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3.5.3 Shell 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.
The basic form of parameter expansion is ${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 that 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. Put another way, if the colon is included,
the operator tests for both existence and that the value is not null;
if the colon is omitted, the operator tests only for existence.
${parameter:-word}
-
If parameter is unset or null, the expansion of
word is substituted. Otherwise, the value of
parameter is substituted.
${parameter:=word}
-
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}
-
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}
-
If parameter
is null or unset, nothing is substituted, otherwise the expansion of
word is substituted.
${parameter:offset}
-
${parameter:offset:length}
-
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 section 6.5 Shell Arithmetic).
This is referred to as Substring Expansion.
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 expanded value of parameter is
substituted.
If parameter is '*' or '@', the value substituted
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 filename
expansion (see section 3.5.8 Filename Expansion). If the pattern matches
the beginning 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
parameter 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
filename expansion.
If the pattern matches a trailing portion of the expanded value of
parameter, then the result of the expansion is the 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 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
filename 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.
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