Use of if in shell, shellif use

Use of if in shell, shellif use
Compare if in shell


Shell determines whether an array contains an element:


Ary = (1 2 3)


A = 2


If [["$ {ary [@]}" = ~ "$ A"]; then
Echo "a in ary"
Else
Echo "a not in ary"
Fi


The method to compare the two strings is as follows:


If ["$ test" x = "test" x]; then


The key points here are:


1. Use a single equal sign


2 Note that there is a space on both sides of the equal sign: This is a unix shell requirement.


3. Note that the last x of "$ test" x is specially arranged, because when $ test is null, the above expression becomes x = testx, obviously, they are not equal. Without this x, the expression returns the error [: =: unary operator expected





Binary comparison operator, which compares variables or numbers. Note the difference between numbers and strings.


Integer comparison


-Eq equals to, for example, if ["$ a"-eq "$ B"]


-Ne is not equal to, for example, if ["$ a"-ne "$ B"]


-Gt is greater than, for example, if ["$ a"-gt "$ B"]


-Ge is greater than or equal to, for example, if ["$ a"-ge "$ B"]


-Lt is less than, for example, if ["$ a"-lt "$ B"]


-Le is less than or equal to, for example, if ["$ a"-le "$ B"]


Greater than (double parentheses required), such as: ("$ a"> "$ B "))


> = Equal to or greater than (double parentheses required), for example: ("$ a"> = "$ B "))


AWK can be used for small data comparison


String comparison


= Equals, for example, if ["$ a" = "$ B"]


= Equal to, such as: if ["$ a" = "$ B"], equivalent to =


Note: The = function has different behaviors in [[] and [], as shown below:


1 [[$ a = z *] # true if $ a starts with "z" (pattern matching)


2 [[$ a = "z *"] # If $ a is equal to z * (character matching), the result is true.


3


4 [$ a = z *] # File globbing and word splitting will occur


5 ["$ a" = "z *"] # If $ a is equal to z * (character matching), the result is true.


For a bit of explanation, File globbing is a stenographer of files, for example, "*. c" is, and then ~ Yes.


But file globbing is not a strict regular expression, although the structure is similar in most cases.


! = Not equal to, such as: if ["$ "! = "$ B"]


This operator uses pattern matching in the [[] structure.


Greater than, in the ASCII alphabetic order. For example:


If [["$ a"> "$ B"]


If ["$ a" \> "$ B"]


Note: ">" needs to be escaped in the [] structure.


For more information, see Example 26-11.


-Z string is "null". The length is 0.


-N string is not "null"


Note:

To use-n in the [] structure, you must use "" To cause variables. Use a string that is not! -Z or a string that is not referenced by "", put it in the [] structure. In general
Work, but this is not safe. It is a good habit to use "" To test strings.

If simplified
If [condition judgment 1] & (|) [condition Judgment 2]; then
Elif [condition judgment 3] & (|) [condition judgment 4]; then
Else
Execute the third content program
Fi