The difference between $ (()) in the shell and $ () and ${}

$ () and ' (anti-quote)

In the bash shell, $ () and "(anti-quote) are used for command substitution.
$ echo the last Sunday was $ (date-d "last Sunday" +%y-%m-%d)
Get the date of the previous Sunday

The reason for using $ ()

1. "Easy to mix with" (single quote). Sometimes in some strange glyphs, the two symbols are identical (vertical two points).
2. In a multi-level compound substitution, ' extra hops (\ ') are required, while $ () is more intuitive. For example:
Command1 ' Command2 ' Command3 '
The original intention is to Command2 ' Command3 ' in the first to replace the Command3 to Command2 processing, and then pass the results to Command1 ' Command2 ... ' to deal with.
However, the real results are divided into ' command2 ' and ' two segments ' in the command line.
The correct input should be as follows:
Command1 ' command2 \ ' command3\ '
Change to $ () at a glance:
Command1 $ (Command2 $ (COMMAND3))

$ () Insufficient

"Basically, it can be used in all of the Unix shells, if it's written in shell script, the transplant is relatively high. and $ () not every shell can be used.

${} used for variable substitution

In general, the $var works the same as ${var}. However, using ${} will be more precise in defining the range of variable names, for example:
$ a=b
$ echo $AB
Originally intended to replace the result of a $ A and then a B-letter followed, but on the command line, the real result is only replace the value of the variable name is AB out.
Using ${} is no problem:
$ echo ${a}b
Bb

Some psychic powers of ${}

Define a variable:
File=/dir1/dir2/dir3/my.file.txt
You can use ${} to replace each other to get different values:
${file#*/} take out the first/its left string: dir1/dir2/dir3/my.file.txt
${file##*/} take out the last/and left string: my.file.txt
${file#*.} Take out the first one. And the string to the left: file.txt
${file##*.} Take off the last one. And the string to the left: txt
${file%/*} take out the last/And right string:/dir1/dir2/dir3
${file%%/*} take out the first/and the right string: (null value)
${file%.*} take off the last one. And the string to the right:/dir1/dir2/dir3/my.file
${file%%.*} take off the first one. And the string to the right:/dir1/dir2/dir3/my
Methods of Memory:
# Remove the left side (on the keyboard # on the left side)
% Remove right (on keyboard% on right of $)
The single symbol is the minimum match, and the two symbols are the maximum matches.
${file:0:5} extracts the leftmost 5 bytes:/dir1
${file:5:5} extracts the 5th byte to the right of 5 consecutive bytes:/DIR2
You can also replace the string in the value of the variable:
${file/dir/path} replaces the first dir with a path:/path1/dir2/dir3/my.file.txt
${file//dir/path} Replace all dir with Path:/path1/path2/path3/my.file.txt
With ${} You can also assign values to different variable states (not set, NULL, non-null values):
${file-my.file.txt} If $file is not set, use My.file.txt as the return value. (null and non-null values are not processed)
${file:-my.file.txt} If $file is not set or null, use My.file.txt to return the value. (Non-null value is not processed)
${file+my.file.txt} If the $file is set to a null value or a non-null value, the value is returned using My.file.txt. (not processed when not set)
${file:+my.file.txt} If $file is a non-null value, use My.file.txt as the return value. (not processed when not set and null value)
${file=my.file.txt} If $file is not set, use My.file.txt as the return value and assign the $file to My.file.txt. (null and non-null values are not processed)
${file:=my.file.txt} If $file is not set or NULL, My.file.txt is used to return the value, and the $file is assigned a value of My.file.txt. (Non-null value is not processed)
${file?my.file.txt}: If the $file is not set, the My.file.txt output to STDERR. (null and non-null values are not processed)
${file:?my.file.txt}: If $file is not set or null, the my.file.txt output to STDERR. (Non-null value is not processed)
The above understanding is, must distinguish Chu unset with null and non-null these three kinds of assignment state.
In general, NULL is concerned, without: null is not affected, if band: then even null is affected.
${#var} to calculate the length of the variable value:
${#file} can get 27,/dir1/dir2/dir3/my.file.txt just 27 bytes.

Bash Arrays (array) processing method

In general, a= "a b C def" simply replaces the $A with a single string, but instead a= (a b C def), the $A is defined as an array.
The array substitution method for Bash can be found in the following ways:
${a[@]} or ${a[*]} to get A b C def (all arrays)
${a[0]} Get a (first element), ${a[1]} second ...
${#A [@]} or ${#A [*]} Get 4 (number of arrays)
${#A [0]} gets 1 (the length of the first element a), ${#A [3]} gets 3 (the length of the fourth element Def)
A[3]=XYZ to redefine the fourth element as XYZ

Purpose of $ (())

Used for integer arithmetic. In bash, the integer operation symbol for $ (()) roughly has these:
+-*/Add, subtract, multiply, divide
% remainder operation
& | ^ ! And, or, XOR, not arithmetic
Example:
$ a=5; b=7; c=2
$ echo $ ((a+b*c))
19
$ echo $ (((a+b)/C))
6
$ echo $ (((a*b)%c))
1
The variable name in $ (()) can also be preceded by a $ symbol: $ (($a + $b * $c)) can also get 19 results.
In addition, $ (()) can also be used for different binary (such as binary, octal, hexadecimal) operations, but the output is only decimal.
echo $ ((16#2A)) result is 42 (16 decimal)
To give a practical example:
The current umask is 022, and the permissions for the new file are:
$ umask 022
$ echo "OBASE=8; $ ((8#666 & (8#777 ^ 8#$ (umask))) "| Bc
644
In fact, it is possible to redefine the value of a variable by simply (()), or as a testing:
a=5; ((a++)) to redefine $a to 6
a=5; ((A –)) a=4
a=5; b=7; ((a < b) will get a return value of 0 (TRUE)
Common test symbols for (()) have the following:
< less than
> Greater than
<= less than or equal to
>= greater than or equal to
= = equals
! = does not equal

The difference between $ (()) in the shell and $ () and ${}