求助变量${i%%pattern} ${i%pattern} ${i##pattern} ${i#pattern} 详解

syttk

${i%%pattern} ${i%pattern} ${i##pattern} ${i#pattern}

详解
例子 谢谢

寂寞烈火

http://bbs.chinaunix.net/viewthr ... p;page=7#pid1617953

vbs100

Parameter Expansion
       The `$' character introduces parameter expansion, command substitution, or arithmetic  expansion.   The  parameter
       name  or  symbol to be expanded may be enclosed in braces, which are optional but serve to protect the variable to
       be expanded from characters immediately following it which could be interpreted as part of the name.

       When braces are used, the matching ending brace is the first `}' not escaped by a backslash  or  within  a  quoted
       string, and not within an embedded arithmetic expansion, command substitution, or parameter expansion.

       ${parameter}
              The  value  of  parameter is substituted.  The braces are required when parameter is a positional parameter
              with more than one digit, or when parameter is followed by a character which is not to  be  interpreted  as
              part of its name.

       If  the first character of parameter is an exclamation point, a level of variable indirection is introduced.  Bash
       uses the value of the variable formed from the rest of parameter as the name of the  variable;  this  variable  is
       then  expanded  and that value is used in the rest of the substitution, rather than the value of parameter itself.
       This is known as indirect expansion.  The exceptions to this are the expansions  of  ${!prefix*}  and  ${!name[@]}
       described  below.  The exclamation point must immediately follow the left brace in order to introduce indirection.

       In each of the cases below, word is subject to tilde expansion, parameter  expansion,  command  substitution,  and
       arithmetic  expansion.  When not performing substring expansion, bash tests for a parameter that is unset or null;
       omitting the colon results in a test only for a parameter that is unset.

       ${parameter:-word}
              Use Default Values.  If parameter is unset or null, the expansion of word is substituted.   Otherwise,  the
              value of parameter is substituted.
       ${parameter:=word}
              Assign Default Values.  If parameter is unset or null, the expansion of word is assigned to parameter.  The
              value of parameter is then substituted.  Positional parameters and special parameters may not  be  assigned
              to in this way.
       ${parameterword}
              Display Error if Null or Unset.  If parameter is null or unset, the expansion of word (or a message to that
              effect if word is not present) is written to the standard error and the shell, if it  is  not  interactive,
              exits.  Otherwise, the value of parameter is substituted.
       ${parameter:+word}
              Use  Alternate  Value.   If  parameter is null or unset, nothing is substituted, otherwise the expansion of
              word is substituted.
       ${parameterffset}
       ${parameterffset:length}
              Substring Expansion.  Expands to up to length characters of parameter starting at the  character  specified
              by offset.  If length is omitted, expands to the substring of parameter starting at the character specified
              by offset.  length and offset are arithmetic expressions (see ARITHMETIC EVALUATION  below).   length  must
              evaluate  to  a  number greater than or equal to zero.  If offset evaluates to a number less than zero, the
              value is used as an offset from the end of the value of parameter.  Arithmetic expressions starting with  a
              -  must  be  separated  by  whitespace from the preceding : to be distinguished from the Use Default Values
              expansion.  If parameter is @, the result is length positional parameters beginning at offset.  If  parame-
              ter  is  an  array  name  indexed  by  @ or *, the result is the length members of the array beginning with
              ${parameter[offset]}.  A negative offset is taken relative to one greater than the  maximum  index  of  the
              specified  array.   Note  that  a negative offset must be separated from the colon by at least one space to
              avoid being confused with the :- expansion.  Substring indexing is zero-based unless the positional parame-
              ters are used, in which case the indexing starts at 1.

       ${!prefix*}
       ${!prefix@}
              Expands  to  the  names of variables whose names begin with prefix, separated by the first character of the
              IFS special variable.

       ${!name[@]}
       ${!name

}
              If name is an array variable, expands to the list of array indices (keys) assigned in name.  If name is not
              an  array, expands to 0 if name is set and null otherwise.  When @ is used and the expansion appears within
              double quotes, each key expands to a separate word.

       ${#parameter}
              The length in characters of the value of parameter is substituted.  If parameter is * or @, the value  sub-
              stituted  is the number of positional parameters.  If parameter is an array name subscripted by * or @, the
              value substituted is the number of elements in the array.

       ${parameter#word}
       ${parameter##word}
              The word is expanded to produce a pattern just as in pathname expansion.  If the pattern matches the begin-
              ning  of  the  value of parameter, then the result of the expansion is the expanded value of parameter with
              the shortest matching pattern (the ``#'' case) or the longest matching pattern (the ``##''  case)  deleted.
              If  parameter is @ or *, the pattern removal operation is applied to each positional parameter in turn, and
              the expansion is the resultant list.  If parameter is an array variable subscripted with @ or *,  the  pat-
              tern  removal  operation is applied to each member of the array in turn, and the expansion is the resultant
              list.

       ${parameter%word}
       ${parameter%%word}
              The word is expanded to produce a pattern just as in pathname expansion.  If the pattern matches a trailing
              portion  of  the  expanded  value  of  parameter, then the result of the expansion is the expanded value of
              parameter with the shortest matching pattern (the ``%'' case) or the longest matching pattern  (the  ``%%''
              case) deleted.  If parameter is @ or *, the pattern removal operation is applied to each positional parame-
              ter in turn, and the expansion is the resultant list.  If parameter is an array variable subscripted with @
              or  *,  the  pattern removal operation is applied to each member of the array in turn, and the expansion is
              the resultant list.

       ${parameter/pattern/string}
              The pattern is expanded to produce a pattern just as in pathname expansion.  Parameter is expanded and  the
              longest match of pattern against its value is replaced with string.  If Ipattern begins with /, all matches
              of pattern are replaced with string.  Normally only the first match is replaced.  If pattern begins with #,
              it must match at the beginning of the expanded value of parameter.  If pattern begins with %, it must match
              at the end of the expanded value of parameter.  If string is null, matches of pattern are deleted and the /
              following  pattern  may  be omitted.  If parameter is @ or *, the substitution operation is applied to each
              positional parameter in turn, and the expansion is the resultant list.  If parameter is an  array  variable
              subscripted with @ or *, the substitution operation is applied to each member of the array in turn, and the
              expansion is the resultant list.