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This article focuses on extensions in bash. Which focuses on the curly braces extension Brace Expansion in bash, a detailed introduction of the definition and format of braces extension, has a certain reference value, friends in need can refer to it. All the expansion of bash are as follows:

Brace Expansion (curly braces extension)

Tilde Expansion (tilde extension)

Parameter and Variable Expansion (parameter and variable extension)

Command Substitution (command replacement)

Arithmetic Expansion (arithmetic extension)

Word Splitting (word segmentation)

Pathname Expansion (path extension)

The order listed above is exactly the order in which bash is extended.

Definition of curly braces expansion (Brace Expansion)

Curly braces extension, also known as curly braces extension, is an extension that allows bash to generate arbitrary strings. It is very similar to path extension, except that the generated string can be a path or file name that does not exist.

In bash, curly braces extension has the highest priority among many extensions, so a statement similar to echo {aforce b} $PATH should have the result of a$PATH b$PATH after completing the curly braces extension, while the extension of PATH environment variables will not start curly braces extension until the subsequent "parameter and variable extension" phase.

3.1 the first type of format is:

Preamble+ {string1,string2,string3,...,stringN} + postcript

Left and right curly braces are required, and the middle string list is separated by commas. Note that there can be no spaces before and after commas. If there are spaces in string, single or double quotation marks are required.

When bash actually extends, it concatenates preamble with all the strings in curly braces (from left to right), and finally adds postscript.

In addition, there is at least one comma in the middle of the curly braces, otherwise bash would not consider the curly braces extensions, for example:

Echo {moneny}. If you want to output money, change it to echo {money,} as follows:

[root@master01 ~] # echo {money}

{money}

[root@master01 ~] # echo {money,}

Money

Check out the complete example (pay attention to the handling of spaces)

[root@master01 ~] # echo sp {el,il,al} l

Spell spill spall

[root@master01 ~] # echo sp {el,il, al} l

Sp {el,il, al} l

[root@master01 ~] # echo sp {el,il,' al'} l

Spell spill sp all

[root@master01 ~] # echo sp {el,il, "" al} l

Spell spill sp all

[root@master01 ~] # echo sp {el,il,''al} l

Spell spill sp all

[root@master01 ~] # echo sp {el,il, "al"} l

Spell spill sp all

3.2 the second type of format is:

Preamble+ {.... [INCR]} + postscript

Among them; among them. The combined expression term is called a sequence expression (sequence expansion), which represents a specific range. When the sum is a number, it represents a range of numbers; when the sum is a single letter, it represents a range of characters (the default LC_ALL character arrangement). And must be numeric or alphabetic, otherwise bash does not think of curly braces as an extension, but output as is.

[root@master01 ~] # echo {1.. 12}

12 3 4 5 6 7 8 9 10 11 12

[root@master01 ~] # echo {3... Mustang 2}

3 2 1 0 1 2

[root@master01 ~] # echo {a.. g}

A b c d e f g

[root@master01 ~] # echo {h.. a}

H g f e d c b a

One of them is optional and represents the increment of the interval range, which must be a number. For example:

[root@master01 ~] # echo {0.10. 2}

0 2 4 6 8 10

Starting at 0, the corresponding number is taken out for every 2 increments.

If it is not specified, then the default is 1 or-1, specifically 1 or-1, depending on whether the range of the previous range is increasing or decreasing. For example, in the example {a. G} above, the default is 1, and {h. A} defaults to-1.

In addition, when the sum is a number, we can keep the output length consistent by adding 0 before the number.

[root@master01 ~] # echo {1.. 10}

1 2 3 4 5 6 7 8 9 10

[root@master01 ~] # echo {01.. 10}

01 02 03 04 05 06 07 08 09 10

[root@master01 ~] # echo {001.. 10}

001 002 003 004 005 006 007 008 009 010

This format is the same length, it will look beautiful after watching the above content, do you have any further understanding of the curly braces extension in bash? If you want to learn more skills or want to know more about it, you are welcome to follow the industry information channel. Thank you for reading.

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