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This article introduces the relevant knowledge of "how shell compares whether two strings are equal". In the operation of actual cases, many people will encounter such a dilemma, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

The way to compare whether two strings are equal is:

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

The key points here are as follows:

1 use a single equal sign

2 notice that there is a space on each side of the equal sign: this is the requirement of unix shell

3 notice the last x of "$test" x, which is deliberately arranged, because when $test is empty, the above expression becomes x = testx, which is obviously not equal. Without this x, the expression will report an error: [: =: unary operator expected

Binary comparison operator, comparison variable or comparison number. Pay attention to the difference between numbers and strings.

Integer comparison

-eq equals, 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, such as: if ["$a"-ge "$b"]

-lt is less than, for example: if ["$a"-lt "$b"]

-le is less than or equal to, such as: if ["$a"-le "$b"]

= greater than or equal (double parentheses are required), such as: (("$a" > = "$b"))

Small data can be compared with AWK.

String comparison

= equal to, for example: if ["$a" = "$b"]

= equal to, for example: if ["$a" = = "$b"], equivalent to =

Note: the function of = = behaves differently in [[]] and [], as follows:

1 [[$a = = z*]] # if $a starts with "z" (pattern match) then it will be true

2 [[$a = "z*"] # if $an equals z * (character match), then the result is true

three

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

5 ["$a" = "z*"] # if $an equals z * (character match), then the result is true

A little explanation, about File globbing is a kind of shorthand about files, such as "* .c" is, and so is also.

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

! = not equal to, for example: if ["$a"! = "$b"]

This operator will use pattern matching in the [[]] structure.

< less than, in ASCII alphabetical order. Such as:

If [["$a" < "$b"]]

If ["$a"\ < "$b"]

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

Refer to Example 26-11 for an example of this operator application.

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

-n string is not "null"

Note:

Using-n to test in the [] structure must use "" to generate variables. Use a string that is not "" to use!-z

Or the string itself that is not referenced by "" is placed in the [] structure. Although under normal circumstances

To work, but it is not safe. It is a good habit to use "" to test strings.

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