functions.html

Shall高级编程

  57 to_roman $num 10 X
  58 num=$?
  59 to_roman $num 9 IX
  60 num=$?
  61 to_roman $num 5 V
  62 num=$?
  63 to_roman $num 4 IV
  64 num=$?
  65 to_roman $num 1 I
  66 
  67 echo
  68 
  69&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>See also <A
HREF="testbranch.html#ISALPHA"
>Example 10-28</A
>.</P
><DIV
CLASS="IMPORTANT"
><TABLE
CLASS="IMPORTANT"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/important.png"
HSPACE="5"
ALT="Important"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The largest positive integer a function can return is
	      255. The <B
CLASS="COMMAND"
>return</B
> command is closely tied
	      to the concept of <A
HREF="exit-status.html#EXITSTATUSREF"
>exit
	      status</A
>, which accounts for this particular
	      limitation.  Fortunately, there are various <A
HREF="assortedtips.html#RVT"
>workarounds</A
> for those situations
	      requiring a large integer return value from a
	      function.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="RETURNTEST"
></A
><P
><B
>Example 23-9. Testing large return values in a function</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# return-test.sh
   3&nbsp;
   4&nbsp;# The largest positive value a function can return is 255.
   5&nbsp;
   6&nbsp;return_test ()         # Returns whatever passed to it.
   7&nbsp;{
   8&nbsp;  return $1
   9&nbsp;}
  10&nbsp;
  11&nbsp;return_test 27         # o.k.
  12&nbsp;echo $?                # Returns 27.
  13&nbsp;  
  14&nbsp;return_test 255        # Still o.k.
  15&nbsp;echo $?                # Returns 255.
  16&nbsp;
  17&nbsp;return_test 257        # Error!
  18&nbsp;echo $?                # Returns 1 (return code for miscellaneous error).
  19&nbsp;
  20&nbsp;# ======================================================
  21&nbsp;return_test -151896    # Do large negative numbers work?
  22&nbsp;echo $?                # Will this return -151896?
  23&nbsp;                       # No! It returns 168.
  24&nbsp;#  Version of Bash before 2.05b permitted
  25&nbsp;#+ large negative integer return values.
  26&nbsp;#  Newer versions of Bash plug this loophole.
  27&nbsp;#  This may break older scripts.
  28&nbsp;#  Caution!
  29&nbsp;# ======================================================
  30&nbsp;
  31&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>A workaround for obtaining large integer <SPAN
CLASS="QUOTE"
>"return
	      values"</SPAN
> is to simply assign the <SPAN
CLASS="QUOTE"
>"return
	      value"</SPAN
> to a global variable.

	        <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;Return_Val=   # Global variable to hold oversize return value of function.
   2&nbsp;
   3&nbsp;alt_return_test ()
   4&nbsp;{
   5&nbsp;  fvar=$1
   6&nbsp;  Return_Val=$fvar
   7&nbsp;  return   # Returns 0 (success).
   8&nbsp;}
   9&nbsp;
  10&nbsp;alt_return_test 1
  11&nbsp;echo $?                              # 0
  12&nbsp;echo "return value = $Return_Val"    # 1
  13&nbsp;
  14&nbsp;alt_return_test 256
  15&nbsp;echo "return value = $Return_Val"    # 256
  16&nbsp;
  17&nbsp;alt_return_test 257
  18&nbsp;echo "return value = $Return_Val"    # 257
  19&nbsp;
  20&nbsp;alt_return_test 25701
  21&nbsp;echo "return value = $Return_Val"    #25701</PRE
></TD
></TR
></TABLE
>
            </P
><P
><A
NAME="CAPTURERETVAL"
></A
></P
><P
>A more elegant method is to have the function
              <B
CLASS="COMMAND"
>echo</B
> its <SPAN
CLASS="QUOTE"
>"return
              value to <TT
CLASS="FILENAME"
>stdout</TT
>,"</SPAN
> and
              then capture it by <A
HREF="commandsub.html#COMMANDSUBREF"
>command
	      substitution</A
>. See the <A
HREF="assortedtips.html#RVT"
>discussion
	      of this</A
> in <A
HREF="assortedtips.html"
>Section 33.8</A
>.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="MAX2"
></A
><P
><B
>Example 23-10. Comparing two large integers</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# max2.sh: Maximum of two LARGE integers.
   3&nbsp;
   4&nbsp;#  This is the previous "max.sh" example,
   5&nbsp;#+ modified to permit comparing large integers.
   6&nbsp;
   7&nbsp;EQUAL=0             # Return value if both params equal.
   8&nbsp;E_PARAM_ERR=-99999  # Not enough params passed to function.
   9&nbsp;#           ^^^^^^    Out of range of any params that might be passed.
  10&nbsp;
  11&nbsp;max2 ()             # "Returns" larger of two numbers.
  12&nbsp;{
  13&nbsp;if [ -z "$2" ]
  14&nbsp;then
  15&nbsp;  echo $E_PARAM_ERR
  16&nbsp;  return
  17&nbsp;fi
  18&nbsp;
  19&nbsp;if [ "$1" -eq "$2" ]
  20&nbsp;then
  21&nbsp;  echo $EQUAL
  22&nbsp;  return
  23&nbsp;else
  24&nbsp;  if [ "$1" -gt "$2" ]
  25&nbsp;  then
  26&nbsp;    retval=$1
  27&nbsp;  else
  28&nbsp;    retval=$2
  29&nbsp;  fi
  30&nbsp;fi
  31&nbsp;
  32&nbsp;echo $retval        # Echoes (to stdout), rather than returning value.
  33&nbsp;                    # Why?
  34&nbsp;}
  35&nbsp;
  36&nbsp;
  37&nbsp;return_val=$(max2 33001 33997)
  38&nbsp;#            ^^^^             Function name
  39&nbsp;#                 ^^^^^ ^^^^^ Params passed
  40&nbsp;#  This is actually a form of command substitution:
  41&nbsp;#+ treating a function as if it were a command,
  42&nbsp;#+ and assigning the stdout of the function to the variable "return_val."
  43&nbsp;
  44&nbsp;
  45&nbsp;# ========================= OUTPUT ========================
  46&nbsp;if [ "$return_val" -eq "$E_PARAM_ERR" ]
  47&nbsp;  then
  48&nbsp;  echo "Error in parameters passed to comparison function!"
  49&nbsp;elif [ "$return_val" -eq "$EQUAL" ]
  50&nbsp;  then
  51&nbsp;    echo "The two numbers are equal."
  52&nbsp;else
  53&nbsp;    echo "The larger of the two numbers is $return_val."
  54&nbsp;fi
  55&nbsp;# =========================================================
  56&nbsp;  
  57&nbsp;exit 0
  58&nbsp;
  59&nbsp;#  Exercises:
  60&nbsp;#  ---------
  61&nbsp;#  1) Find a more elegant way of testing
  62&nbsp;#+    the parameters passed to the function.
  63&nbsp;#  2) Simplify the if/then structure at "OUTPUT."
  64&nbsp;#  3) Rewrite the script to take input from command-line parameters.</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>Here is another example of capturing a function
	      <SPAN
CLASS="QUOTE"
>"return value."</SPAN
> Understanding it requires some
	      knowledge of <A
HREF="awk.html#AWKREF"
>awk</A
>.

	    <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;month_length ()  # Takes month number as an argument.
   2&nbsp;{                # Returns number of days in month.
   3&nbsp;monthD="31 28 31 30 31 30 31 31 30 31 30 31"  # Declare as local?
   4&nbsp;echo "$monthD" | awk '{ print $'"${1}"' }'    # Tricky.
   5&nbsp;#                             ^^^^^^^^^
   6&nbsp;# Parameter passed to function  ($1 -- month number), then to awk.
   7&nbsp;# Awk sees this as "print $1 . . . print $12" (depending on month number)
   8&nbsp;# Template for passing a parameter to embedded awk script:
   9&nbsp;#                                 $'"${script_parameter}"'
  10&nbsp;
  11&nbsp;#  Needs error checking for correct parameter range (1-12)
  12&nbsp;#+ and for February in leap year.
  13&nbsp;}
  14&nbsp;
  15&nbsp;# ----------------------------------------------
  16&nbsp;# Usage example:
  17&nbsp;month=4        # April, for example (4th month).
  18&nbsp;days_in=$(month_length $month)
  19&nbsp;echo $days_in  # 30
  20&nbsp;# ----------------------------------------------</PRE
></TD
></TR
></TABLE
></P
><P
>See also <A
HREF="contributed-scripts.html#DAYSBETWEEN"
>Example A-7</A
>.</P
><P
><TT
CLASS="USERINPUT"
><B
>Exercise:</B
></TT
> Using what we have
	      just learned, extend the previous <A
HREF="functions.html#EX61"
>Roman numerals example</A
> to accept
	      arbitrarily large input.</P
></TD
></TR
></TABLE
></DIV
></DD
></DL
></DIV
><DIV
CLASS="VARIABLELIST"
><P
><B
><A
NAME="REDSTDINFUNC1"
></A
>Redirection</B
></P
><DL
><DT
><TT
CLASS="REPLACEABLE"
><I
>Redirecting the stdin
	    of a function</I
></TT
></DT
><DD
><P
>A function is essentially a <A
HREF="special-chars.html#CODEBLOCKREF"
>code block</A
>, which means its
	      <TT
CLASS="FILENAME"
>stdin</TT
> can be redirected (as in <A
HREF="special-chars.html#EX8"
>Example 3-1</A
>).</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="REALNAME"
></A
><P
><B
>Example 23-11. Real name from username</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# realname.sh
   3&nbsp;#
   4&nbsp;# From username, gets "real name" from /etc/passwd.
   5&nbsp;
   6&nbsp;
   7&nbsp;ARGCOUNT=1       # Expect one arg.
   8&nbsp;E_WRONGARGS=65
   9&nbsp;
  10&nbsp;file=/etc/passwd
  11&nbsp;pattern=$1
  12&nbsp;
  13&nbsp;if [ $# -ne "$ARGCOUNT" ]
  14&nbsp;then
  15&nbsp;  echo "Usage: `basename $0` USERNAME"
  16&nbsp;  exit $E_WRONGARGS
  17&nbsp;fi  
  18&nbsp;
  19&nbsp;file_excerpt ()  # Scan file for pattern, then print relevant portion of line.
  20&nbsp;{
  21&nbsp;while read line  # "while" does not necessarily need "[ condition ]"
  22&nbsp;do
  23&nbsp;  echo "$line" | grep $1 | awk -F":" '{ print $5 }'  # Have awk use ":" delimiter.
  24&nbsp;done
  25&nbsp;} &#60;$file  # Redirect into function's stdin.
  26&nbsp;
  27&nbsp;file_excerpt $pattern
  28&nbsp;
  29&nbsp;# Yes, this entire script could be reduced to
  30&nbsp;#       grep PATTERN /etc/passwd | awk -F":" '{ print $5 }'
  31&nbsp;# or
  32&nbsp;#       awk -F: '/PATTERN/ {print $5}'
  33&nbsp;# or
  34&nbsp;#       awk -F: '($1 == "username") { print $5 }' # real name from username
  35&nbsp;# However, it might not be as instructive.
  36&nbsp;
  37&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>There is an alternate, and perhaps less confusing
	     method of redirecting a function's
	     <TT
CLASS="FILENAME"
>stdin</TT
>.  This involves redirecting the
	     <TT
CLASS="FILENAME"
>stdin</TT
> to an embedded bracketed code
	     block within the function.

	       <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;# Instead of:
   2&nbsp;Function ()
   3&nbsp;{
   4&nbsp; ...
   5&nbsp; } &#60; file
   6&nbsp;
   7&nbsp;# Try this:
   8&nbsp;Function ()
   9&nbsp;{
  10&nbsp;  {
  11&nbsp;    ...
  12&nbsp;   } &#60; file
  13&nbsp;}
  14&nbsp;
  15&nbsp;# Similarly,
  16&nbsp;
  17&nbsp;Function ()  # This works.
  18&nbsp;{
  19&nbsp;  {
  20&nbsp;   echo $*
  21&nbsp;  } | tr a b
  22&nbsp;}
  23&nbsp;
  24&nbsp;Function ()  # This doesn't work.
  25&nbsp;{
  26&nbsp;  echo $*
  27&nbsp;} | tr a b   # A nested code block is mandatory here.
  28&nbsp;
  29&nbsp;
  30&nbsp;# Thanks, S.C.</PRE
></TD
></TR
></TABLE
>
           </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.AEN17024"
HREF="functions.html#AEN17024"
>[1]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>The <B
CLASS="COMMAND"
>return</B
> command is a
		 Bash <A
HREF="internal.html#BUILTINREF"
>builtin</A
>.</P
></TD
></TR
></TABLE
><DIV
CLASS="NAVFOOTER"
><HR
ALIGN="LEFT"
WIDTH="100%"><TABLE
SUMMARY="Footer navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
><A
HREF="process-sub.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="index.html"
ACCESSKEY="H"
>Home</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
><A
HREF="localvar.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>Process Substitution</TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="part5.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
>Local Variables</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>