Shell parentheses, square brackets, use double brackets and double square brackets scene summary

Foreword
In a recent study Shell scripting, others found the program if-then block there is a conditional statement if the double parentheses (()), double brackets [[]] of use, and thus access to relevant information, but also to see to a good blog, we made a summary of the use Shell script brackets, hereby record and share

1. Figures in parentheses ()
Brackets generally used interchangeably in the command, use the dollar sign $ cooperate, such as
```
#!/bin/bash
# 输出今年的年份
year=$(date +%Y)
echo "This year is $year"
```

2. Square brackets []
Since the if-then statement can not test conditions other than the command status code, it provides Bash Shell test command for assisting if-then statement testing of other conditions such as the comparison value, compare strings, file comparison, etc. , and the test commands is shorthand square brackets [] , which must be put spaces before the first and second square brackets square brackets, otherwise it will error
2.1 numerical comparison

Compare	description
n1 -eq n2	Check n1 and n2 are equal
n1 -ge n2	Check if n1 n2 is greater than or equal to
n1 -gt n2	Check if n1 is greater than n2
n1 n2 -The	Check n1 n2 is less than or equal to
n1 -lt n2	Check n1 is less than n2
n1 -ne n2	Check n1 n2 not equal

Examples are as follows:

#!/bin/bash
# 1. 数值比较
n1=20
n2=10
if [ $n1 -ge $n2 ]; then
    echo "n1 is greater than or euqal to n2"
else
    echo "n1 is less than n2"
fi

Note: Bash Shell can only deal directly with integer, floating point numbers will be assigned an error
2.2 String comparison

Compare	description
str1 = str2	Check that same str1 and str2
str1 != str2	Check that str1 and str2 different
str1 < str2	Check if str1 str2 smaller than
str1 > str2	Check str1 is greater than str2
-n str1	Check whether the length of the non-0 str1
-with str1	Check whether the length of str1 0

Examples are as follows:

# 2. 字符串比较
user=root
if [ $(whoami)=$user ]; then
    echo "root is online"
else
    echo "root is offline"
fi

File Compare 2.3

Compare	description
-d file	Check the file exists and is a directory
-e file	Check the file if there is
-f file	Check the file exists and is a file
-r file	Check whether the file exists and read
-s file	Check the file if there is not empty
-w file	Check whether the file exists and write
-x file	Check whether the file exists and execute
-O file	Check the file exists and the current user belongs to all
-x file	Check whether the file exists and execute
-G file	Check the file exists and the current user default group same
file1 file2 -nt	Check if file1 is newer than file2
file1 file2 -ot	Check if file1 is older than file2

Examples are as follows:

# 3. 文件比较
fileName=test3
if [ -e $fileName ]; then
    echo "$fileName  exists"
else
    echo "$fileName doesn't exists"
fi

3. double parentheses (())
Dual brackets allow use in comparative statement Advanced Math

symbol	description
val++	After the increase
val--	After Less
++val	To increase
--val	First cut
！	Logical negation
～	Bitwise negation
**	Exponentiation
<<	Left Shift
>>	Right shift
&	Boolean AND
\|	Boolean or
&&	Logic and
\|\|	Logical or

Examples are as follows:

#!/bin/bash
# 双括号使用练习
var1=10
if (($var1 >= 10)); then
    for ((i = 0; i < 3; i++)); do
        echo $i
    done
fi

4. double square brackets [[]]

The two sides brace provides advanced features for string comparisons, mathematical symbols can be used to compare strings, and implements pattern matching

#!/bin/bash
# 双方括号使用练习
fileName=test5
if [[ $fileName==test* ]]; then
    echo "This is a test file!"
    if [[ $fileName==test5 ]]; then
        echo "This file is test5!"
    fi
fi