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- Brace Expansion
- Brace expansion is a mechanism by which arbitrary strings may be generated. This mechanism is similar to pathname expansion, but the
- filenames generated need not exist. Patterns to be brace expanded take the form of 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; left to right order is preserved. For example,
- a{d,c,b}e expands into ‘ade ace abe’.
- A sequence expression takes the form {x..y[..incr]}, where x and y are either integers or single characters, and incr, an optional incre-
- ment, is an integer. When integers are supplied, the expression expands to each number between x and y, inclusive. Supplied integers
- may be prefixed with 0 to force each term to have the same width. When either x or y begins with a zero, the shell attempts to force all
- generated terms to contain the same number of digits, zero-padding where necessary. When characters are supplied, the expression expands
- to each character lexicographically between x and y, inclusive. Note that both x and y must be of the same type. 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 con-
- sidered part of a brace expression. To avoid conflicts with parameter expansion, the string ${ is not considered eligible for brace
- expansion.
- 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 spe-
- cially 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. The same word is output as file1 file2
- after expansion by bash. If strict compatibility with sh is desired, start bash with the +B option or disable brace expansion with the
- +B option to the set command (see SHELL BUILTIN COMMANDS below).
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发表于 2014-09-13 17:07
