functions.html

Shall高级编程

  27 
  28 echo
  29    
  30 echo "Nothing passed."   
  31 func2                          # Called with no params
  32 echo
  33 
  34 
  35 echo "Zero-length parameter passed."
  36 func2 ""                       # Called with zero-length param
  37 echo
  38 
  39 echo "Null parameter passed."
  40 func2 "$uninitialized_param"   # Called with uninitialized param
  41 echo
  42 
  43 echo "One parameter passed."   
  44 func2 first           # Called with one param
  45 echo
  46 
  47 echo "Two parameters passed."   
  48 func2 first second    # Called with two params
  49 echo
  50 
  51 echo "\"\" \"second\" passed."
  52 func2 "" second       # Called with zero-length first parameter
  53 echo                  # and ASCII string as a second one.
  54 
  55&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
><A
NAME="FSHIFTREF"
></A
></P
><DIV
CLASS="IMPORTANT"
><TABLE
CLASS="IMPORTANT"
WIDTH="100%"
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 <A
HREF="othertypesv.html#SHIFTREF"
>shift</A
>
        command works on arguments passed to functions (see <A
HREF="assortedtips.html#MULTIPLICATION"
>Example 33-15</A
>).</P
></TD
></TR
></TABLE
></DIV
><P
>But, what about command-line arguments passed to the script? 
        Does a function see them? Well, let's clear up the confusion.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="FUNCCMDLINEARG"
></A
><P
><B
>Example 23-3. Functions and command-line args passed to the script</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# func-cmdlinearg.sh
   3&nbsp;#  Call this script with a command-line argument,
   4&nbsp;#+ something like $0 arg1.
   5&nbsp;
   6&nbsp;
   7&nbsp;func ()
   8&nbsp;
   9&nbsp;{
  10&nbsp;echo "$1"
  11&nbsp;}
  12&nbsp;
  13&nbsp;echo "First call to function: no arg passed."
  14&nbsp;echo "See if command-line arg is seen."
  15&nbsp;func
  16&nbsp;# No! Command-line arg not seen.
  17&nbsp;
  18&nbsp;echo "============================================================"
  19&nbsp;echo
  20&nbsp;echo "Second call to function: command-line arg passed explicitly."
  21&nbsp;func $1
  22&nbsp;# Now it's seen!
  23&nbsp;
  24&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>In contrast to certain other programming languages,
	shell scripts normally pass only value parameters to
	functions. Variable names (which are actually
	<I
CLASS="FIRSTTERM"
>pointers</I
>), if
	passed as parameters to functions, will be treated as string
	literals.  <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>Functions interpret their arguments
	literally.</I
></SPAN
></P
><P
><A
NAME="FUNCPOINTERS"
></A
></P
><P
><A
HREF="ivr.html#IVRREF"
>Indirect variable
	    references</A
> (see <A
HREF="bash2.html#EX78"
>Example 34-2</A
>) provide a clumsy
	    sort of mechanism for passing variable pointers to
	    functions.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="INDFUNC"
></A
><P
><B
>Example 23-4. Passing an indirect reference to a function</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# ind-func.sh: Passing an indirect reference to a function.
   3&nbsp;
   4&nbsp;echo_var ()
   5&nbsp;{
   6&nbsp;echo "$1"
   7&nbsp;}
   8&nbsp;
   9&nbsp;message=Hello
  10&nbsp;Hello=Goodbye
  11&nbsp;
  12&nbsp;echo_var "$message"        # Hello
  13&nbsp;# Now, let's pass an indirect reference to the function.
  14&nbsp;echo_var "${!message}"     # Goodbye
  15&nbsp;
  16&nbsp;echo "-------------"
  17&nbsp;
  18&nbsp;# What happens if we change the contents of "hello" variable?
  19&nbsp;Hello="Hello, again!"
  20&nbsp;echo_var "$message"        # Hello
  21&nbsp;echo_var "${!message}"     # Hello, again!
  22&nbsp;
  23&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>The next logical question is whether parameters can be
	  dereferenced <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>after</I
></SPAN
> being passed to a
	  function.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="DEREFERENCECL"
></A
><P
><B
>Example 23-5. Dereferencing a parameter passed to a function</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# dereference.sh
   3&nbsp;# Dereferencing parameter passed to a function.
   4&nbsp;# Script by Bruce W. Clare.
   5&nbsp;
   6&nbsp;dereference ()
   7&nbsp;{
   8&nbsp;     y=\$"$1"   # Name of variable.
   9&nbsp;     echo $y    # $Junk
  10&nbsp;
  11&nbsp;     x=`eval "expr \"$y\" "`
  12&nbsp;     echo $1=$x
  13&nbsp;     eval "$1=\"Some Different Text \""  # Assign new value.
  14&nbsp;}
  15&nbsp;
  16&nbsp;Junk="Some Text"
  17&nbsp;echo $Junk "before"    # Some Text before
  18&nbsp;
  19&nbsp;dereference Junk
  20&nbsp;echo $Junk "after"     # Some Different Text after
  21&nbsp;
  22&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="REFPARAMS"
></A
><P
><B
>Example 23-6. Again, dereferencing a parameter passed to a function</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# ref-params.sh: Dereferencing a parameter passed to a function.
   3&nbsp;#                (Complex Example)
   4&nbsp;
   5&nbsp;ITERATIONS=3  # How many times to get input.
   6&nbsp;icount=1
   7&nbsp;
   8&nbsp;my_read () {
   9&nbsp;  #  Called with my_read varname,
  10&nbsp;  #+ outputs the previous value between brackets as the default value,
  11&nbsp;  #+ then asks for a new value.
  12&nbsp;
  13&nbsp;  local local_var
  14&nbsp;
  15&nbsp;  echo -n "Enter a value "
  16&nbsp;  eval 'echo -n "[$'$1'] "'  #  Previous value.
  17&nbsp;# eval echo -n "[\$$1] "     #  Easier to understand,
  18&nbsp;                             #+ but loses trailing space in user prompt.
  19&nbsp;  read local_var
  20&nbsp;  [ -n "$local_var" ] &#38;&#38; eval $1=\$local_var
  21&nbsp;
  22&nbsp;  # "And-list": if "local_var" then set "$1" to its value.
  23&nbsp;}
  24&nbsp;
  25&nbsp;echo
  26&nbsp;
  27&nbsp;while [ "$icount" -le "$ITERATIONS" ]
  28&nbsp;do
  29&nbsp;  my_read var
  30&nbsp;  echo "Entry #$icount = $var"
  31&nbsp;  let "icount += 1"
  32&nbsp;  echo
  33&nbsp;done  
  34&nbsp;
  35&nbsp;
  36&nbsp;# Thanks to Stephane Chazelas for providing this instructive example.
  37&nbsp;
  38&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="VARIABLELIST"
><P
><B
><A
NAME="EXITRETURN1"
></A
>Exit and Return</B
></P
><DL
><DT
><B
CLASS="COMMAND"
>exit status</B
></DT
><DD
><P
>Functions return a value, called an <I
CLASS="FIRSTTERM"
>exit
	      status</I
>. The exit status may be explicitly
	      specified by a <B
CLASS="COMMAND"
>return</B
> statement,
	      otherwise it is the exit status of the last command
	      in the function (<SPAN
CLASS="RETURNVALUE"
>0</SPAN
> if
	      successful, and a non-zero error code if not). This
	      <A
HREF="exit-status.html#EXITSTATUSREF"
>exit status</A
>
	      may be used in the script by referencing it as
	      <A
HREF="variables2.html#XSTATVARREF"
>$?</A
>.  This mechanism
	      effectively permits script functions to have a <SPAN
CLASS="QUOTE"
>"return
	      value"</SPAN
> similar to C functions.</P
></DD
><DT
><B
CLASS="COMMAND"
>return</B
></DT
><DD
><P
><A
NAME="RETURNREF"
></A
></P
><P
>Terminates a function. A <B
CLASS="COMMAND"
>return</B
> command
	       <A
NAME="AEN17024"
HREF="#FTN.AEN17024"
>[1]</A
>
	      optionally takes an <I
CLASS="FIRSTTERM"
>integer</I
>
	      argument, which is returned to the calling script as
	      the <SPAN
CLASS="QUOTE"
>"exit status"</SPAN
> of the function, and
	      this exit status is assigned to the variable <A
HREF="variables2.html#XSTATVARREF"
>$?</A
>.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="MAX"
></A
><P
><B
>Example 23-7. Maximum of two numbers</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# max.sh: Maximum of two integers.
   3&nbsp;
   4&nbsp;E_PARAM_ERR=250    # If less than 2 params passed to function.
   5&nbsp;EQUAL=251          # Return value if both params equal.
   6&nbsp;#  Error values out of range of any
   7&nbsp;#+ params that might be fed to the function.
   8&nbsp;
   9&nbsp;max2 ()             # Returns larger of two numbers.
  10&nbsp;{                   # Note: numbers compared must be less than 257.
  11&nbsp;if [ -z "$2" ]
  12&nbsp;then
  13&nbsp;  return $E_PARAM_ERR
  14&nbsp;fi
  15&nbsp;
  16&nbsp;if [ "$1" -eq "$2" ]
  17&nbsp;then
  18&nbsp;  return $EQUAL
  19&nbsp;else
  20&nbsp;  if [ "$1" -gt "$2" ]
  21&nbsp;  then
  22&nbsp;    return $1
  23&nbsp;  else
  24&nbsp;    return $2
  25&nbsp;  fi
  26&nbsp;fi
  27&nbsp;}
  28&nbsp;
  29&nbsp;max2 33 34
  30&nbsp;return_val=$?
  31&nbsp;
  32&nbsp;if [ "$return_val" -eq $E_PARAM_ERR ]
  33&nbsp;then
  34&nbsp;  echo "Need to pass two parameters to the function."
  35&nbsp;elif [ "$return_val" -eq $EQUAL ]
  36&nbsp;  then
  37&nbsp;    echo "The two numbers are equal."
  38&nbsp;else
  39&nbsp;    echo "The larger of the two numbers is $return_val."
  40&nbsp;fi  
  41&nbsp;
  42&nbsp;  
  43&nbsp;exit 0
  44&nbsp;
  45&nbsp;#  Exercise (easy):
  46&nbsp;#  ---------------
  47&nbsp;#  Convert this to an interactive script,
  48&nbsp;#+ that is, have the script ask for input (two numbers).</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="TIP"
><TABLE
CLASS="TIP"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/tip.png"
HSPACE="5"
ALT="Tip"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>For a function to return a string or array, use a
	      dedicated variable.
	        <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;count_lines_in_etc_passwd()
   2&nbsp;{
   3&nbsp;  [[ -r /etc/passwd ]] &#38;&#38; REPLY=$(echo $(wc -l &#60; /etc/passwd))
   4&nbsp;  #  If /etc/passwd is readable, set REPLY to line count.
   5&nbsp;  #  Returns both a parameter value and status information.
   6&nbsp;  #  The 'echo' seems unnecessary, but . . .
   7&nbsp;  #+ it removes excess whitespace from the output.
   8&nbsp;}
   9&nbsp;
  10&nbsp;if count_lines_in_etc_passwd
  11&nbsp;then
  12&nbsp;  echo "There are $REPLY lines in /etc/passwd."
  13&nbsp;else
  14&nbsp;  echo "Cannot count lines in /etc/passwd."
  15&nbsp;fi  
  16&nbsp;
  17&nbsp;# Thanks, S.C.</PRE
></TD
></TR
></TABLE
>
	    </P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX61"
></A
><P
><B
>Example 23-8. Converting numbers to Roman numerals</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;# Arabic number to Roman numeral conversion
   4&nbsp;# Range: 0 - 200
   5&nbsp;# It's crude, but it works.
   6&nbsp;
   7&nbsp;# Extending the range and otherwise improving the script is left as an exercise.
   8&nbsp;
   9&nbsp;# Usage: roman number-to-convert
  10&nbsp;
  11&nbsp;LIMIT=200
  12&nbsp;E_ARG_ERR=65
  13&nbsp;E_OUT_OF_RANGE=66
  14&nbsp;
  15&nbsp;if [ -z "$1" ]
  16&nbsp;then
  17&nbsp;  echo "Usage: `basename $0` number-to-convert"
  18&nbsp;  exit $E_ARG_ERR
  19&nbsp;fi  
  20&nbsp;
  21&nbsp;num=$1
  22&nbsp;if [ "$num" -gt $LIMIT ]
  23&nbsp;then
  24&nbsp;  echo "Out of range!"
  25&nbsp;  exit $E_OUT_OF_RANGE
  26&nbsp;fi  
  27&nbsp;
  28&nbsp;to_roman ()   # Must declare function before first call to it.
  29&nbsp;{
  30&nbsp;number=$1
  31&nbsp;factor=$2
  32&nbsp;rchar=$3
  33&nbsp;let "remainder = number - factor"
  34&nbsp;while [ "$remainder" -ge 0 ]
  35&nbsp;do
  36&nbsp;  echo -n $rchar
  37&nbsp;  let "number -= factor"
  38&nbsp;  let "remainder = number - factor"
  39&nbsp;done  
  40&nbsp;
  41&nbsp;return $number
  42&nbsp;       # Exercise:
  43&nbsp;       # --------
  44&nbsp;       # Explain how this function works.
  45&nbsp;       # Hint: division by successive subtraction.
  46&nbsp;}
  47&nbsp;   
  48&nbsp;
  49&nbsp;to_roman $num 100 C
  50&nbsp;num=$?
  51&nbsp;to_roman $num 90 LXXXX
  52&nbsp;num=$?
  53&nbsp;to_roman $num 50 L
  54&nbsp;num=$?
  55&nbsp;to_roman $num 40 XL
  56&nbsp;num=$?