tests.html

Shall高级编程

	The <TT
CLASS="USERINPUT"
><B
>if test condition-true</B
></TT
> construct is the
	exact equivalent of <TT
CLASS="USERINPUT"
><B
>if [ condition-true ]</B
></TT
>.
	As it happens, the left bracket, <B
CLASS="COMMAND"
>[</B
> , is a token
	which invokes the <B
CLASS="COMMAND"
>test</B
> command.  The closing
	right bracket, <B
CLASS="COMMAND"
>]</B
> , in an if/test should not
	therefore be strictly necessary, however newer versions of Bash
	require it.</P
><P
><A
NAME="TTESTREF"
></A
></P
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="100%"
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"
>test</B
> command is a Bash <A
HREF="internal.html#BUILTINREF"
>builtin</A
> which tests file
	  types and compares strings. Therefore, in a Bash script,
	  <B
CLASS="COMMAND"
>test</B
> does <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
> call
	  the external <TT
CLASS="FILENAME"
>/usr/bin/test</TT
> binary,
	  which is part of the <I
CLASS="FIRSTTERM"
>sh-utils</I
>
	  package. Likewise, <B
CLASS="COMMAND"
>[</B
> does not call
	  <TT
CLASS="FILENAME"
>/usr/bin/[</TT
>, which is linked to
	  <TT
CLASS="FILENAME"
>/usr/bin/test</TT
>.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>type test</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>test is a shell builtin</TT
>
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>type '['</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>[ is a shell builtin</TT
>
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>type '[['</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>[[ is a shell keyword</TT
>
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>type ']]'</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>]] is a shell keyword</TT
>
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>type ']'</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>bash: type: ]: not found</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	    </P
><P
><A
NAME="USRBINTEST"
></A
></P
><P
>If, for some reason, you wish to use
	  <TT
CLASS="FILENAME"
>/usr/bin/test</TT
> in a Bash script,
	  then specify it by full pathname.</P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX11"
></A
><P
><B
>Example 7-2. Equivalence of <I
CLASS="FIRSTTERM"
>test</I
>,
	  <TT
CLASS="FILENAME"
>/usr/bin/test</TT
>, <SPAN
CLASS="TOKEN"
>[ ]</SPAN
>,
	  and <TT
CLASS="FILENAME"
>/usr/bin/[</TT
></B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><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"      # Equivalent to "test" builtin.
  15&nbsp;#  ^^^^^^^^^^^^^              # Specifying full pathname.
  16&nbsp;then
  17&nbsp;  echo "No command-line arguments."
  18&nbsp;else
  19&nbsp;  echo "First command-line argument is $1."
  20&nbsp;fi
  21&nbsp;
  22&nbsp;echo
  23&nbsp;
  24&nbsp;if [ -z "$1" ]                # Functionally identical to above code blocks.
  25&nbsp;#   if [ -z "$1"                should work, but...
  26&nbsp;#+  Bash responds to a missing close-bracket with an error message.
  27&nbsp;then
  28&nbsp;  echo "No command-line arguments."
  29&nbsp;else
  30&nbsp;  echo "First command-line argument is $1."
  31&nbsp;fi
  32&nbsp;
  33&nbsp;echo
  34&nbsp;
  35&nbsp;
  36&nbsp;if /usr/bin/[ -z "$1" ]       # Again, functionally identical to above.
  37&nbsp;# if /usr/bin/[ -z "$1"       # Works, but gives an error message.
  38&nbsp;#                             # Note:
  39&nbsp;#                               This has been fixed in Bash, version 3.x.
  40&nbsp;then
  41&nbsp;  echo "No command-line arguments."
  42&nbsp;else
  43&nbsp;  echo "First command-line argument is $1."
  44&nbsp;fi
  45&nbsp;
  46&nbsp;echo
  47&nbsp;
  48&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
><A
NAME="DBLBRACKETS"
></A
>The <SPAN
CLASS="TOKEN"
>[[  ]]</SPAN
> construct
	is the more versatile Bash version of <SPAN
CLASS="TOKEN"
>[	]</SPAN
>. This
	is the <I
CLASS="FIRSTTERM"
>extended test command</I
>, adopted from
	<I
CLASS="FIRSTTERM"
>ksh88</I
>.</P
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="100%"
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
>No filename expansion or word splitting takes place
	between <SPAN
CLASS="TOKEN"
>[[</SPAN
> and <SPAN
CLASS="TOKEN"
>]]</SPAN
>, but there is
	parameter expansion and command substitution.</P
></TD
></TR
></TABLE
></DIV
><P
>	<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><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
></TD
></TR
></TABLE
>
      </P
><DIV
CLASS="TIP"
><TABLE
CLASS="TIP"
WIDTH="100%"
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
>Using the <B
CLASS="COMMAND"
>[[ ... ]]</B
> test construct,
	rather than <B
CLASS="COMMAND"
>[ ... ]</B
> can prevent many
	logic errors in scripts. For example, the <SPAN
CLASS="TOKEN"
>&#38;&#38;</SPAN
>,
	<SPAN
CLASS="TOKEN"
>||</SPAN
>, <SPAN
CLASS="TOKEN"
>&#60;</SPAN
>, and <SPAN
CLASS="TOKEN"
>&#62;</SPAN
>
	operators work within a <SPAN
CLASS="TOKEN"
>[[  ]]</SPAN
> test, despite
	giving an error within a <SPAN
CLASS="TOKEN"
>[  ]</SPAN
> construct.</P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="100%"
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
>Following an <B
CLASS="COMMAND"
>if</B
>, neither the
        <B
CLASS="COMMAND"
>test</B
> command nor the test brackets ( [ ] or [[ ]] )
	are strictly necessary.

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

        The "if COMMAND" construct returns the exit status of COMMAND.</P
><P
>Similarly, a condition within test brackets may stand alone
	  without an <B
CLASS="COMMAND"
>if</B
>, when used in combination
	  with a <A
HREF="list-cons.html#LISTCONSREF"
>list construct</A
>.

	  <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><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
></TD
></TR
></TABLE
></P
></TD
></TR
></TABLE
></DIV
><P
>The <A
HREF="dblparens.html"
>(( )) construct</A
> expands
	and evaluates an arithmetic expression. If
	the expression evaluates as zero, it returns an
	<A
HREF="exit-status.html#EXITSTATUSREF"
>exit status</A
> of
	<SPAN
CLASS="RETURNVALUE"
>1</SPAN
>, or <SPAN
CLASS="QUOTE"
>"false"</SPAN
>. A non-zero
	expression returns an exit status of <SPAN
CLASS="RETURNVALUE"
>0</SPAN
>,
	or <SPAN
CLASS="QUOTE"
>"true"</SPAN
>. This is in marked contrast to using
	the <B
CLASS="COMMAND"
>test</B
> and <SPAN
CLASS="TOKEN"
>[ ]</SPAN
> constructs
	previously discussed.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="ARITHTESTS"
></A
><P
><B
>Example 7-3. Arithmetic Tests using <SPAN
CLASS="TOKEN"
>(( ))</SPAN
></B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# Arithmetic tests.
   3&nbsp;
   4&nbsp;# The (( ... )) construct evaluates and tests numerical expressions.
   5&nbsp;# Exit status opposite from [ ... ] construct!
   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 ))                                      # true
  14&nbsp;echo "Exit status of \"(( 5 &#62; 4 ))\" is $?."     # 0
  15&nbsp;
  16&nbsp;(( 5 &#62; 9 ))                                      # false
  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 ))                                      # Division o.k.
  23&nbsp;echo "Exit status of \"(( 5 / 4 ))\" is $?."     # 0
  24&nbsp;
  25&nbsp;(( 1 / 2 ))                                      # Division result &#60; 1.
  26&nbsp;echo "Exit status of \"(( 1 / 2 ))\" is $?."     # Rounded off to 0.
  27&nbsp;                                                 # 1
  28&nbsp;
  29&nbsp;(( 1 / 0 )) 2&#62;/dev/null                          # Illegal division by 0.
  30&nbsp;#           ^^^^^^^^^^^
  31&nbsp;echo "Exit status of \"(( 1 / 0 ))\" is $?."     # 1
  32&nbsp;
  33&nbsp;# What effect does the "2&#62;/dev/null" have?
  34&nbsp;# What would happen if it were removed?
  35&nbsp;# Try removing it, then rerunning the script.
  36&nbsp;
  37&nbsp;exit 0</PRE
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