variables2.html

Shall高级编程

><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;E_WRONG_DIRECTORY=73
   4&nbsp;
   5&nbsp;clear # Clear 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;# Filenames may contain all characters in the 0 - 255 range, except "/".
  27&nbsp;# Deleting files beginning with weird characters is left as an exercise.
  28&nbsp;
  29&nbsp;# Various other operations here, as necessary.
  30&nbsp;
  31&nbsp;echo
  32&nbsp;echo "Done."
  33&nbsp;echo "Old files deleted in $TargetDirectory."
  34&nbsp;echo
  35&nbsp;
  36&nbsp;
  37&nbsp;exit 0</PRE
></TD
></TR
></TABLE
></P
></DD
><DT
><A
NAME="REPLYREF"
></A
><TT
CLASS="VARNAME"
>$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 "$TIME_LIMIT" ]
  11&nbsp;do
  12&nbsp;  if [ "$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="AEN4581"
HREF="#FTN.AEN4581"
>[2]</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 $song."</PRE
></TD
></TR
></TABLE
>
	  </P
><P
><A
NAME="TIMINGLOOP"
></A
></P
><P
>There are other, more complex, ways of implementing
	    timed input in a script. One alternative is to set up
	    a timing loop to signal the script when it times out.
	    This also requires a signal handling routine to trap (see
	    <A
HREF="debugging.html#EX76"
>Example 29-5</A
>) the interrupt generated by the timing
	    loop (whew!).</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="TMDIN"
></A
><P
><B
>Example 9-2. Timed Input</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# timed-input.sh
   3&nbsp;
   4&nbsp;# TMOUT=3    Also works, as of newer versions of Bash.
   5&nbsp;
   6&nbsp;
   7&nbsp;TIMELIMIT=3  # Three seconds in this instance. May be set to different value.
   8&nbsp;
   9&nbsp;PrintAnswer()
  10&nbsp;{
  11&nbsp;  if [ "$answer" = TIMEOUT ]
  12&nbsp;  then
  13&nbsp;    echo $answer
  14&nbsp;  else       # Don't want to mix up the two instances. 
  15&nbsp;    echo "Your favorite veggie is $answer"
  16&nbsp;    kill $!  # Kills no longer needed TimerOn function running in background.
  17&nbsp;             # $! is PID of last job running in background.
  18&nbsp;  fi
  19&nbsp;
  20&nbsp;}  
  21&nbsp;
  22&nbsp;
  23&nbsp;
  24&nbsp;TimerOn()
  25&nbsp;{
  26&nbsp;  sleep $TIMELIMIT &#38;&#38; kill -s 14 $$ &#38;
  27&nbsp;  # Waits 3 seconds, then sends sigalarm to script.
  28&nbsp;}  
  29&nbsp;
  30&nbsp;Int14Vector()
  31&nbsp;{
  32&nbsp;  answer="TIMEOUT"
  33&nbsp;  PrintAnswer
  34&nbsp;  exit 14
  35&nbsp;}  
  36&nbsp;
  37&nbsp;trap Int14Vector 14   # Timer interrupt (14) subverted for our purposes.
  38&nbsp;
  39&nbsp;echo "What is your favorite vegetable "
  40&nbsp;TimerOn
  41&nbsp;read answer
  42&nbsp;PrintAnswer
  43&nbsp;
  44&nbsp;
  45&nbsp;#  Admittedly, this is a kludgy implementation of timed input,
  46&nbsp;#+ however the "-t" option to "read" simplifies this task.
  47&nbsp;#  See "t-out.sh", below.
  48&nbsp;
  49&nbsp;#  If you need something really elegant...
  50&nbsp;#+ consider writing the application in C or C++,
  51&nbsp;#+ using appropriate library functions, such as 'alarm' and 'setitimer'.
  52&nbsp;
  53&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
   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 $TIMELIMIT variable &#60;&#38;1
   9&nbsp;#                           ^^^
  10&nbsp;#  In this instance, "&#60;&#38;1" is needed for Bash 1.x and 2.x,
  11&nbsp;#  but unnecessary for Bash 3.x.
  12&nbsp;
  13&nbsp;echo
  14&nbsp;
  15&nbsp;if [ -z "$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 <TT
CLASS="FILENAME"
>/etc/passwd</TT
>
	    </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"
>$USERNAME</TT
> are <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
>
	  Bash <A
HREF="internal.html#BUILTINREF"
>builtins</A
>. These are,
	  however, often set as <A
HREF="othertypesv.html#ENVREF"
>environmental
	  variables</A
> in one of the Bash <A
HREF="files.html#FILESREF1"
>startup files</A
>.  <A
NAME="SHELLVARREF"
></A
><TT
CLASS="VARNAME"
>$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"
>$2</TT
>, etc.</DT
><DD
><P
>positional parameters, passed from command
	      line to script, passed to a function, or <A
HREF="internal.html#SETREF"
>set</A
> to a variable (see <A
HREF="othertypesv.html#EX17"
>Example 4-5</A
> and <A
HREF="internal.html#EX34"
>Example 14-16</A
>)</P
></DD
><DT
><A
NAME="CLACOUNTREF"
></A
><TT
CLASS="VARNAME"
>$#</TT
></DT
><DD
><P
>number of command line arguments
		<A
NAME="AEN4737"
HREF="#FTN.AEN4737"
>[3]</A
>
	    or positional parameters (see <A
HREF="wrapper.html#EX4"
>Example 33-2</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,