你可以man bash或者sh看看，下面这个是bash的。 你这个的意思是吧 /前面的*全部截去。
${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 exception to this is the expansion of ${!prefix*} described below.
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.
${parameter:?word}
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.
${parameter
ffset}
${parameter
ffset: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. If parameter is @, the result is length
positional parameters beginning at offset. If parameter is an array name indexed by @ or *, the result is
the length members of the array beginning with ${parameter[offset]}. Substring indexing is zero-based
unless the positional parameters are used, in which case the indexing starts at 1.
${!prefix*}
Expands to the names of variables whose names begin with prefix, separated by the first character of the
IFS special variable.
${#parameter}
The length in characters of the value of parameter is substituted. If parameter is * or @, the value sub颅
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颅
ning of the value of parameter, then the result of the expansion is the expanded value of parameter with
the shortest matching pattern (the 鈥樷�?[1m#'' case) or the longest matching pattern (the 鈥樷�?[1m##'' 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颅
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 鈥樷�?[1m%'' case) or the longest matching pattern (the 鈥樷�?[1m%%''
case) deleted. If parameter is @ or *, the pattern removal operation is applied to each positional parame颅
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}
${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. In the first form, only the first
match is replaced. The second form causes all matches of pattern to be replaced with string. 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.
