functions.html

一本完整的描述Unix Shell 编程的工具书的所有范例

   5 
   6 
   7 func ()
   8 
   9 {
  10 echo "$1"
  11 }
  12 
  13 echo "First call to function: no arg passed."
  14 echo "See if command-line arg is seen."
  15 func
  16 # No! Command-line arg not seen.
  17 
  18 echo "============================================================"
  19 echo
  20 echo "Second call to function: command-line arg passed explicitly."
  21 func $1
  22 # Now it's seen!
  23 
  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 pointers), if
	passed as parameters to functions, will be treated as string
	literals.  <I
CLASS="EMPHASIS"
>Functions interpret their arguments
	literally.</I
></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 <I
CLASS="EMPHASIS"
>after</I
> 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="AEN14191"
HREF="#FTN.AEN14191"
>[1]</A
>
	      optionally takes an <I
CLASS="EMPHASIS"
>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=-198    # If less than 2 params passed to function.
   5&nbsp;EQUAL=-199          # 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 $num 40 XL
  56&nbsp;num=$?
  57&nbsp;to_roman $num 10 X
  58&nbsp;num=$?
  59&nbsp;to_roman $num 9 IX
  60&nbsp;num=$?
  61&nbsp;to_roman $num 5 V
  62&nbsp;num=$?
  63&nbsp;to_roman $num 4 IV
  64&nbsp;num=$?
  65&nbsp;to_roman $num 1 I
  66&nbsp;
  67&nbsp;echo
  68&nbsp;
  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