bash: Shell Parameter Expansion
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 PARAMETER is a shell parameter as
DONTPRINTYET described above (
Shell Parameters) or an array reference (*noteDONTPRINTYET described above (Shell Parameters) or an array reference (
Arrays). 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 (!), and
PARAMETER is not a NAMEREF, it introduces a level of variable
indirection. 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'. If
PARAMETER is a nameref, this expands to the name of the variable
referenced by PARAMETER instead of performing the complete indirect
expansion. The exceptions to this are the expansions of ${!PREFIX*} and
${!NAME[@]} described below. The exclamation point must immediately
follow the left brace in order to introduce indirection.
In each of the cases below, WORD is subject to tilde expansion,
parameter expansion, command substitution, and arithmetic expansion.
When not performing substring expansion, using the form described
below (e.g., ':-'), Bash tests for a parameter that is unset or null.
Omitting the colon results in a test only for a parameter that is unset.
Put another way, if the colon is included, the operator tests for both
PARAMETER's existence and that its value is not null; if the colon is
omitted, the operator tests only for existence.
'${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}'
This is referred to as Substring Expansion. It expands to up to
LENGTH characters of the value of PARAMETER starting at the
character specified by OFFSET. If PARAMETER is '@', an indexed
array subscripted by '@' or '*', or an associative array name, the
results differ as described below. If LENGTH is omitted, it
expands to the substring of the value of PARAMETER starting at the
character specified by OFFSET and extending to the end of the
value. LENGTH and OFFSET are arithmetic expressions (Shell
Arithmetic).
If OFFSET evaluates to a number less than zero, the value is used
as an offset in characters from the end of the value of PARAMETER.
If LENGTH evaluates to a number less than zero, it is interpreted
as an offset in characters from the end of the value of PARAMETER
rather than a number of characters, and the expansion is the
characters between OFFSET and that result. Note that a negative
offset must be separated from the colon by at least one space to
avoid being confused with the ':-' expansion.
Here are some examples illustrating substring expansion on
parameters and subscripted arrays:
$ string=01234567890abcdefgh
$ echo ${string:7}
7890abcdefgh
$ echo ${string:7:0}
$ echo ${string:7:2}
78
$ echo ${string:7:-2}
7890abcdef
$ echo ${string: -7}
bcdefgh
$ echo ${string: -7:0}
$ echo ${string: -7:2}
bc
$ echo ${string: -7:-2}
bcdef
$ set -- 01234567890abcdefgh
$ echo ${1:7}
7890abcdefgh
$ echo ${1:7:0}
$ echo ${1:7:2}
78
$ echo ${1:7:-2}
7890abcdef
$ echo ${1: -7}
bcdefgh
$ echo ${1: -7:0}
$ echo ${1: -7:2}
bc
$ echo ${1: -7:-2}
bcdef
$ array[0]=01234567890abcdefgh
$ echo ${array[0]:7}
7890abcdefgh
$ echo ${array[0]:7:0}
$ echo ${array[0]:7:2}
78
$ echo ${array[0]:7:-2}
7890abcdef
$ echo ${array[0]: -7}
bcdefgh
$ echo ${array[0]: -7:0}
$ echo ${array[0]: -7:2}
bc
$ echo ${array[0]: -7:-2}
bcdef
If PARAMETER is '@', the result is LENGTH positional parameters
beginning at OFFSET. A negative OFFSET is taken relative to one
greater than the greatest positional parameter, so an offset of -1
evaluates to the last positional parameter. It is an expansion
error if LENGTH evaluates to a number less than zero.
The following examples illustrate substring expansion using
positional parameters:
$ set -- 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
$ echo ${@:7}
7 8 9 0 a b c d e f g h
$ echo ${@:7:0}
$ echo ${@:7:2}
7 8
$ echo ${@:7:-2}
bash: -2: substring expression < 0
$ echo ${@: -7:2}
b c
$ echo ${@:0}
./bash 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
$ echo ${@:0:2}
./bash 1
$ echo ${@: -7:0}
If PARAMETER is an indexed array name subscripted by '@' or '*',
the result is the LENGTH members of the array beginning with
'${PARAMETER[OFFSET]}'. A negative OFFSET is taken relative to one
greater than the maximum index of the specified array. It is an
expansion error if LENGTH evaluates to a number less than zero.
These examples show how you can use substring expansion with
indexed arrays:
$ array=(0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h)
$ echo ${array[@]:7}
7 8 9 0 a b c d e f g h
$ echo ${array[@]:7:2}
7 8
$ echo ${array[@]: -7:2}
b c
$ echo ${array[@]: -7:-2}
bash: -2: substring expression < 0
$ echo ${array[@]:0}
0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
$ echo ${array[@]:0:2}
0 1
$ echo ${array[@]: -7:0}
Substring expansion applied to an associative array produces
undefined results.
Substring indexing is zero-based unless the positional parameters
are used, in which case the indexing starts at 1 by default. If
OFFSET is 0, and the positional parameters are used, '$@' is
prefixed to the list.
'${!PREFIX*}'
'${!PREFIX@}'
Expands to the names of variables whose names begin with PREFIX,
separated by the first character of the 'IFS' special variable.
When '@' is used and the expansion appears within double quotes,
each variable name expands to a separate word.
'${!NAME[@]}'
'${!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.
'${#PARAMETER}'
The length in characters of the expanded value of PARAMETER is
substituted. If PARAMETER is '*' or '@', the value substituted is
the number of positional parameters. If PARAMETER is an array name
subscripted by '*' or '@', the value substituted is the number of
elements in the array. If PARAMETER is an indexed array name
subscripted by a negative number, that number is interpreted as
relative to one greater than the maximum index of PARAMETER, so
negative indices count back from the end of the array, and an index
of -1 references the last element.
'${PARAMETER#WORD}'
'${PARAMETER##WORD}'
The WORD is expanded to produce a pattern just as in filename
expansion (Filename Expansion). If the pattern matches the
beginning of the expanded value of PARAMETER, then the result of
the expansion is the expanded value of PARAMETER with the shortest
matching pattern (the '#' case) or the longest matching pattern
(the '##' case) deleted. If PARAMETER is '@' or '*', the pattern
removal operation is applied to each positional parameter in turn,
and the expansion is the resultant list. If PARAMETER is an array
variable subscripted with '@' or '*', the pattern removal operation
is applied to each member of the array in turn, and the expansion
is the resultant list.
'${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}'
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. If PATTERN 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 the
'nocasematch' shell option (see the description of 'shopt' in
The Shopt Builtin) is enabled, the match is performed without
regard to the case of alphabetic characters. 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.
'${PARAMETER^PATTERN}'
'${PARAMETER^^PATTERN}'
'${PARAMETER,PATTERN}'
'${PARAMETER,,PATTERN}'
This expansion modifies the case of alphabetic characters in
PARAMETER. The PATTERN is expanded to produce a pattern just as in
filename expansion. Each character in the expanded value of
PARAMETER is tested against PATTERN, and, if it matches the
pattern, its case is converted. The pattern should not attempt to
match more than one character. The '^' operator converts lowercase
letters matching PATTERN to uppercase; the ',' operator converts
matching uppercase letters to lowercase. The '^^' and ',,'
expansions convert each matched character in the expanded value;
the '^' and ',' expansions match and convert only the first
character in the expanded value. If PATTERN is omitted, it is
treated like a '?', which matches every character. If PARAMETER is
'@' or '*', the case modification 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 case modification operation is applied to each member of
the array in turn, and the expansion is the resultant list.
'${PARAMETER@OPERATOR}'
The expansion is either a transformation of the value of PARAMETER
or information about PARAMETER itself, depending on the value of
OPERATOR. Each OPERATOR is a single letter:
'Q'
The expansion is a string that is the value of PARAMETER
quoted in a format that can be reused as input.
'E'
The expansion is a string that is the value of PARAMETER with
backslash escape sequences expanded as with the '$'...''
quoting mechansim.
'P'
The expansion is a string that is the result of expanding the
value of PARAMETER as if it were a prompt string (
Controlling the Prompt).
'A'
The expansion is a string in the form of an assignment
statement or 'declare' command that, if evaluated, will
recreate PARAMETER with its attributes and value.
'a'
The expansion is a string consisting of flag values
representing PARAMETER's attributes.
If PARAMETER is '@' or '*', the 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 operation is applied to each member of the array in turn,
and the expansion is the resultant list.
The result of the expansion is subject to word splitting and
pathname expansion as described below.
Info Catalog
bash: Tilde Expansion
bash: Shell Expansions
bash: Command Substitution