In distinction bash (), {}, (()), [], [[]] of

HTTPS: // www.cnblogs.com/marility/p/7259821.html 

least bit of difference hack away 

needs carefully

Introduction: bash encountered in the various brackets, while when judging character numeric comparison is performed, constantly a problem, then, at the same time be tested on centos version 6.7 by referring to "advanced bash-scripting guide", the current situation is summarized as follows . If flawed, look correct.

One.()

A combination of the command, a command group corresponding to

[root@mghuee~chunlanyy testdir]# I=123;(I=xyz;echo $I;);echo $I
xyz
123

two.{}

With "{}", but also for a combination of the command, the difference between "()" is that the former is currently the shell, while the latter are inside the shell

[root@mghuee~chunlanyy testdir]# I=123;(echo $I;I=xyz;echo $I;);echo $I
123
xyz
123
[root@mghuee~chunlanyy testdir]# I=123;{ echo $I;I=xyz;echo $I; };echo $I
123
xyz
xyz

Can be seen from the example, "()" is assigned only affect its own sub-shell, and the shell will not be assigned to the parent, and "{}" is only performed in the same shell

Also note that, "{}" is a keyword, so that in the "{" space isolation required, and on both sides simultaneously using the command ";" indicates the end of the command.

[root@mghuee~chunlanyy testdir]# type {
{ is a shell keyword

three.[]

"[]" It is mainly used as a determination condition, the determination target comprises comparing the file type and assignment

[root@mghuee~chunlanyy testdir]# type [
[ is a shell builtin

We can see the "[" is an internal command, can be considered equivalent to the basic test command

[root@mghuee~chunlanyy testdir]# [ 4 -lt 3 ] && echo yes || echo no
no
[root@mghuee~chunlanyy testdir]#  test 4 -lt 3  && echo yes || echo no
no

Common comparison test as follows:

Digital test: -eq -ne -lt -le -gt -ge

File test: -r, -l, -w, -x, -f, -d, -s, -nt, -ot

Test String: =, =, - n, -z, \>, \ <!

Logic test: -a, -o ,!

Math: support for integer arithmetic, but has been "((...))" instead of

Points to note:

1. Only such a digital comparison -LT-character comparison, but you can not use the "<" Such a comparison operator, even if added escapes "\ <"

[root@mghuee~chunlanyy testdir]# [ 1 > 2 ] && echo yes || echo no
yes

Obviously, "1> 2" should be wrong, but show "yes", in fact, here ">" is not greater than the number of arithmetic comparison, and to redirect output to view the current directory by ls command more a file named file 2

[root@mghuee~chunlanyy testdir]# ll -l
total 0
-rw-r--r--. 1 root root 0 Jun 17 20:53 2

Imagine someone will escape numerical comparison

[root@mghuee~chunlanyy testdir]# [ 1 \> 2 ] && echo yes || echo no
no

Size numeric comparison by comparing this line with our expectations, but "\>" not here to be understood in our daily

[root@mghuee~chunlanyy testdir]# [ a \> 2 ] && echo yes || echo no
yes

Here is the ASCII value comparison size comparison, at the same time here can only support single-character size comparison

[root@mghuee~chunlanyy testdir]# [ 5 \> 20 ] && echo yes || echo no
yes

2. "[]" Is a logical operator support -a, -o ,! Where -a, -o parameter test is equivalent to a command, you can not use &&, ||

[root@mghuee~chunlanyy testdir]# [ 1 -lt  2 && 1 -lt 3 ] && echo yes || echo no
-bash: [: missing `]'
no

&& and || strong logical operation, will split command, has been split into the above example, "[1 -lt 2" && "1 -lt 3]", it will show no pairing "["

In the logical operations, [expre1 -a expre 2] and [[expre1 && expre2]] is not entirely consistent, logical && will short-circuit operation, but not -a

[root@mghuee~chunlanyy testdir]# [ 1 -gt 3 -a 2 > test1.txt ] && echo yes || echo no
no
[root@mghuee~chunlanyy testdir]# ls
test1.txt

No short-circuiting operation logic performs all determination, even if "1 -gt 3" has an error, the subsequent operation is still performed, although the result is no, but also generates file test1.txt

[root@mghuee~chunlanyy testdir]# [[ 1 -gt 3 && 2 > test2.txt ]] && echo yes || echo no
no
[root@mghuee~chunlanyy testdir]# ls
test1.txt

Logical arithmetic operation, and the command, "1 -gt 3" is false i.e. without subsequent judgment, it appears no test2.txt

3. "[]" Is a command, it is a reference variable or constant therein, need to use double quotes or single quotes, because there will be split word (Word-Splitting) Phenomenon

[root@mghuee~chunlanyy testdir]# I=" 29";[ -f $I ] && echo yes || echo no
no
[root@mghuee~chunlanyy testdir]# I=" 29";[ -f "$I" ] && echo yes || echo no
yes
[root@mghuee~chunlanyy testdir]# ll -a
total 8
drwxr-xr-x. 2 root root 4096 Jun 17 23:20 .
drwxrwxrwt. 6 root root 4096 Jun 17 20:53 ..
-rw-r--r--. 1 root root    0 Jun 17 23:17  29

There is a file in the current directory named "29" file (29 in front of a space), in the [] is not added when the reference to the variable double quotation marks, [] -f internal execution is determined, "29", because the current directory file does not exist file name "29", showing result is no

four.(())

It supports four operations, equivalent to let function

[root@mghuee~chunlanyy testdir]# let I=2+4;echo $I
6
[root@mghuee~chunlanyy testdir]# I=$((2+4));echo $I
6

Fives.[[]]

"[[" Is a keyword, for comparison and determination, Basic Support "[]" is determined in all of the comparison operators.

[root@mghuee~chunlanyy testdir]# type [[
[[ is a shell keyword

"[[" And "[" differences:

1. The logical operator inconsistency, "[[]]" is the "&&", "||", "[]" to "-a", "- o". "[[]]" Shorted support logic, and "[]" is not supported

2. "[[]]" Support for regular expression matching.

3. "[[]]" As a keyword, the same bracket with intermediate spaces must be isolated expression

4. "[[]]" Can not escape when used in comparison operators, while not appear Word-Splitting

[root@mghuee~chunlanyy testdir]# I=" 29";[[ -f $I ]] && echo yes || echo no
yes
[root@mghuee~chunlanyy testdir]# ll -a
total 8
drwxr-xr-x. 2 root root 4096 Jun 17 23:20 .
drwxrwxrwt. 6 root root 4096 Jun 17 20:53 ..
-rw-r--r--. 1 root root    0 Jun 17 23:17  29

Compared with the result of the "[]", it is clear, in the "[[]]" and does not appear in the Word-Splitting

[root@mghuee~chunlanyy testdir]# [[ a > 1 ]] && echo yes || echo no
yes
[root@mghuee~chunlanyy testdir]# [[ a \> 1 ]] && echo yes || echo no
-bash: conditional binary operator expected
-bash: syntax error near `\>'

As can be seen, error after using the escape

5. [[]] In the "==" using "= -" in

Reference "advanced bash-scripting guide" interpretation of both

== String comparison operator <== string comparison

= ~ Regular Expression match operator <== regular expression matching

But this is not representative of the regular expression pattern matching "==" in the "[[]]" can not be achieved, part of the match can be achieved

[root@mghuee~chunlanyy tmp]# [[ abc == a*b***c ]] && echo yes || echo no
yes

But only to achieve a match between the characters

[root@mghuee~chunlanyy tmp]# [[ `cat /etc/passwd` == ^root ]] && echo yes || echo no
no
[root@mghuee~chunlanyy tmp]# [[ `cat /etc/passwd` =~ ^root ]] && echo yes || echo no
yes

The "= ~" is able to complete by the regular expression to match

Note: In the regular expression pattern matching, pattern matching the right not to use double quotes , after the bash 3.2 version has been clearly stated when using regular expressions to match patterns can not be combined with double quotes

[root@mghuee~chunlanyy tmp]# [[ `cat /tmp/123` =~ "^root" ]] && echo yes || echo no
yes
[root@mghuee~chunlanyy tmp]# cat 123
"^root"
[root@mghuee~chunlanyy tmp]# [[ `cat /tmp/123` =~ ^root ]] && echo yes || echo no
no
[root@mghuee~chunlanyy tmp]# cat 123
"^root"

Because "[[]]" does not appear Word-Splitting, after joining but let the match pattern becomes "" ^ root ""

summary:

Use "let command" when an arithmetic operation, or "$ (())" "
Files exist or whether the character is empty "[]"
Logical operator "&&", "||" to be used in the "[[]]" in, "- a", "- o" in "[]" in
In "[[]]" regular expression matching pattern not matched when using double quotes
Numerical size in comparison "[]" in comparison to the use of "-lt", "- gt" Such a comparison operator, can use the "<", ">"
"[[]]" And "[]" are the best holding space on both sides, "[[]]" is a space on both sides must be isolated
The above are customary use, it is not explained in combination with other modes must not use, but can avoid some puzzling errors