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>[</B
> , 
	是调用<B
CLASS="COMMAND"
>test</B
>命令的标识. 
	而关闭条件判断用的的右中括号, <B
CLASS="COMMAND"
>]</B
>, 在if/test结构中并不是严格必需的, 
	但是在Bash的新版本中必须要求使用. </P
><DIV
CLASS="NOTE"
><P
></P
><TABLE
CLASS="NOTE"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="./images/note.gif"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
><B
CLASS="COMMAND"
>test</B
>命令在Bash中是<A
HREF="internal.html#BUILTINREF"
>内建</A
>命令, 用来测试文件类型, 或者用来比较字符串. 
	  因此, 在Bash脚本中, 
	  <B
CLASS="COMMAND"
>test</B
>命令并<EM
>不会</EM
>调用外部的<TT
CLASS="FILENAME"
>/usr/bin/test</TT
>中的test命令,
	  这是<EM
>sh-utils</EM
>工具包中的一部分. 
	  同样的, <SPAN
CLASS="TOKEN"
>[</SPAN
>也并不会调用<TT
CLASS="FILENAME"
>/usr/bin/[</TT
>, 
	  这是<TT
CLASS="FILENAME"
>/usr/bin/test</TT
>的符号链接.</P
><P
>	      <TABLE
BORDER="1"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>bash$ </SAMP
><KBD
CLASS="USERINPUT"
>type test</KBD
>
<SAMP
CLASS="COMPUTEROUTPUT"
>test is a shell builtin</SAMP
>
<SAMP
CLASS="PROMPT"
>bash$ </SAMP
><KBD
CLASS="USERINPUT"
>type '['</KBD
>
<SAMP
CLASS="COMPUTEROUTPUT"
>[ is a shell builtin</SAMP
>
<SAMP
CLASS="PROMPT"
>bash$ </SAMP
><KBD
CLASS="USERINPUT"
>type '[['</KBD
>
<SAMP
CLASS="COMPUTEROUTPUT"
>[[ is a shell keyword</SAMP
>
<SAMP
CLASS="PROMPT"
>bash$ </SAMP
><KBD
CLASS="USERINPUT"
>type ']]'</KBD
>
<SAMP
CLASS="COMPUTEROUTPUT"
>]] is a shell keyword</SAMP
>
<SAMP
CLASS="PROMPT"
>bash$ </SAMP
><KBD
CLASS="USERINPUT"
>type ']'</KBD
>
<SAMP
CLASS="COMPUTEROUTPUT"
>bash: type: ]: not found</SAMP
>
	      </PRE
></FONT
></TD
></TR
></TABLE
>
	    </P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX11"
></A
><P
><B
>例子 7-2. <SPAN
CLASS="TOKEN"
>test</SPAN
>,
	  <TT
CLASS="FILENAME"
>/usr/bin/test</TT
>, <SPAN
CLASS="TOKEN"
>[ ]</SPAN
>,
	  和<TT
CLASS="FILENAME"
>/usr/bin/[</TT
>都是等价命令</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>  1&nbsp;#!/bin/bash
  2&nbsp;
  3&nbsp;echo
  4&nbsp;
  5&nbsp;if test -z "$1"
  6&nbsp;then
  7&nbsp;  echo "No command-line arguments."
  8&nbsp;else
  9&nbsp;  echo "First command-line argument is $1."
 10&nbsp;fi
 11&nbsp;
 12&nbsp;echo
 13&nbsp;
 14&nbsp;if /usr/bin/test -z "$1"      # 与内建的"test"命令结果相同. 
 15&nbsp;then
 16&nbsp;  echo "No command-line arguments."
 17&nbsp;else
 18&nbsp;  echo "First command-line argument is $1."
 19&nbsp;fi
 20&nbsp;
 21&nbsp;echo
 22&nbsp;
 23&nbsp;if [ -z "$1" ]                # 与上边的代码块作用相同.
 24&nbsp;#   if [ -z "$1"                应该能够运行, 但是...
 25&nbsp;#+  Bash报错, 提示缺少关闭条件测试的右中括号. 
 26&nbsp;then
 27&nbsp;  echo "No command-line arguments."
 28&nbsp;else
 29&nbsp;  echo "First command-line argument is $1."
 30&nbsp;fi
 31&nbsp;
 32&nbsp;echo
 33&nbsp;
 34&nbsp;
 35&nbsp;if /usr/bin/[ -z "$1" ]       # 再来一个, 与上边的代码块作用相同.
 36&nbsp;# if /usr/bin/[ -z "$1"       # 能够工作, 但是还是给出一个错误消息.
 37&nbsp;#                             # 注意:
 38&nbsp;#                               在版本3.x的Bash中, 这个bug已经被修正了.
 39&nbsp;then
 40&nbsp;  echo "No command-line arguments."
 41&nbsp;else
 42&nbsp;  echo "First command-line argument is $1."
 43&nbsp;fi
 44&nbsp;
 45&nbsp;echo
 46&nbsp;
 47&nbsp;exit 0</PRE
></FONT
></TD
></TR
></TABLE
><HR></DIV
><P
><A
NAME="DBLBRACKETS"
></A
><SPAN
CLASS="TOKEN"
>[[  ]]</SPAN
>结构比<SPAN
CLASS="TOKEN"
>[	]结构更加通用</SPAN
>. 
			这是一个<EM
>扩展的test命令</EM
>, 是从<EM
>ksh88</EM
>中引进的.</P
><DIV
CLASS="NOTE"
><P
></P
><TABLE
CLASS="NOTE"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="./images/note.gif"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>在<SPAN
CLASS="TOKEN"
>[[</SPAN
>和<SPAN
CLASS="TOKEN"
>]]</SPAN
>之间所有的字符都不会发生文件名扩展或者单词分割,
	但是会发生参数扩展和命令替换. </P
></TD
></TR
></TABLE
></DIV
><P
>	<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>  1&nbsp;file=/etc/passwd
  2&nbsp;
  3&nbsp;if [[ -e $file ]]
  4&nbsp;then
  5&nbsp;  echo "Password file exists."
  6&nbsp;fi</PRE
></FONT
></TD
></TR
></TABLE
>
      </P
><DIV
CLASS="TIP"
><P
></P
><TABLE
CLASS="TIP"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="./images/tip.gif"
HSPACE="5"
ALT="Tip"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>使用<B
CLASS="COMMAND"
>[[ ... ]]</B
>条件判断结构,
	而不是<B
CLASS="COMMAND"
>[ ... ]</B
>, 能够防止脚本中的许多逻辑错误. 
	比如, <SPAN
CLASS="TOKEN"
>&#38;&#38;</SPAN
>,
	<SPAN
CLASS="TOKEN"
>||</SPAN
>, <SPAN
CLASS="TOKEN"
>&#60;</SPAN
>, 和<SPAN
CLASS="TOKEN"
>&#62;</SPAN
>
	操作符能够正常存在于<SPAN
CLASS="TOKEN"
>[[  ]]</SPAN
>条件判断结构中, 
	但是如果出现在<SPAN
CLASS="TOKEN"
>[  ]</SPAN
>结构中的话, 会报错.</P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="NOTE"
><P
></P
><TABLE
CLASS="NOTE"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="./images/note.gif"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>在<B
CLASS="COMMAND"
>if</B
>后面也不一定非得是<B
CLASS="COMMAND"
>test</B
>命令或者是用于条件判断的中括号结构( [ ] 或 [[ ]] ).

	<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>  1&nbsp;dir=/home/bozo
  2&nbsp;
  3&nbsp;if cd "$dir" 2&#62;/dev/null; then   # "2&#62;/dev/null" 会隐藏错误信息.
  4&nbsp;  echo "Now in $dir."
  5&nbsp;else
  6&nbsp;  echo "Can't change to $dir."
  7&nbsp;fi</PRE
></FONT
></TD
></TR
></TABLE
>

        "if COMMAND"结构将会返回COMMAND的退出状态码.</P
><P
>与此相似, 在中括号中的条件判断也不一定非得要<B
CLASS="COMMAND"
>if</B
>不可, 
	也可以使用<A
HREF="list-cons.html#LISTCONSREF"
>列表结构</A
>.

	  <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>  1&nbsp;var1=20
  2&nbsp;var2=22
  3&nbsp;[ "$var1" -ne "$var2" ] &#38;&#38; echo "$var1 is not equal to $var2"
  4&nbsp;
  5&nbsp;home=/home/bozo
  6&nbsp;[ -d "$home" ] || echo "$home directory does not exist."</PRE
></FONT
></TD
></TR
></TABLE
></P
></TD
></TR
></TABLE
></DIV
><P
><A
HREF="dblparens.html"
>(( ))结构</A
>扩展并计算一个算术表达式的值. 
		  如果表达式的结果为0, 那么返回的<A
HREF="exit-status.html#EXITSTATUSREF"
>退出状态码</A
>为<SPAN
CLASS="RETURNVALUE"
>1</SPAN
>, 
		  或者是<SPAN
CLASS="QUOTE"
>"假"</SPAN
>. 
		  而一个非零值的表达式所返回的退出状态码将为<SPAN
CLASS="RETURNVALUE"
>0</SPAN
>,
		  或者是<SPAN
CLASS="QUOTE"
>"true"</SPAN
>. 
		  这种情况和先前所讨论的<B
CLASS="COMMAND"
>test</B
>命令和<SPAN
CLASS="TOKEN"
>[ ]</SPAN
>结构的行为正好相反. </P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="ARITHTESTS"
></A
><P
><B
>例子 7-3. 算术测试需要使用<SPAN
CLASS="TOKEN"
>(( ))</SPAN
></B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>  1&nbsp;#!/bin/bash
  2&nbsp;# 算术测试.
  3&nbsp;
  4&nbsp;# (( ... ))结构可以用来计算并测试算术表达式的结果. 
  5&nbsp;# 退出状态将会与[ ... ]结构完全相反!
  6&nbsp;
  7&nbsp;(( 0 ))
  8&nbsp;echo "Exit status of \"(( 0 ))\" is $?."         # 1
  9&nbsp;
 10&nbsp;(( 1 ))
 11&nbsp;echo "Exit status of \"(( 1 ))\" is $?."         # 0
 12&nbsp;
 13&nbsp;(( 5 &#62; 4 ))                                      # 真
 14&nbsp;echo "Exit status of \"(( 5 &#62; 4 ))\" is $?."     # 0
 15&nbsp;
 16&nbsp;(( 5 &#62; 9 ))                                      # 假
 17&nbsp;echo "Exit status of \"(( 5 &#62; 9 ))\" is $?."     # 1
 18&nbsp;
 19&nbsp;(( 5 - 5 ))                                      # 0
 20&nbsp;echo "Exit status of \"(( 5 - 5 ))\" is $?."     # 1
 21&nbsp;
 22&nbsp;(( 5 / 4 ))                                      # 除法也可以.
 23&nbsp;echo "Exit status of \"(( 5 / 4 ))\" is $?."     # 0
 24&nbsp;
 25&nbsp;(( 1 / 2 ))                                      # 除法的计算结果 &#60; 1.
 26&nbsp;echo "Exit status of \"(( 1 / 2 ))\" is $?."     # 截取之后的结果为 0.
 27&nbsp;                                                 # 1
 28&nbsp;
 29&nbsp;(( 1 / 0 )) 2&#62;/dev/null                          # 除数为0, 非法计算. 
 30&nbsp;#           ^^^^^^^^^^^
 31&nbsp;echo "Exit status of \"(( 1 / 0 ))\" is $?."     # 1
 32&nbsp;
 33&nbsp;# "2&#62;/dev/null"起了什么作用?
 34&nbsp;# 如果这句被删除会怎样?
 35&nbsp;# 尝试删除这句, 然后在运行这个脚本. 
 36&nbsp;
 37&nbsp;exit 0</PRE
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