Brace expansion is a mechanism to generate arbitrary strings sharing a common prefix and suffix, either of which can be empty. This mechanism is similar to filename expansion (see Filename Expansion), but the filenames generated need not exist. Patterns to be brace expanded are formed from an optional preamble, followed by either a series of comma-separated strings or a sequence expression between a pair of braces, followed by an optional postscript. The preamble is prefixed to each string contained within the braces, and the postscript is then appended to each resulting string, expanding left to right.
Brace expansions may be nested. The results of each expanded string are not sorted; brace expansion preserves left to right order. For example,
bash$ echo a{d,c,b}e
ade ace abe
A sequence expression takes the form
x..y[..incr],
where x and y are either integers or letters,
and incr, an optional increment, is an integer.
When integers are supplied, the expression expands to each number between
x and y, inclusive.
If either x or y begins with a zero,
each generated term will contain the same number of digits,
zero-padding where necessary.
When letters are supplied, the expression expands to each character
lexicographically between x and y, inclusive,
using the C locale.
Note that both x and y must be of the same type
(integer or letter).
When the increment is supplied, it is used as the difference between
each term.
The default increment is 1 or -1 as appropriate.
Brace expansion is performed before any other expansions, and any characters special to other expansions are preserved in the result. It is strictly textual. Bash does not apply any syntactic interpretation to the context of the expansion or the text between the braces.
A correctly-formed brace expansion must contain unquoted opening and closing braces, and at least one unquoted comma or a valid sequence expression. Any incorrectly formed brace expansion is left unchanged.
A ‘{’ or ‘,’ may be quoted with a backslash to prevent its being considered part of a brace expression. To avoid conflicts with parameter expansion, the string ‘${’ is not considered eligible for brace expansion, and inhibits brace expansion until the closing ‘}’.
This construct is typically used as shorthand when the common prefix of the strings to be generated is longer than in the above example:
mkdir /usr/local/src/bash/{old,new,dist,bugs}
or
chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
Brace expansion introduces a slight incompatibility with
historical versions of
sh.
sh
does not treat opening or closing braces specially when they
appear as part of a word, and preserves them in the output.
Bash
removes braces from words as a consequence of brace
expansion.
For example, a word entered to
sh
as
‘file{1,2}’
appears identically in the output.
Bash
outputs that word as
‘file1 file2’
after brace expansion.
Start
Bash
with the
+B
option or disable brace expansion with the
+B
option to the
set
command
(see Shell Builtin Commands)
for strict
sh
compatibility.