internal.html

Shall高级编程

><A
NAME="READTIMED"
></A
></P
><P
>The <TT
CLASS="OPTION"
>-t</TT
> option to <B
CLASS="COMMAND"
>read</B
>
	      permits timed input (see <A
HREF="variables2.html#TOUT"
>Example 9-4</A
>).</P
><P
><A
NAME="READREDIR0"
></A
></P
><P
>The <B
CLASS="COMMAND"
>read</B
> command may also
	      <SPAN
CLASS="QUOTE"
>"read"</SPAN
> its variable value from a file
	      <A
HREF="io-redirection.html#IOREDIRREF"
>redirected</A
> to
	      <TT
CLASS="FILENAME"
>stdin</TT
>.  If the file contains
	      more than one line, only the first line is assigned
	      to the variable. If <B
CLASS="COMMAND"
>read</B
>
	      has more than one parameter, then each of
	      these variables gets assigned a successive <A
HREF="special-chars.html#WHITESPACEREF"
>whitespace-delineated</A
>
	      string. Caution!</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="READREDIR"
></A
><P
><B
>Example 14-7. Using <I
CLASS="FIRSTTERM"
>read</I
> with
	        <A
HREF="io-redirection.html#IOREDIRREF"
>file redirection</A
></B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;read var1 &#60;data-file
   4&nbsp;echo "var1 = $var1"
   5&nbsp;# var1 set to the entire first line of the input file "data-file"
   6&nbsp;
   7&nbsp;read var2 var3 &#60;data-file
   8&nbsp;echo "var2 = $var2   var3 = $var3"
   9&nbsp;# Note non-intuitive behavior of "read" here.
  10&nbsp;# 1) Rewinds back to the beginning of input file.
  11&nbsp;# 2) Each variable is now set to a corresponding string,
  12&nbsp;#    separated by whitespace, rather than to an entire line of text.
  13&nbsp;# 3) The final variable gets the remainder of the line.
  14&nbsp;# 4) If there are more variables to be set than whitespace-terminated strings
  15&nbsp;#    on the first line of the file, then the excess variables remain empty.
  16&nbsp;
  17&nbsp;echo "------------------------------------------------"
  18&nbsp;
  19&nbsp;# How to resolve the above problem with a loop:
  20&nbsp;while read line
  21&nbsp;do
  22&nbsp;  echo "$line"
  23&nbsp;done &#60;data-file
  24&nbsp;# Thanks, Heiner Steven for pointing this out.
  25&nbsp;
  26&nbsp;echo "------------------------------------------------"
  27&nbsp;
  28&nbsp;# Use $IFS (Internal Field Separator variable) to split a line of input to
  29&nbsp;# "read", if you do not want the default to be whitespace.
  30&nbsp;
  31&nbsp;echo "List of all users:"
  32&nbsp;OIFS=$IFS; IFS=:       # /etc/passwd uses ":" for field separator.
  33&nbsp;while read name passwd uid gid fullname ignore
  34&nbsp;do
  35&nbsp;  echo "$name ($fullname)"
  36&nbsp;done &#60;/etc/passwd   # I/O redirection.
  37&nbsp;IFS=$OIFS              # Restore original $IFS.
  38&nbsp;# This code snippet also by Heiner Steven.
  39&nbsp;
  40&nbsp;
  41&nbsp;
  42&nbsp;#  Setting the $IFS variable within the loop itself
  43&nbsp;#+ eliminates the need for storing the original $IFS
  44&nbsp;#+ in a temporary variable.
  45&nbsp;#  Thanks, Dim Segebart, for pointing this out.
  46&nbsp;echo "------------------------------------------------"
  47&nbsp;echo "List of all users:"
  48&nbsp;
  49&nbsp;while IFS=: read name passwd uid gid fullname ignore
  50&nbsp;do
  51&nbsp;  echo "$name ($fullname)"
  52&nbsp;done &#60;/etc/passwd   # I/O redirection.
  53&nbsp;
  54&nbsp;echo
  55&nbsp;echo "\$IFS still $IFS"
  56&nbsp;
  57&nbsp;exit 0</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
><A
HREF="special-chars.html#PIPEREF"
>Piping</A
> output
	      to a <I
CLASS="FIRSTTERM"
>read</I
>, using <A
HREF="internal.html#ECHOREF"
>echo</A
> to set variables <A
HREF="gotchas.html#BADREAD0"
>will fail</A
>.</P
><P
><A
NAME="READPIPEREF"
></A
>Yet, piping the output of <A
HREF="external.html#CATREF"
>cat</A
> <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>seems</I
></SPAN
> to
	      work.</P
><P
><A
NAME="WHILEREADREF"
></A
></P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;cat file1 file2 |
   2&nbsp;while read line
   3&nbsp;do
   4&nbsp;echo $line
   5&nbsp;done</PRE
></TD
></TR
></TABLE
></P
><P
>However, as Bj鰊 Eriksson shows:</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="READPIPE"
></A
><P
><B
>Example 14-8. Problems reading from a pipe</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/sh
   2&nbsp;# readpipe.sh
   3&nbsp;# This example contributed by Bjon Eriksson.
   4&nbsp;
   5&nbsp;last="(null)"
   6&nbsp;cat $0 |
   7&nbsp;while read line
   8&nbsp;do
   9&nbsp;    echo "{$line}"
  10&nbsp;    last=$line
  11&nbsp;done
  12&nbsp;printf "\nAll done, last:$last\n"
  13&nbsp;
  14&nbsp;exit 0  # End of code.
  15&nbsp;        # (Partial) output of script follows.
  16&nbsp;        # The 'echo' supplies extra brackets.
  17&nbsp;
  18&nbsp;#############################################
  19&nbsp;
  20&nbsp;./readpipe.sh 
  21&nbsp;
  22&nbsp;{#!/bin/sh}
  23&nbsp;{last="(null)"}
  24&nbsp;{cat $0 |}
  25&nbsp;{while read line}
  26&nbsp;{do}
  27&nbsp;{echo "{$line}"}
  28&nbsp;{last=$line}
  29&nbsp;{done}
  30&nbsp;{printf "nAll done, last:$lastn"}
  31&nbsp;
  32&nbsp;
  33&nbsp;All done, last:(null)
  34&nbsp;
  35&nbsp;The variable (last) is set within the subshell but unset outside.</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>The <I
CLASS="FIRSTTERM"
>gendiff</I
> script, usually
              found in <TT
CLASS="FILENAME"
>/usr/bin</TT
> on
              many Linux distros, pipes the output of <A
HREF="moreadv.html#FINDREF"
>find</A
> to a <I
CLASS="FIRSTTERM"
>while
              read</I
> construct.

	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;find $1 \( -name "*$2" -o -name ".*$2" \) -print |
   2&nbsp;while read f; do
   3&nbsp;. . .</PRE
></TD
></TR
></TABLE
>
            </P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="TIP"
><TABLE
CLASS="TIP"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/tip.png"
HSPACE="5"
ALT="Tip"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>It is possible to <I
CLASS="FIRSTTERM"
>paste</I
> text into
	        the input field of a <I
CLASS="FIRSTTERM"
>read</I
>. See <A
HREF="contributed-scripts.html#PADSW"
>Example A-39</A
>.</P
></TD
></TR
></TABLE
></DIV
></DD
></DL
></DIV
><DIV
CLASS="VARIABLELIST"
><P
><B
><A
NAME="INTFILESYSTEM1"
></A
>Filesystem</B
></P
><DL
><DT
><A
NAME="CDREF"
></A
><B
CLASS="COMMAND"
>cd</B
></DT
><DD
><P
>The familiar <B
CLASS="COMMAND"
>cd</B
> change directory
	      command finds use in scripts where execution of a command
	      requires being in a specified directory.</P
><P
>  
	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;(cd /source/directory &#38;&#38; tar cf - . ) | (cd /dest/directory &#38;&#38; tar xpvf -)</PRE
></TD
></TR
></TABLE
>
	      [from the <A
HREF="special-chars.html#COXEX"
>previously cited</A
>
	      example by Alan Cox]</P
><P
>The <TT
CLASS="OPTION"
>-P</TT
> (physical) option to
	      <B
CLASS="COMMAND"
>cd</B
> causes it to ignore symbolic
	      links.</P
><P
><B
CLASS="COMMAND"
>cd -</B
> changes to <A
HREF="variables2.html#OLDPWD"
>$OLDPWD</A
>, the previous working
	    directory.</P
><P
><A
NAME="DOUBLESLASHREF"
></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 <B
CLASS="COMMAND"
>cd</B
> command does not function
	      as expected when presented with two forward slashes.

	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>cd //</B
></TT
>
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>pwd</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>//</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	      The output should, of course, be <TT
CLASS="COMPUTEROUTPUT"
>/</TT
>.
	      This is a problem both from the command line and in a script.</P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><A
NAME="PWD2REF"
></A
><B
CLASS="COMMAND"
>pwd</B
></DT
><DD
><P
>Print Working Directory. This gives the user's
	      (or script's) current directory (see <A
HREF="internal.html#EX37"
>Example 14-9</A
>). The effect is identical to
	      reading the value of the builtin variable <A
HREF="variables2.html#PWDREF"
>$PWD</A
>.</P
></DD
><DT
><A
NAME="DIRSD"
></A
><B
CLASS="COMMAND"
>pushd</B
>, <B
CLASS="COMMAND"
>popd</B
>, <B
CLASS="COMMAND"
>dirs</B
></DT
><DD
><P
>This command set is a mechanism for bookmarking working directories,
	      a means of moving back and forth through directories in an orderly
	      manner.  A pushdown stack is used to keep track of directory names.
	      Options allow various manipulations of the directory stack.</P
><P
><A
NAME="PUSHDREF"
></A
><TT
CLASS="USERINPUT"
><B
>pushd
	      dir-name</B
></TT
> pushes the path
	      <TT
CLASS="REPLACEABLE"
><I
>dir-name</I
></TT
> onto the directory
	      stack and simultaneously changes the current working
	      directory to <TT
CLASS="REPLACEABLE"
><I
>dir-name</I
></TT
></P
><P
><A
NAME="POPDREF"
></A
><B
CLASS="COMMAND"
>popd</B
> removes
	      (pops) the top directory path name off the directory stack
	      and simultaneously changes the current working directory
	      to that directory popped from the stack.</P
><P
><A
NAME="DIRSREF"
></A
><B
CLASS="COMMAND"
>dirs</B
> lists
	      the contents of the directory stack (compare this
	      with the <A
HREF="variables2.html#DIRSTACKREF"
>$DIRSTACK</A
>
	      variable).  A successful <B
CLASS="COMMAND"
>pushd</B
>
	      or <B
CLASS="COMMAND"
>popd</B
> will automatically invoke
	      <B
CLASS="COMMAND"
>dirs</B
>.</P
><P
>Scripts that require various changes to the current
	      working directory without hard-coding the directory name
	      changes can make good use of these commands. Note that
	      the implicit <TT
CLASS="VARNAME"
>$DIRSTACK</TT
> array variable,
	      accessible from within a script, holds the contents of
	      the directory stack.
	    </P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX37"
></A
><P
><B
>Example 14-9. Changing the current working directory</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;dir1=/usr/local
   4&nbsp;dir2=/var/spool
   5&nbsp;
   6&nbsp;pushd $dir1
   7&nbsp;# Will do an automatic 'dirs' (list directory stack to stdout).
   8&nbsp;echo "Now in directory `pwd`." # Uses back-quoted 'pwd'.
   9&nbsp;
  10&nbsp;# Now, do some stuff in directory 'dir1'.
  11&nbsp;pushd $dir2
  12&nbsp;echo "Now in directory `pwd`."
  13&nbsp;
  14&nbsp;# Now, do some stuff in directory 'dir2'.
  15&nbsp;echo "The top entry in the DIRSTACK array is $DIRSTACK."
  16&nbsp;popd
  17&nbsp;echo "Now back in directory `pwd`."
  18&nbsp;
  19&nbsp;# Now, do some more stuff in directory 'dir1'.
  20&nbsp;popd
  21&nbsp;echo "Now back in original working directory `pwd`."
  22&nbsp;
  23&nbsp;exit 0
  24&nbsp;
  25&nbsp;# What happens if you don't 'popd' -- then exit the script?
  26&nbsp;# Which directory do you end up in? Why?</PRE
></TD
></TR
></TABLE
><HR></DIV
></DD
></DL
></DIV
><DIV
CLASS="VARIABLELIST"
><P
><B
><A
NAME="INTVAR1"
></A
>Variables</B
></P
><DL
><DT
><A
NAME="LETREF"
></A
><B
CLASS="COMMAND"
>let</B
></DT
><DD
><P
>The <B
CLASS="COMMAND"
>let</B
> command carries out arithmetic
	      operations on variables. In many cases, it functions as a less
	      complex version of <A
HREF="moreadv.html#EXPRREF"
>expr</A
>.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX46"
></A
><P
><B
>Example 14-10. Letting <I
CLASS="FIRSTTERM"
>let</I
> do arithmetic.</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;echo
   4&nbsp;
   5&nbsp;let a=11            # Same as 'a=11'
   6&nbsp;let a=a+5           # Equivalent to  let "a = a + 5"