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Extracting Parts of String "#" vs "%"
09-05-2015
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 embed-
ded 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 substi-
tution, rather than the value of parameter itself. This is known as indirect expansion. The exceptions to this are the expansions of ${!pre-
fix*} 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 substi-
tuted.
${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 substi-
tuted.
${parameter:+word}
Use Alternate Value. If parameter is null or unset, nothing is substituted, otherwise the expansion of word is substituted.
${parameter:offset}
${parameter:offset:length}
Substring Expansion. Expands to up to length characters of parameter starting at the character specified by offset. If length is omit-
ted, 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]}. 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 index-
ing is zero-based unless the positional parameters 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 substituted is the number of posi-
tional 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 beginning 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 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%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 posi-
tional 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/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.
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