Linux System Bash (Shell) Basics (3)

Arithmetic operations are undoubtedly very important in shell scripting programming;

The command to perform an integer arithmetic operation in bash is let, which has the syntax format:

Let Arg ...

Arg is a complete arithmetic expression consisting of a separate arithmetic expression, such as +,-,*,/,%,^;

where ^ is the square operation, such as 2^3 represents 2 of the three-time side;

The Let command can be overridden in the following ways:

((arithmetic expression))

For example

[[email protected] wjq]# let "a=2*3"

[Email protected] wjq]# echo $a

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[[Email protected] wjq]# (("a=3*3"))

[Email protected] wjq]# echo $a

9

In addition to the simple subtraction operation, the arithmetic operation also includes the enhanced operation mode and the special enhanced operation mode.

Enhanced mode of operation:

+ =: such as A+=b-->a=a+b

-+: such as A-=b-->a=a-b

*=: such as A*=b-->a=a*b

%=: such as A%=b-->a=a%b

/=: such as a/=b-->a=a/b

The Convert to let command operation is:

[Email protected] wjq]# b=5

[[email protected] wjq]# let "a+=b"

[Email protected] wjq]# echo $a

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[Email protected] wjq]#

Special-Enhanced operations:

A++:A first assigns the value to the variable, then +1;

++A:A first +1, then assigns the value to the variable;

A--:A assigns the value to the variable first, then-1;

--A:A first-1, then assigns the value to the variable;

The convert to let command is calculated as:

[Email protected] wjq]# a=4

[Email protected] wjq]# b=5

[[email protected] wjq]# let "a++"

[[email protected] wjq]# let "a+=b"

[Email protected] wjq]# echo $a

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Arithmetic Operation Method:

①let var= Arithmetic expression

#let "B=5+4"

#echo $B

The arithmetic expression is first performed, and the result of the operation is saved to the variable var;

Variables: containers for storing data

Weakly variable: Weakly typed variable, default is character type variable, numeric type (not including float type)

②var=$[Arithmetic expression]

[[email protected] wjq]# A=$[a*b]

[Email protected] wjq]# echo $a

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③var=$ ((arithmetic expression))

In an arithmetic expression, if there is a reference, you can omit the

[Email protected] wjq]# a=$ ((a*b))

[Email protected] wjq]# echo $a

250

④exper ARGU1 ARGU2 ARGU3

ARGU1 and ARGU3 must be numeric

ARGU2 is an operator

⑤echo "Arithmetic expression" | BC (BC Command equivalent Calculator)

[Email protected] wjq]# echo "3*4" | Bc

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Conditional Test Statement:

There are three test statements for conditional testing

One is the test command;

The second is to enclose it in a pile of square brackets, which is completely equivalent;

Test-f "/etc/passwd" is exactly equivalent to [-F "/etc/passwd"]

Three is [[conditional expression]];

These three expressions can be used in conjunction with a variety of system operators, which can be divided into four classes: Numeric test operators, string test operators, file state test operators, and logical test operators;

The following four operators are represented here using the second expression;

Data test operator: operation with binocular operator;

[N1-eq n2]: Test if N1 and N2 are equal, equal to true, and not as false

[N1-ne n2]: Test N1 and N2 are not equal, unequal to true, equal to false;

[n1-lt n2]: Test N1 and N2, the left is less than the right, less than true, greater than false;

[N1-GT n2]: Test N1 and N2, the left is greater than the right, greater than true, less than false;

[N1-le n2]: Test N1 and N2, whether the left is less than or equal to the right;

Cases

[[Email protected] wjq]# ["$a"-eq 0] && echo "a equals 0" | | echo "A is not equal to 0"

A is not equal to 0

String test operators:

Operate with a single-mesh operator;

-Z ' N1 ': If the string length is 0, the test condition is true;

-N ' N1 ': If the string length is not 0, the non-null is true, and the null is false;

[[email protected] wjq]# a= ' string '

[[Email protected] wjq]# [-Z "$a"] && echo "a equals 0" | | echo "A is not equal to 0"

A is not equal to 0

To operate with the binocular operator:

==|=: The two strings tested are equal, the same is true, and the differences are false;

！ =: The two characters tested are not the same, the difference is true, the same is false;

: The binary data of the two strings being tested is converted to ASCLL code; The left is greater than the right, greater than true, less than false;

: The two strings tested in the binary data after conversion to ASCLL code, the left is less than the right, less than true, greater than false;

Cases

①

[[email protected] wjq]# a= ' string '

[[email protected] wjq]# b= ' string '

[[Email protected] wjq]# ["$a" = = "$b"] && echo "a equals B" | | echo "A is not equal to B"

A equals b

Ii

[[email protected] wjq]# a= ' string '

[[email protected] wjq]# b= ' str '

[[Email protected] wjq]# ["$a"! = "$b"] && echo "A not equal to B" | | echo "a equals B"

A is not equal to B

③

[[email protected] wjq]# a= ' string '

[[email protected] wjq]# b= ' str '

[[Email protected] wjq]# ["$a" > "$b"] && echo "A greater than B" | | echo "A is less than B"

A greater than B

File status test operator: Monocular test operator;

File existence test, if the test file exists for true, does not exist for false;

-A|-E:

Cases

[[Email protected] wjq]# [-A./8yu] && echo "file exists" | | echo "file does not exist"

File exists

File type Test (test presence)

-B File: If the file exists and is a block device, the test is true;

-C File: If the file exists and is a character device, the test is true;

-D File: If the file exists and is a directory device, the test is true;

-F File: The test is true if it exists and is a text document;

-h|-l file: The test is true if it is present and the file is linked to a symbol;

-P File: If the file exists and is a socket file, the test is true;

Cases

[[Email protected] wjq]# [-F./8yu] && echo "File as text document" | | echo "File is not a text document"

File is a text document

Access permission settings for the file:

-R file: If the file exists and is user readable, the test is true;

-W File: If the file exists and is user writable, the test is true;

-X file: If the file exists and is user-executable, the test is true;

Cases

[[Email protected] wjq]# [-R./8yu] && echo "File readable" | | echo "File not readable"

File readable

[Email protected] wjq]# [-W./8yu] && echo "File writable" | | echo "File not writable"

File can be written

[[Email protected] wjq]# [-x/8yu] && echo "File executable" | | echo "File not executable"

File executable

File special permission Identification test:

-U file: If the file exists and the SUID permission is set, the test is true; otherwise false;

-G File: If the file exists and the Sgid permission is set, the test is true; otherwise false;

-K File: If the file exists and the sticky permission is set, the test is true; otherwise false;

Cases

[[Email protected] wjq]# [-U./8yu] && echo "file exists suid permissions" | | echo "file does not exist SUID permissions"

File does not exist SUID permissions

[[Email protected] wjq]# [-G./8yu] && echo "file exists Sgid permissions" | | echo "file does not exist Sgid permissions"

File does not exist Sgid permissions

[[Email protected] wjq]# [-K./8yu] && echo "file exists Sticky permissions" | | echo "file does not exist sticky permissions"

File does not exist SUID permissions

File ownership test:

-O File: If the file exists and its owner is the current active user, the test is true; otherwise false;

-G File: If the file exists and its genus is the current active user, the test is true; otherwise false;

Cases

[[Email protected] wjq]# [-O./8yu] && echo "file exists and its owner is the user" | | echo "file does not exist"

File does not exist

[[email protected] wjq]# ls./8yu

./8yu

[Email protected] wjq]# ls-l./8yu

-rwxrw-r--. 1 WJQ WJQ 71 March 20:39/8yu

Whether the file contents are empty:

-S file: True if the document exists and the content is not empty, and the content is not empty; otherwise false;

Time Stamp test:

-N File: The test file has been modified since the last time it was modified;

Logical Test operators:

①! : Logical non (not), he placed before any expression, is the original true expression becomes false, so that the original false expression becomes true;

②-a: Logic and (and), he placed between two logical expressions, only if two are true, the result is true;

Cases:

[-F./8yu-a-F/AA]

③-o: Logic or (or), he placed in the middle of two logical expressions, in which as long as one is true, the result is true;

Cases:

[-F./8yu-o-F/AA]

Two types of expressions are equivalent:

The first type of expression:

[-o/tmp/test] && [-s/tmp/test]

[-o/tmp/test] | | [-s/tmp/test]

The second type of expression:

[-o/tmp/test-a-s/tmp/test]

[-o/tmp/test-o-s/tmp/test]

[!-o/tmp/test]

Linux System Bash (Shell) Basics (3)