parameter-substitution.html

Shall高级编程

  47 #  Compare these methods of checking whether a variable has been set
  48 #+ with "set -u" . . .
  49 
  50 
  51 
  52 echo "You will not see this message, because script already terminated."
  53 
  54 HERE=0
  55 exit $HERE   # Will NOT exit here.
  56 
  57&nbsp;# In fact, this script will return an exit status (echo $?) of 1.</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="USAGEMESSAGE"
></A
><P
><B
>Example 9-17. Parameter substitution and <SPAN
CLASS="QUOTE"
>"usage"</SPAN
> messages</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# usage-message.sh
   3&nbsp;
   4&nbsp;: ${1?"Usage: $0 ARGUMENT"}
   5&nbsp;#  Script exits here if command-line parameter absent,
   6&nbsp;#+ with following error message.
   7&nbsp;#    usage-message.sh: 1: Usage: usage-message.sh ARGUMENT
   8&nbsp;
   9&nbsp;echo "These two lines echo only if command-line parameter given."
  10&nbsp;echo "command line parameter = \"$1\""
  11&nbsp;
  12&nbsp;exit 0  # Will exit here only if command-line parameter present.
  13&nbsp;
  14&nbsp;# Check the exit status, both with and without command-line parameter.
  15&nbsp;# If command-line parameter present, then "$?" is 0.
  16&nbsp;# If not, then "$?" is 1.</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="FORMALPARA"
><P
><B
>Parameter substitution and/or expansion. </B
><A
NAME="PSUB2"
></A
>The following expressions are
		the complement to the <B
CLASS="COMMAND"
>match</B
>
		<TT
CLASS="REPLACEABLE"
><I
>in</I
></TT
> <B
CLASS="COMMAND"
>expr</B
>
		string operations (see <A
HREF="moreadv.html#EX45"
>Example 15-9</A
>).
		These particular ones are used mostly in parsing file
		path names.</P
></DIV
><DIV
CLASS="VARIABLELIST"
><P
><B
><A
NAME="PSOREX1"
></A
>Variable length / Substring removal</B
></P
><DL
><DT
><TT
CLASS="USERINPUT"
><B
>${#var}</B
></TT
></DT
><DD
><P
><TT
CLASS="USERINPUT"
><B
>String length</B
></TT
> (number
		    of characters in <TT
CLASS="VARNAME"
>$var</TT
>). For
		    an <A
HREF="arrays.html#ARRAYREF"
>array</A
>,
		    <B
CLASS="COMMAND"
>${#array}</B
> is the length of the
		    first element in the array.</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
>		    Exceptions:

		  <UL
><LI
><P
><A
NAME="NUMPOSPARAM"
></A
></P
><P
>		      <B
CLASS="COMMAND"
>${#*}</B
> and
		      <B
CLASS="COMMAND"
>${#@}</B
> give the <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>number
		      of positional parameters</I
></SPAN
>.
		    </P
></LI
><LI
><P
>		      For an array, <B
CLASS="COMMAND"
>${#array[*]}</B
> and
		      <B
CLASS="COMMAND"
>${#array[@]}</B
> give the number
		      of elements in the array.
		    </P
></LI
></UL
>
		  </P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="LENGTH"
></A
><P
><B
>Example 9-18. Length of a variable</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# length.sh
   3&nbsp;
   4&nbsp;E_NO_ARGS=65
   5&nbsp;
   6&nbsp;if [ $# -eq 0 ]  # Must have command-line args to demo script.
   7&nbsp;then
   8&nbsp;  echo "Please invoke this script with one or more command-line arguments."
   9&nbsp;  exit $E_NO_ARGS
  10&nbsp;fi  
  11&nbsp;
  12&nbsp;var01=abcdEFGH28ij
  13&nbsp;echo "var01 = ${var01}"
  14&nbsp;echo "Length of var01 = ${#var01}"
  15&nbsp;# Now, let's try embedding a space.
  16&nbsp;var02="abcd EFGH28ij"
  17&nbsp;echo "var02 = ${var02}"
  18&nbsp;echo "Length of var02 = ${#var02}"
  19&nbsp;
  20&nbsp;echo "Number of command-line arguments passed to script = ${#@}"
  21&nbsp;echo "Number of command-line arguments passed to script = ${#*}"
  22&nbsp;
  23&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
></DD
><DT
><A
NAME="PSOREX2"
></A
><TT
CLASS="USERINPUT"
><B
>${var#Pattern}</B
></TT
>, <TT
CLASS="USERINPUT"
><B
>${var##Pattern}</B
></TT
></DT
><DD
><P
><A
NAME="PSOREXSH"
></A
></P
><P
><B
CLASS="COMMAND"
>${var#Pattern} </B
>
		  Remove from <TT
CLASS="VARNAME"
>$var</TT
>
		  the <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>shortest</I
></SPAN
> part of
		  <TT
CLASS="VARNAME"
>$Pattern</TT
> that matches
		  the <TT
CLASS="REPLACEABLE"
><I
>front end</I
></TT
> of
		  <TT
CLASS="VARNAME"
>$var</TT
>.
		</P
><P
><A
NAME="PSOREXLO"
></A
></P
><P
><B
CLASS="COMMAND"
>${var##Pattern} </B
>
		  Remove from <TT
CLASS="VARNAME"
>$var</TT
>
		  the <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>longest</I
></SPAN
> part of
		  <TT
CLASS="VARNAME"
>$Pattern</TT
> that matches
		  the <TT
CLASS="REPLACEABLE"
><I
>front end</I
></TT
> of
		  <TT
CLASS="VARNAME"
>$var</TT
>.
		</P
><P
>A usage illustration from <A
HREF="contributed-scripts.html#DAYSBETWEEN"
>Example A-7</A
>:
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;# Function from "days-between.sh" example.  # Strips
   2&nbsp;leading zero(s) from argument passed.
   3&nbsp;
   4&nbsp;strip_leading_zero () #  Strip possible leading zero(s)
   5&nbsp;{                     #+ from argument passed.
   6&nbsp;  return=${1#0}       #  The "1" refers to "$1" -- passed arg.
   7&nbsp;}                     #  The "0" is what to remove from "$1" -- strips zeros.</PRE
></TD
></TR
></TABLE
>
		</P
><P
>Manfred Schwarb's more elaborate variation of the above:
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;strip_leading_zero2 () # Strip possible leading zero(s), since otherwise
   2&nbsp;{                      # Bash will interpret such numbers as octal values.
   3&nbsp;  shopt -s extglob     # Turn on extended globbing.
   4&nbsp;  local val=${1##+(0)} # Use local variable, longest matching series of 0's.
   5&nbsp;  shopt -u extglob     # Turn off extended globbing.
   6&nbsp;  _strip_leading_zero2=${val:-0}
   7&nbsp;                       # If input was 0, return 0 instead of "".
   8&nbsp;}</PRE
></TD
></TR
></TABLE
>
		</P
><P
>Another usage illustration: 
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;echo `basename $PWD`        # Basename of current working directory.
   2&nbsp;echo "${PWD##*/}"           # Basename of current working directory.
   3&nbsp;echo
   4&nbsp;echo `basename $0`          # Name of script.
   5&nbsp;echo $0                     # Name of script.
   6&nbsp;echo "${0##*/}"             # Name of script.
   7&nbsp;echo
   8&nbsp;filename=test.data
   9&nbsp;echo "${filename##*.}"      # data
  10&nbsp;                            # Extension of filename.</PRE
></TD
></TR
></TABLE
>
		</P
></DD
><DT
><A
NAME="PCTPATREF"
></A
><TT
CLASS="USERINPUT"
><B
>${var%Pattern}</B
></TT
>, <TT
CLASS="USERINPUT"
><B
>${var%%Pattern}</B
></TT
></DT
><DD
><P
><A
NAME="PCTREP1"
></A
></P
><P
><B
CLASS="COMMAND"
>{$var%Pattern}</B
>
		  Remove from <TT
CLASS="VARNAME"
>$var</TT
>
		  the <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>shortest</I
></SPAN
> part of
		  <TT
CLASS="VARNAME"
>$Pattern</TT
> that matches
		  the <TT
CLASS="REPLACEABLE"
><I
>back end</I
></TT
> of
		  <TT
CLASS="VARNAME"
>$var</TT
>.  </P
><P
><A
NAME="PCTREP2"
></A
></P
><P
><B
CLASS="COMMAND"
>{$var%%Pattern}</B
>
		  Remove from <TT
CLASS="VARNAME"
>$var</TT
>
		  the <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>longest</I
></SPAN
> part of
		  <TT
CLASS="VARNAME"
>$Pattern</TT
> that matches
		  the <TT
CLASS="REPLACEABLE"
><I
>back end</I
></TT
> of
		  <TT
CLASS="VARNAME"
>$var</TT
>.  </P
></DD
></DL
></DIV
><P
><A
HREF="bash2.html#BASH2REF"
>Version 2</A
> of Bash added
	      additional options.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="PATTMATCHING"
></A
><P
><B
>Example 9-19. Pattern matching in parameter substitution</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# patt-matching.sh
   3&nbsp;
   4&nbsp;# Pattern matching  using the # ## % %% parameter substitution operators.
   5&nbsp;
   6&nbsp;var1=abcd12345abc6789
   7&nbsp;pattern1=a*c  # * (wild card) matches everything between a - c.
   8&nbsp;
   9&nbsp;echo
  10&nbsp;echo "var1 = $var1"           # abcd12345abc6789
  11&nbsp;echo "var1 = ${var1}"         # abcd12345abc6789
  12&nbsp;                              # (alternate form)
  13&nbsp;echo "Number of characters in ${var1} = ${#var1}"
  14&nbsp;echo
  15&nbsp;
  16&nbsp;echo "pattern1 = $pattern1"   # a*c  (everything between 'a' and 'c')
  17&nbsp;echo "--------------"
  18&nbsp;echo '${var1#$pattern1}  =' "${var1#$pattern1}"    #         d12345abc6789
  19&nbsp;# Shortest possible match, strips out first 3 characters  abcd12345abc6789
  20&nbsp;#                                     ^^^^^               |-|
  21&nbsp;echo '${var1##$pattern1} =' "${var1##$pattern1}"   #                  6789      
  22&nbsp;# Longest possible match, strips out first 12 characters  abcd12345abc6789
  23&nbsp;#                                    ^^^^^                |----------|
  24&nbsp;
  25&nbsp;echo; echo; echo
  26&nbsp;
  27&nbsp;pattern2=b*9            # everything between 'b' and '9'
  28&nbsp;echo "var1 = $var1"     # Still  abcd12345abc6789
  29&nbsp;echo
  30&nbsp;echo "pattern2 = $pattern2"
  31&nbsp;echo "--------------"
  32&nbsp;echo '${var1%pattern2}  =' "${var1%$pattern2}"     #     abcd12345a
  33&nbsp;# Shortest possible match, strips out last 6 characters  abcd12345abc6789
  34&nbsp;#                                     ^^^^                         |----|
  35&nbsp;echo '${var1%%pattern2} =' "${var1%%$pattern2}"    #     a
  36&nbsp;# Longest possible match, strips out last 12 characters  abcd12345abc6789
  37&nbsp;#                                    ^^^^                 |-------------|
  38&nbsp;
  39&nbsp;# Remember, # and ## work from the left end (beginning) of string,
  40&nbsp;#           % and %% work from the right end.
  41&nbsp;
  42&nbsp;echo
  43&nbsp;
  44&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="RFE"
></A
><P
><B
>Example 9-20. Renaming file extensions<SPAN
CLASS="TOKEN"
>:</SPAN
></B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# rfe.sh: Renaming file extensions.
   3&nbsp;#
   4&nbsp;#         rfe old_extension new_extension
   5&nbsp;#
   6&nbsp;# Example:
   7&nbsp;# To rename all *.gif files in working directory to *.jpg,
   8&nbsp;#          rfe gif jpg
   9&nbsp;
  10&nbsp;
  11&nbsp;E_BADARGS=65