters to the script. See <A
HREF="textproc.html#ROT13"
>Example 16-24</A
> and <A
HREF="textproc.html#CRYPTOQUOTE"
>Example 16-25</A
>.</P
><DIV
CLASS="CAUTION"
><TABLE
CLASS="CAUTION"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/caution.png"
HSPACE="5"
ALT="Caution"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The <TT
CLASS="VARNAME"
>$*</TT
> and <TT
CLASS="VARNAME"
>$@</TT
>
	      parameters sometimes display inconsistent and
	      puzzling behavior, depending on the setting of <A
HREF="variables2.html#IFSREF"
>$IFS</A
>.</P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="INCOMPAT"
></A
><P
><B
>Example 9-7. Inconsistent <TT
CLASS="VARNAME"
>$*</TT
> and <TT
CLASS="VARNAME"
>$@</TT
> behavior</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;#  Erratic behavior of the "$*" and "$@" internal Bash variables,
   4&nbsp;#+ depending on whether or not they are quoted.
   5&nbsp;#  Demonstrates inconsistent handling of word splitting and linefeeds.
   6&nbsp;
   7&nbsp;
   8&nbsp;set -- "First one" "second" "third:one" "" "Fifth: :one"
   9&nbsp;# Setting the script arguments, $1, $2, $3, etc.
  10&nbsp;
  11&nbsp;echo
  12&nbsp;
  13&nbsp;echo 'IFS unchanged, using "$*"'
  14&nbsp;c=0
  15&nbsp;for i in "$*"               # quoted
  16&nbsp;do echo "$((c+=1)): [$i]"   # This line remains the same in every instance.
  17&nbsp;                            # Echo args.
  18&nbsp;done
  19&nbsp;echo ---
  20&nbsp;
  21&nbsp;echo 'IFS unchanged, using $*'
  22&nbsp;c=0
  23&nbsp;for i in $*                 # unquoted
  24&nbsp;do echo "$((c+=1)): [$i]"
  25&nbsp;done
  26&nbsp;echo ---
  27&nbsp;
  28&nbsp;echo 'IFS unchanged, using "$@"'
  29&nbsp;c=0
  30&nbsp;for i in "$@"
  31&nbsp;do echo "$((c+=1)): [$i]"
  32&nbsp;done
  33&nbsp;echo ---
  34&nbsp;
  35&nbsp;echo 'IFS unchanged, using $@'
  36&nbsp;c=0
  37&nbsp;for i in $@
  38&nbsp;do echo "$((c+=1)): [$i]"
  39&nbsp;done
  40&nbsp;echo ---
  41&nbsp;
  42&nbsp;IFS=:
  43&nbsp;echo 'IFS=":", using "$*"'
  44&nbsp;c=0
  45&nbsp;for i in "$*"
  46&nbsp;do echo "$((c+=1)): [$i]"
  47&nbsp;done
  48&nbsp;echo ---
  49&nbsp;
  50&nbsp;echo 'IFS=":", using $*'
  51&nbsp;c=0
  52&nbsp;for i in $*
  53&nbsp;do echo "$((c+=1)): [$i]"
  54&nbsp;done
  55&nbsp;echo ---
  56&nbsp;
  57&nbsp;var=$*
  58&nbsp;echo 'IFS=":", using "$var" (var=$*)'
  59&nbsp;c=0
  60&nbsp;for i in "$var"
  61&nbsp;do echo "$((c+=1)): [$i]"
  62&nbsp;done
  63&nbsp;echo ---
  64&nbsp;
  65&nbsp;echo 'IFS=":", using $var (var=$*)'
  66&nbsp;c=0
  67&nbsp;for i in $var
  68&nbsp;do echo "$((c+=1)): [$i]"
  69&nbsp;done
  70&nbsp;echo ---
  71&nbsp;
  72&nbsp;var="$*"
  73&nbsp;echo 'IFS=":", using $var (var="$*")'
  74&nbsp;c=0
  75&nbsp;for i in $var
  76&nbsp;do echo "$((c+=1)): [$i]"
  77&nbsp;done
  78&nbsp;echo ---
  79&nbsp;
  80&nbsp;echo 'IFS=":", using "$var" (var="$*")'
  81&nbsp;c=0
  82&nbsp;for i in "$var"
  83&nbsp;do echo "$((c+=1)): [$i]"
  84&nbsp;done
  85&nbsp;echo ---
  86&nbsp;
  87&nbsp;echo 'IFS=":", using "$@"'
  88&nbsp;c=0
  89&nbsp;for i in "$@"
  90&nbsp;do echo "$((c+=1)): [$i]"
  91&nbsp;done
  92&nbsp;echo ---
  93&nbsp;
  94&nbsp;echo 'IFS=":", using $@'
  95&nbsp;c=0
  96&nbsp;for i in $@
  97&nbsp;do echo "$((c+=1)): [$i]"
  98&nbsp;done
  99&nbsp;echo ---
 100&nbsp;
 101&nbsp;var=$@
 102&nbsp;echo 'IFS=":", using $var (var=$@)'
 103&nbsp;c=0
 104&nbsp;for i in $var
 105&nbsp;do echo "$((c+=1)): [$i]"
 106&nbsp;done
 107&nbsp;echo ---
 108&nbsp;
 109&nbsp;echo 'IFS=":", using "$var" (var=$@)'
 110&nbsp;c=0
 111&nbsp;for i in "$var"
 112&nbsp;do echo "$((c+=1)): [$i]"
 113&nbsp;done
 114&nbsp;echo ---
 115&nbsp;
 116&nbsp;var="$@"
 117&nbsp;echo 'IFS=":", using "$var" (var="$@")'
 118&nbsp;c=0
 119&nbsp;for i in "$var"
 120&nbsp;do echo "$((c+=1)): [$i]"
 121&nbsp;done
 122&nbsp;echo ---
 123&nbsp;
 124&nbsp;echo 'IFS=":", using $var (var="$@")'
 125&nbsp;c=0
 126&nbsp;for i in $var
 127&nbsp;do echo "$((c+=1)): [$i]"
 128&nbsp;done
 129&nbsp;
 130&nbsp;echo
 131&nbsp;
 132&nbsp;# Try this script with ksh or zsh -y.
 133&nbsp;
 134&nbsp;exit 0
 135&nbsp;
 136&nbsp;#  This example script written by Stephane Chazelas,
 137&nbsp;#+ and slightly modified by the document author.</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The <B
CLASS="COMMAND"
>$@</B
> and <B
CLASS="COMMAND"
>$*</B
>
	      parameters differ only when between double quotes.</P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="IFSEMPTY"
></A
><P
><B
>Example 9-8. <TT
CLASS="VARNAME"
>$*</TT
> and <TT
CLASS="VARNAME"
>$@</TT
> when
	        <TT
CLASS="VARNAME"
>$IFS</TT
> is empty</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;#  If $IFS set, but empty,
   4&nbsp;#+ then "$*" and "$@" do not echo positional params as expected.
   5&nbsp;
   6&nbsp;mecho ()       # Echo positional parameters.
   7&nbsp;{
   8&nbsp;echo "$1,$2,$3";
   9&nbsp;}
  10&nbsp;
  11&nbsp;
  12&nbsp;IFS=""         # Set, but empty.
  13&nbsp;set a b c      # Positional parameters.
  14&nbsp;
  15&nbsp;mecho "$*"     # abc,,
  16&nbsp;#                   ^^
  17&nbsp;mecho $*       # a,b,c
  18&nbsp;
  19&nbsp;mecho $@       # a,b,c
  20&nbsp;mecho "$@"     # a,b,c
  21&nbsp;
  22&nbsp;#  The behavior of $* and $@ when $IFS is empty depends
  23&nbsp;#+ on which Bash or sh version being run.
  24&nbsp;#  It is therefore inadvisable to depend on this "feature" in a script.
  25&nbsp;
  26&nbsp;
  27&nbsp;# Thanks, Stephane Chazelas.
  28&nbsp;
  29&nbsp;exit</PRE
></TD
></TR
></TABLE
><HR></DIV
></DD
></DL
></DIV
><DIV
CLASS="VARIABLELIST"
><P
><B
>Other Special Parameters</B
></P
><DL
><DT
><A
NAME="FLPREF"
></A
><TT
CLASS="VARNAME"
>$-</TT
></DT
><DD
><P
>Flags passed to script (using <A
HREF="internal.html#SETREF"
>set</A
>). See <A
HREF="internal.html#EX34"
>Example 15-16</A
>.</P
><DIV
CLASS="CAUTION"
><TABLE
CLASS="CAUTION"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/caution.png"
HSPACE="5"
ALT="Caution"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>This was originally a <I
CLASS="FIRSTTERM"
>ksh</I
>
	      construct adopted into Bash, and unfortunately it does not
	      seem to work reliably in Bash scripts. One possible use
	      for it is to have a script <A
HREF="miscellany.html#IITEST"
>self-test
	      whether it is interactive</A
>.</P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><A
NAME="PIDVARREF"
></A
><TT
CLASS="VARNAME"
>$!</TT
></DT
><DD
><P
><A
HREF="special-chars.html#PROCESSIDDEF"
>PID</A
> (process ID) of last
	      job run in background</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;LOG=$0.log
   2&nbsp;
   3&nbsp;COMMAND1="sleep 100"
   4&nbsp;
   5&nbsp;echo "Logging PIDs background commands for script: $0" &#62;&#62; "$LOG"
   6&nbsp;# So they can be monitored, and killed as necessary.
   7&nbsp;echo &#62;&#62; "$LOG"
   8&nbsp;
   9&nbsp;# Logging commands.
  10&nbsp;
  11&nbsp;echo -n "PID of \"$COMMAND1\":  " &#62;&#62; "$LOG"
  12&nbsp;${COMMAND1} &#38;
  13&nbsp;echo $! &#62;&#62; "$LOG"
  14&nbsp;# PID of "sleep 100":  1506
  15&nbsp;
  16&nbsp;# Thank you, Jacques Lederer, for suggesting this.</PRE
></TD
></TR
></TABLE
>
            </P
><P
>Using <TT
CLASS="VARNAME"
>$!</TT
> for job control:</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;possibly_hanging_job &#38; { sleep ${TIMEOUT}; eval 'kill -9 $!' &#38;&#62; /dev/null; }
   2&nbsp;# Forces completion of an ill-behaved program.
   3&nbsp;# Useful, for example, in init scripts.
   4&nbsp;
   5&nbsp;# Thank you, Sylvain Fourmanoit, for this creative use of the "!" variable.</PRE
></TD
></TR
></TABLE
>

            </P
><P
>Or, alternately:</P
><P
>              <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;# This example by Matthew Sage.
   2&nbsp;# Used with permission.
   3&nbsp;
   4&nbsp;TIMEOUT=30   # Timeout value in seconds
   5&nbsp;count=0
   6&nbsp;
   7&nbsp;possibly_hanging_job &#38; {
   8&nbsp;        while ((count &#60; TIMEOUT )); do
   9&nbsp;                eval '[ ! -d "/proc/$!" ] &#38;&#38; ((count = TIMEOUT))'
  10&nbsp;                # /proc is where information about running processes is found.
  11&nbsp;                # "-d" tests whether it exists (whether directory exists).
  12&nbsp;                # So, we're waiting for the job in question to show up.
  13&nbsp;                ((count++))
  14&nbsp;                sleep 1
  15&nbsp;        done
  16&nbsp;        eval '[ -d "/proc/$!" ] &#38;&#38; kill -15 $!'
  17&nbsp;        # If the hanging job is running, kill it.
  18&nbsp;}
  19&nbsp;
  20&nbsp;#  -------------------------------------------------------------- #
  21&nbsp;
  22&nbsp;#  However, this may not not work as specified if another process
  23&nbsp;#+ begins to run after the "hanging_job" . . .
  24&nbsp;#  In such a case, the wrong job may be killed.
  25&nbsp;#  Ariel Meragelman suggests the following fix.
  26&nbsp;
  27&nbsp;TIMEOUT=30
  28&nbsp;count=0
  29&nbsp;# Timeout value in seconds
  30&nbsp;possibly_hanging_job &#38; {
  31&nbsp;
  32&nbsp;while ((count &#60; TIMEOUT )); do
  33&nbsp;  eval '[ ! -d "/proc/$lastjob" ] &#38;&#38; ((count = TIMEOUT))'
  34&nbsp;  lastjob=$!
  35&nbsp;  ((count++))
  36&nbsp;  sleep 1
  37&nbsp;done
  38&nbsp;eval '[ -d "/proc/$lastjob" ] &#38;&#38; kill -15 $lastjob'
  39&nbsp;
  40&nbsp;}
  41&nbsp;
  42&nbsp;exit</PRE
></TD
></TR
></TABLE
>
	    </P
></DD
><DT
><A
NAME="UNDERSCOREREF"
></A
><TT
CLASS="VARNAME"
>$_</TT
></DT
><DD
><P
>Special variable set to final argument of previous command
	        executed.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="USCREF"
></A
><P
><B
>Example 9-9. Underscore variable</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;echo $_              #  /bin/bash
   4&nbsp;                     #  Just called /bin/bash to run the script.
   5&nbsp;                     #  Note that this will vary according to
   6&nbsp;                     #+ how the script is invoked.
   7&nbsp;
   8&nbsp;du &#62;/dev/null        #  So no output from command.
   9&nbsp;echo $_              #  du
  10&nbsp;
  11&nbsp;ls -al &#62;/dev/null    #  So no output from command.
  12&nbsp;echo $_              #  -al  (last argument)
  13&nbsp;
  14&nbsp;:
  15&nbsp;echo $_              #  :</PRE
></TD
></TR
></TABLE
><HR></DIV
></DD
><DT
><A
NAME="XSTATVARREF"
></A
><TT
CLASS="VARNAME"
>$?</TT
></DT
><DD
><P
><A
HREF="exit-status.html#EXITSTATUSREF"
>Exit status</A
>
	    of a command, <A
HREF="functions.html#FUNCTIONREF"
>function</A
>,
	    or the script itself (see <A
HREF="functions.html#MAX"
>Example 24-7</A
>)</P
></DD
><DT
><A
NAME="PROCCID"
></A
><TT
CLASS="VARNAME"
>$$</TT
></DT
><DD
><P
>Process ID (<I
CLASS="FIRSTTERM"
>PID</I
>) of
            the script itself.

	      <A
NAME="AEN5654"
HREF="#FTN.AEN5654"
>[5]</A
>

            The <TT
CLASS="VARNAME"
>$$</TT
> variable often
	    finds use in scripts to construct <SPAN
CLASS="QUOTE"
>"unique"</SPAN
>
	    temp file names (see <A
HREF="debugging.html#ONLINE"
>Example 32-6</A
>, <A
HREF="filearchiv.html#DERPM"
>Example 16-31</A
>, and <A
HREF="internal.html#SELFDESTRUCT"
>Example 15-27</A
>).
	    This is usually simpler than invoking <A
HREF="filearchiv.html#MKTEMPREF"
>mktemp</A
>.</P
></DD
></DL
></DIV
></DIV
></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
><TABLE
BORDER="0"
CLASS="FOOTNOTES"
WIDTH="100%"
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN4671"
HREF="variables2.html#AEN4671"
>[1]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
><A
NAME="STACKDEFREF"
></A
>
              A <I
CLASS="FIRSTTERM"
>stack register</I
>
              is a set of consecutive memory locations, such that
	      the values stored (<I
CLASS="FIRSTTERM"
>pushed</I
>)
	      are retrieved (<I
CLASS="FIRSTTERM"
>popped</I
>)
	      in <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>reverse</I
></SPAN
> order. The last
	      value stored is the first retrieved. This is
	      sometimes called a <TT
CLASS="REPLACEABLE"
><I
>LIFO</I
></TT
>
	      (<I
CLASS="FIRSTTERM"
>last-in-first-out</I
>) or
	      <I
CLASS="FIRSTTERM"
>pushdown</I
> stack.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN5154"
HREF="variables2.html#AEN5154"
>[2]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>The PID of the currently running script is
		  <TT
CLASS="VARNAME"
>$$</TT
>, of course.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN5320"
HREF="variables2.html#AEN5320"
>[3]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>		Somewhat analogous to <A
HREF="localvar.html#RECURSIONREF"
>recursion</A
>, in this context
		<I
CLASS="FIRSTTERM"
>nesting</I
> refers to a pattern
		embedded within a larger pattern. One of the definitions
		of <I
CLASS="FIRSTTERM"
>nest</I
>, according to the 1913
		edition of <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>Webster's Dictionary</I
></SPAN
>,
		illustrates this beautifully: <SPAN
CLASS="QUOTE"
>"<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>A collection of
		boxes, cases, or the like, of graduated size, each put
		within the one next larger.</I
></SPAN
>"</SPAN
>
	      </P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN5479"
HREF="variables2.html#AEN5479"
>[4]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>The words <SPAN
CLASS="QUOTE"
>"argument"</SPAN
>
		and <SPAN
CLASS="QUOTE"
>"parameter"</SPAN
> are often used
		interchangeably. In the context of this document, they
		have the same precise meaning: <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>a variable passed
		to a script or function.</I
></SPAN
></P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN5654"
HREF="variables2.html#AEN5654"
>[5]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>Within a script, inside a subshell,
              <TT
CLASS="VARNAME"
>$$</TT
> <A
HREF="variables2.html#BASHPIDREF"
>returns
              the PID of the script</A
>, not the
              subshell.</P
></TD
></TR
></TABLE
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> or
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