operations.html

Shall高级编程

>As of version 2.05b, Bash supports 64-bit integers.</P
></TD
></TR
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></DIV
><DIV
CLASS="CAUTION"
><TABLE
CLASS="CAUTION"
WIDTH="100%"
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
><A
NAME="NOFLOATINGPOINT"
></A
></P
><P
>Bash does not understand floating point arithmetic. It
        treats numbers containing a decimal point as strings.</P
><P
>        <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;a=1.5
   2&nbsp;
   3&nbsp;let "b = $a + 1.3"  # Error.
   4&nbsp;# t2.sh: let: b = 1.5 + 1.3: syntax error in expression
   5&nbsp;#                            (error token is ".5 + 1.3")
   6&nbsp;
   7&nbsp;echo "b = $b"       # b=1</PRE
></TD
></TR
></TABLE
>
      </P
><P
>Use <A
HREF="mathc.html#BCREF"
>bc</A
> in scripts that that need floating
	point calculations or math library functions.</P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="FORMALPARA"
><P
><B
>bitwise operators. </B
>The bitwise operators seldom make an appearance in shell scripts.
	Their chief use seems to be manipulating and testing values read
	from ports or <A
HREF="devproc.html#SOCKETREF"
>sockets</A
>. <SPAN
CLASS="QUOTE"
>"Bit
	flipping"</SPAN
> is more relevant to compiled languages, such
	as C and C++, which provide direct access to system
	hardware.</P
></DIV
><DIV
CLASS="VARIABLELIST"
><P
><B
><A
NAME="BITWSOPS1"
></A
>bitwise operators</B
></P
><DL
><DT
><SPAN
CLASS="TOKEN"
>&#60;&#60;</SPAN
></DT
><DD
><P
>bitwise left shift (multiplies by <TT
CLASS="LITERAL"
>2</TT
>
	      for each shift position)</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>&#60;&#60;=</SPAN
></DT
><DD
><P
><I
CLASS="FIRSTTERM"
>left-shift-equal</I
></P
><P
><TT
CLASS="USERINPUT"
><B
>let "var &#60;&#60;= 2"</B
></TT
> results in <TT
CLASS="PARAMETER"
><I
>var</I
></TT
>
	    left-shifted <TT
CLASS="LITERAL"
>2</TT
> bits (multiplied by <TT
CLASS="LITERAL"
>4</TT
>)</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>&#62;&#62;</SPAN
></DT
><DD
><P
>bitwise right shift (divides by <TT
CLASS="LITERAL"
>2</TT
>
	      for each shift position)</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>&#62;&#62;=</SPAN
></DT
><DD
><P
><I
CLASS="FIRSTTERM"
>right-shift-equal</I
>
	    (inverse of <SPAN
CLASS="TOKEN"
>&#60;&#60;=</SPAN
>)</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>&#38;</SPAN
></DT
><DD
><P
>bitwise AND</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>&#38;=</SPAN
></DT
><DD
><P
>bitwise <I
CLASS="FIRSTTERM"
>AND-equal</I
></P
></DD
><DT
><SPAN
CLASS="TOKEN"
>|</SPAN
></DT
><DD
><P
>bitwise OR</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>|=</SPAN
></DT
><DD
><P
>bitwise <I
CLASS="FIRSTTERM"
>OR-equal</I
></P
></DD
><DT
><SPAN
CLASS="TOKEN"
>~</SPAN
></DT
><DD
><P
>bitwise NOT</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>^</SPAN
></DT
><DD
><P
>bitwise XOR</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>^=</SPAN
></DT
><DD
><P
>bitwise <I
CLASS="FIRSTTERM"
>XOR-equal</I
></P
></DD
></DL
></DIV
><DIV
CLASS="VARIABLELIST"
><P
><B
><A
NAME="LOGOPS1"
></A
>logical (boolean) operators</B
></P
><DL
><DT
><SPAN
CLASS="TOKEN"
>!</SPAN
></DT
><DD
><P
>NOT</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;if [ ! -f $FILENAME ]
   2&nbsp;then
   3&nbsp;  ...</PRE
></TD
></TR
></TABLE
></P
></DD
><DT
><SPAN
CLASS="TOKEN"
>&#38;&#38;</SPAN
></DT
><DD
><P
>AND</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;if [ $condition1 ] &#38;&#38; [ $condition2 ]
   2&nbsp;# Same as:  if [ $condition1 -a $condition2 ]
   3&nbsp;# Returns true if both condition1 and condition2 hold true...
   4&nbsp;
   5&nbsp;if [[ $condition1 &#38;&#38; $condition2 ]]    # Also works.
   6&nbsp;# Note that &#38;&#38; operator not permitted within [ ... ] construct.</PRE
></TD
></TR
></TABLE
></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="TOKEN"
>&#38;&#38;</SPAN
> may also, depending on context, be
	    used in an <A
HREF="list-cons.html#LISTCONSREF"
>and list</A
>
	    to concatenate commands.</P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><A
NAME="ORREF"
></A
><SPAN
CLASS="TOKEN"
>||</SPAN
></DT
><DD
><P
>OR</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;if [ $condition1 ] || [ $condition2 ]
   2&nbsp;# Same as:  if [ $condition1 -o $condition2 ]
   3&nbsp;# Returns true if either condition1 or condition2 holds true...
   4&nbsp;
   5&nbsp;if [[ $condition1 || $condition2 ]]    # Also works.
   6&nbsp;# Note that || operator not permitted within [ ... ] construct.</PRE
></TD
></TR
></TABLE
></P
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>Bash tests the <A
HREF="exit-status.html#EXITSTATUSREF"
>exit
	    status</A
> of each statement linked with a logical
	    operator.</P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="ANDOR"
></A
><P
><B
>Example 8-3. Compound Condition Tests Using &#38;&#38; and ||</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;a=24
   4&nbsp;b=47
   5&nbsp;
   6&nbsp;if [ "$a" -eq 24 ] &#38;&#38; [ "$b" -eq 47 ]
   7&nbsp;then
   8&nbsp;  echo "Test #1 succeeds."
   9&nbsp;else
  10&nbsp;  echo "Test #1 fails."
  11&nbsp;fi
  12&nbsp;
  13&nbsp;# ERROR:   if [ "$a" -eq 24 &#38;&#38; "$b" -eq 47 ]
  14&nbsp;#+         attempts to execute  ' [ "$a" -eq 24 '
  15&nbsp;#+         and fails to finding matching ']'.
  16&nbsp;#
  17&nbsp;#  Note:  if [[ $a -eq 24 &#38;&#38; $b -eq 24 ]]  works.
  18&nbsp;#  The double-bracket if-test is more flexible
  19&nbsp;#+ than the single-bracket version.       
  20&nbsp;#    (The "&#38;&#38;" has a different meaning in line 17 than in line 6.)
  21&nbsp;#    Thanks, Stephane Chazelas, for pointing this out.
  22&nbsp;
  23&nbsp;
  24&nbsp;if [ "$a" -eq 98 ] || [ "$b" -eq 47 ]
  25&nbsp;then
  26&nbsp;  echo "Test #2 succeeds."
  27&nbsp;else
  28&nbsp;  echo "Test #2 fails."
  29&nbsp;fi
  30&nbsp;
  31&nbsp;
  32&nbsp;#  The -a and -o options provide
  33&nbsp;#+ an alternative compound condition test.
  34&nbsp;#  Thanks to Patrick Callahan for pointing this out.
  35&nbsp;
  36&nbsp;
  37&nbsp;if [ "$a" -eq 24 -a "$b" -eq 47 ]
  38&nbsp;then
  39&nbsp;  echo "Test #3 succeeds."
  40&nbsp;else
  41&nbsp;  echo "Test #3 fails."
  42&nbsp;fi
  43&nbsp;
  44&nbsp;
  45&nbsp;if [ "$a" -eq 98 -o "$b" -eq 47 ]
  46&nbsp;then
  47&nbsp;  echo "Test #4 succeeds."
  48&nbsp;else
  49&nbsp;  echo "Test #4 fails."
  50&nbsp;fi
  51&nbsp;
  52&nbsp;
  53&nbsp;a=rhino
  54&nbsp;b=crocodile
  55&nbsp;if [ "$a" = rhino ] &#38;&#38; [ "$b" = crocodile ]
  56&nbsp;then
  57&nbsp;  echo "Test #5 succeeds."
  58&nbsp;else
  59&nbsp;  echo "Test #5 fails."
  60&nbsp;fi
  61&nbsp;
  62&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>The <SPAN
CLASS="TOKEN"
>&#38;&#38;</SPAN
> and <SPAN
CLASS="TOKEN"
>||</SPAN
> operators also
	      find use in an arithmetic context.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo $(( 1 &#38;&#38; 2 )) $((3 &#38;&#38; 0)) $((4 || 0)) $((0 || 0))</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>1 0 1 0</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	    </P
></DD
></DL
></DIV
><DIV
CLASS="VARIABLELIST"
><P
><B
><A
NAME="MISCOP1"
></A
>miscellaneous operators</B
></P
><DL
><DT
><A
NAME="COMMAOP"
></A
><SPAN
CLASS="TOKEN"
>,</SPAN
></DT
><DD
><P
>Comma operator</P
><P
>The <B
CLASS="COMMAND"
>comma operator</B
> chains together
	      two or more arithmetic operations. All the operations are
	      evaluated (with possible <I
CLASS="FIRSTTERM"
>side effects</I
>
	        <A
NAME="AEN3825"
HREF="#FTN.AEN3825"
>[1]</A
>
	      ), but only the last operation is returned.</P
><P
>	     <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;let "t1 = ((5 + 3, 7 - 1, 15 - 4))"
   2&nbsp;echo "t1 = $t1"               # t1 = 11
   3&nbsp;
   4&nbsp;let "t2 = ((a = 9, 15 / 3))"  # Set "a" and calculate "t2".
   5&nbsp;echo "t2 = $t2    a = $a"     # t2 = 5    a = 9</PRE
></TD
></TR
></TABLE
>
	     </P
><P
>The comma operator finds use mainly in <A
HREF="loops.html#FORLOOPREF1"
>for loops</A
>. See <A
HREF="loops.html#FORLOOPC"
>Example 10-12</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.AEN3825"
HREF="operations.html#AEN3825"
>[1]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
><I
CLASS="FIRSTTERM"
>Side effects</I
>
		are, of course, unintended and usually undesirable
		consequences.</P
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