parameter-substitution.html

一本完整的描述Unix Shell 编程的工具书的所有范例

>${var##Pattern}</B
></TT
></DT
><DD
><P
>Remove from <TT
CLASS="VARNAME"
>$var</TT
>
		  the shortest/longest 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
>:
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><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;# Function from "days-between.sh" example.
   2&nbsp;# Strips 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 "$1" -- strips zeros.</PRE
></TD
></TR
></TABLE
>
		</P
><P
>Manfred Schwarb's more elaborate variation of the above:
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><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;strip_leading_zero2 () # Strip possible leading zero(s), since otherwise
   2&nbsp;{                      # Bash will interpret such numbers as octal values.
   3&nbsp;  shopt -s extglob     # Turn on extended globbing.
   4&nbsp;  local val=${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: 
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><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 "${filename##*.}"      # data
  10&nbsp;                            # Extension of filename.</PRE
></TD
></TR
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>
		</P
></DD
><DT
><A
NAME="PCTPATREF"
></A
><TT
CLASS="USERINPUT"
><B
>${var%Pattern}</B
></TT
>, <TT
CLASS="USERINPUT"
><B
>${var%%Pattern}</B
></TT
></DT
><DD
><P
>Remove from <TT
CLASS="VARNAME"
>$var</TT
>
		  the shortest/longest 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 9-17. Pattern matching in parameter substitution</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
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><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 9-18. 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
  12&nbsp;
  13&nbsp;case $# in
  14&nbsp;  0|1)             # The vertical bar means "or" in this context.
  15&nbsp;  echo "Usage: `basename $0` old_file_suffix new_file_suffix"
  16&nbsp;  exit $E_BADARGS  # If 0 or 1 arg, then bail out.
  17&nbsp;  ;;
  18&nbsp;esac
  19&nbsp;
  20&nbsp;
  21&nbsp;for filename in *.$1
  22&nbsp;# Traverse list of files ending with 1st argument.
  23&nbsp;do
  24&nbsp;  mv $filename ${filename%$1}$2
  25&nbsp;  #  Strip off part of filename matching 1st argument,
  26&nbsp;  #+ then append 2nd argument.
  27&nbsp;done
  28&nbsp;
  29&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="VARIABLELIST"
><P
><B
><A
NAME="EXPREPL1"
></A
>Variable expansion / Substring
	      replacement</B
></P
><DL
><DT
></DT
><DD
><P
>These constructs have been adopted from
		  <I
CLASS="EMPHASIS"
>ksh</I
>.</P
></DD
><DT
><TT
CLASS="USERINPUT"
><B
>${var:pos}</B
></TT
></DT
><DD
><P
>Variable <TT
CLASS="REPLACEABLE"
><I
>var</I
></TT
> expanded,
		    starting from offset <TT
CLASS="REPLACEABLE"
><I
>pos</I
></TT
>.
		  </P
></DD
><DT
><TT
CLASS="USERINPUT"
><B
>${var:pos:len}</B
></TT
></DT
><DD
><P
>Expansion to a max of <TT
CLASS="REPLACEABLE"
><I
>len</I
></TT
>
		    characters of variable <TT
CLASS="REPLACEABLE"
><I
>var</I
></TT
>, from offset
		    <TT
CLASS="REPLACEABLE"
><I
>pos</I
></TT
>. See <A
HREF="contributed-scripts.html#PW"
>Example A-14</A
>
		    for an example of the creative use of this operator.
		  </P
></DD
><DT
><TT
CLASS="USERINPUT"
><B
>${var/Pattern/Replacement}</B
></TT
></DT
><DD
><P
>First match of <TT
CLASS="REPLACEABLE"
><I
>Pattern</I
></TT
>,
		    within <TT
CLASS="REPLACEABLE"
><I
>var</I
></TT
> replaced with
		    <TT
CLASS="REPLACEABLE"
><I
>Replacement</I
></TT
>.</P
><P
>If <TT
CLASS="REPLACEABLE"
><I
>Replacement</I
></TT
> is
		    omitted, then the first match of
		    <TT
CLASS="REPLACEABLE"
><I
>Pattern</I
></TT
> is replaced by
		    <I
CLASS="EMPHASIS"
>nothing</I
>, that is, deleted.</P
></DD
><DT
><TT
CLASS="USERINPUT"
><B
>${var//Pattern/Replacement}</B
></TT
></DT
><DD
><DIV
CLASS="FORMALPARA"
><P
><B
>Global replacement. </B
>All matches of <TT
CLASS="REPLACEABLE"
><I
>Pattern</I
></TT
>,
		    within <TT
CLASS="REPLACEABLE"
><I
>var</I
></TT
> replaced with
		    <TT
CLASS="REPLACEABLE"
><I
>Replacement</I
></TT
>.</P
></DIV
><P
>As above, if <TT
CLASS="REPLACEABLE"
><I
>Replacement</I
></TT
>
		    is omitted, then all occurrences of
		    <TT
CLASS="REPLACEABLE"
><I
>Pattern</I
></TT
> are replaced by
		    <I
CLASS="EMPHASIS"
>nothing</I
>, that is, deleted.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX7"
></A
><P
><B
>Example 9-19. Using pattern matching to parse arbitrary strings</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;var1=abcd-1234-defg
   4&nbsp;echo "var1 = $var1"
   5&nbsp;
   6&nbsp;t=${var1#*-*}
   7&nbsp;echo "var1 (with everything, up to and including first - stripped out) = $t"
   8&nbsp;#  t=${var1#*-}  works just the same,
   9&nbsp;#+ since # matches the shortest string,
  10&nbsp;#+ and * matches everything preceding, including an empty string.
  11&nbsp;# (Thanks, Stephane Chazelas, for pointing this out.)
  12&nbsp;
  13&nbsp;t=${var1##*-*}
  14&nbsp;echo "If var1 contains a \"-\", returns empty string...   var1 = $t"
  15&nbsp;
  16&nbsp;
  17&nbsp;t=${var1%*-*}
  18&nbsp;echo "var1 (with everything from the last - on stripped out) = $t"
  19&nbsp;
  20&nbsp;echo
  21&nbsp;
  22&nbsp;# -------------------------------------------
  23&nbsp;path_name=/home/bozo/ideas/thoughts.for.today
  24&nbsp;# -------------------------------------------
  25&nbsp;echo "path_name = $path_name"
  26&nbsp;t=${path_name##/*/}
  27&nbsp;echo "path_name, stripped of prefixes = $t"
  28&nbsp;# Same effect as   t=`basename $path_name` in this particular case.
  29&nbsp;#  t=${path_name%/}; t=${t##*/}   is a more general solution,
  30&nbsp;#+ but still fails sometimes.
  31&nbsp;#  If $path_name ends with a newline, then `basename $path_name` will not work,
  32&nbsp;#+ but the above expression will.
  33&nbsp;# (Thanks, S.C.)
  34&nbsp;
  35&nbsp;t=${path_name%/*.*}
  36&nbsp;# Same effect as   t=`dirname $path_name`
  37&nbsp;echo "path_name, stripped of suffixes = $t"
  38&nbsp;# These will fail in some cases, such as "../", "/foo////", # "foo/", "/".
  39&nbsp;#  Removing suffixes, especially when the basename has no suffix,
  40&nbsp;#+ but the dirname does, also complicates matters.
  41&nbsp;# (Thanks, S.C.)
  42&nbsp;
  43&nbsp;echo
  44&nbsp;
  45&nbsp;t=${path_name:11}
  46&nbsp;echo "$path_name, with first 11 chars stripped off = $t"
  47&nbsp;t=${path_name:11:5}
  48&nbsp;echo "$path_name, with first 11 chars stripped off, length 5 = $t"
  49&nbsp;
  50&nbsp;echo
  51&nbsp;
  52&nbsp;t=${path_name/bozo/clown}
  53&nbsp;echo "$path_name with \"bozo\" replaced  by \"clown\" = $t"
  54&nbsp;t=${path_name/today/}
  55&nbsp;echo "$path_name with \"today\" deleted = $t"
  56&nbsp;t=${path_name//o/O}
  57&nbsp;echo "$path_name with all o's capitalized = $t"
  58&nbsp;t=${path_name//o/}
  59&nbsp;echo "$path_name with all o's deleted = $t"
  60&nbsp;
  61&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
></DD
><DT
><TT
CLASS="USERINPUT"
><B
>${var/#Pattern/Replacement}</B
></TT
></DT
><DD
><P
>If <I
CLASS="EMPHASIS"
>prefix</I
> of
		  <TT
CLASS="REPLACEABLE"
><I
>var</I
></TT
> matches
		  <TT
CLASS="REPLACEABLE"
><I
>Pattern</I
></TT
>, then substitute
		  <TT
CLASS="REPLACEABLE"
><I
>Replacement</I
></TT
> for
		  <TT
CLASS="REPLACEABLE"
><I
>Pattern</I
></TT
>.</P
></DD
><DT
><TT
CLASS="USERINPUT"
><B
>${var/%Pattern/Replacement}</B
></TT
></DT
><DD
><P
>If <I
CLASS="EMPHASIS"
>suffix</I
> of
		  <TT
CLASS="REPLACEABLE"
><I
>var</I
></TT
> matches
		  <TT
CLASS="REPLACEABLE"
><I
>Pattern</I
></TT
>, then substitute
		  <TT
CLASS="REPLACEABLE"
><I
>Replacement</I
></TT
> for
		  <TT
CLASS="REPLACEABLE"
><I
>Pattern</I
></TT
>.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="VARMATCH"
></A
><P
><B
>Example 9-20. Matching patterns at prefix or suffix of string</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# var-match.sh:
   3&nbsp;# Demo of pattern replacement at prefix / suffix of string.
   4&nbsp;
   5&nbsp;v0=abc1234zip1234abc    # Original variable.
   6&nbsp;echo "v0 = $v0"         # abc1234zip1234abc
   7&nbsp;echo
   8&nbsp;
   9&nbsp;# Match at prefix (beginning) of string.
  10&nbsp;v1=${v0/#abc/ABCDEF}    # abc1234zip1234abc
  11&nbsp;                        # |-|
  12&nbsp;echo "v1 = $v1"         # ABCDEF1234zip1234abc
  13&nbsp;                        # |----|
  14&nbsp;
  15&nbsp;# Match at suffix (end) of string.
  16&nbsp;v2=${v0/%abc/ABCDEF}    # abc1234zip123abc
  17&nbsp;                        #              |-|
  18&nbsp;echo "v2 = $v2"         # abc1234zip1234ABCDEF
  19&nbsp;                        #               |----|
  20&nbsp;
  21&nbsp;echo
  22&nbsp;
  23&nbsp;#  ----------------------------------------------------
  24&nbsp;#  Must match at beginning / end of string,
  25&nbsp;#+ otherwise no replacement results.
  26&nbsp;#  ----------------------------------------------------
  27&nbsp;v3=${v0/#123/000}       # Matches, but not at beginning.
  28&nbsp;echo "v3 = $v3"         # abc1234zip1234abc
  29&nbsp;                        # NO REPLACEMENT.
  30&nbsp;v4=${v0/%123/000}       # Matches, but not at end.
  31&nbsp;echo "v4 = $v4"         # abc1234zip1234abc
  32&nbsp;                        # NO REPLACEMENT.
  33&nbsp;
  34&nbsp;exit 0			</PRE
></TD
></TR
></TABLE
><HR></DIV
></DD
><DT
><TT
CLASS="USERINPUT"
><B
>${!varprefix*}</B
></TT
>, <TT
CLASS="USERINPUT"
><B
>${!varprefix@}</B
></TT
></DT
><DD
><P
>Matches all previously declared variables beginning
		    with <I
CLASS="EMPHASIS"
>varprefix</I
>.
		      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;xyz23=whatever
   2&nbsp;xyz24=
   3&nbsp;
   4&nbsp;a=${!xyz*}      # Expands to names of declared variables beginning with "xyz".
   5&nbsp;echo "a = $a"   # a = xyz23 xyz24
   6&nbsp;a=${!xyz@}      # Same as above.
   7&nbsp;echo "a = $a"   # a = xyz23 xyz24
   8&nbsp;
   9&nbsp;# Bash, version 2.04, adds this feature.</PRE
></TD
></TR
></TABLE
>
                  </P
></DD
></DL
></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.AEN4837"
HREF="parameter-substitution.html#AEN4837"
>[1]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>If $parameter is null in a
		      non-interactive script, it will terminate with a <A
HREF="exitcodes.html#EXITCODESREF"
><SPAN
CLASS="RETURNVALUE"
>127</SPAN
>
		      exit status</A
> (the Bash error code for
		      <SPAN
CLASS="QUOTE"
>"command not found"</SPAN
>).</P
></TD
></TR
></TABLE
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>Typing variables: <B
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