1. The test command has the following two forms:
1.1. The test [operator option] expression
1.2, [operator option expression] [] must have Spaces inside
Tips: The shell environment depends on the return value of the command ($?). When 0 is returned, the command is successfully executed (or the expression is true). Failure if the return value is non-zero (expression false)
1.3. File test operator
-e Tests whether the specified file or directory exists
-f Tests whether the specified object is a file. Whether the specified file exists
-d Tests whether the specified object is a directory. Whether the specified directory exists
-r Tests whether the current user has the read permission
-w Tests whether the current user has the write permission
-x Tests whether the current user has the execution permission
-l Tests whether a file is a symbolic link (soft link file)
Example: [-f /opt/abc.txt]
2. Comparison of integer values in shell
Integer comparison refers to the determination of the relationship between two given integer values: for example, whether they are greater than, equal to, less than…
[Integer variable 1 operator integer variable 2]
2.1. Operators
— -eq: The first number is Equal to the second
– -ne: the first number is Not Equal to the second
– -gt: the first value is Greater Than the second value
– -ge: the first number is Greater or Equal to the second number
-lt: The first number is Lesser Than the second
– -le: the first term is less than or Equal to the second term
3. String comparison
3.1. Common operators
– – = : The characters contain the same contents
– -! = : Different string contents! The sign means the opposite
— z: The string content is empty
— n: Indicates whether the character exists
Format:
[” string 1″ = “string 2″] or [” string 1″ ==” string 2″]
[” string 1″!= “string 2”]
[-z “string”] -z: Checks whether the string is empty or the specified variable is null
[-n “string”] -n: Checks whether the string exists
3.2 Floating point numerical operations
1, the echo “string” of floating point calculation | BC
Awk ‘BEGIN {print string} ‘
4. Logical operation
– – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – –
Or – o or | | : logic, the meaning of “or” a true is true Just before and after the condition of an established
-! :
[expression 1] && [expression 2] is equivalent to [expression 1 -a expression 2] [[expression 1 && expression 2]]
Expression [1] | | expression [2] is equal to 1-2 o expression [expression] [[expression 1 | | expression 2]]
5. Ternary operators
Java, c environment:
Conditional expression? a:b
When the conditional expression is true, the value of A is taken; When the conditional expression is false, take the value b Shell script:
[expression] && | a | b
If the expression is true take a value: otherwise take b value
6. If conditional statement
6.1. Single branch
If [expression]; then
.
Fi
6.2. Double branches
If [expression]; then
.
else
.
fi
6.3. Multiple branches
If [expression]; then
.
Elif [expression]; then
.
Elif [expression]; then
.
[else]
fi
7. Case statement
In case variables
Model 1)
.
;;
Model 2)
.
;;
.
*)
.
esca
