Shell string comparison to determine whether it is a number

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"]
<Less than (double parentheses are required), such as: ("$ a" <"$ B "))
<= Less than or equal to (double parentheses are required), for example: ("$ a" <= "$ B "))
> Greater than (double parentheses are required), such as: ("$ a"> "$ B "))
> = Equal to or greater than (double parentheses required), for example: ("$ a"> = "$ B "))

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.
<Less than, in the ASCII alphabetic order. For example:
If [["$ a" <"$ B"]
If ["$ a" \ <"$ B"]
Note: "<" must be escaped 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 the strings that are not referenced by "" are put in the [] structure. In general
Work, but this is not safe. It is a good habit to use "" To test strings.

Awk '{print $2}' class.txt | grep '^ [0-9.]'> res

-B. returns true if the file exists and is a block file.
-C: returns true if the file exists and is a character file.
-D. If pathname exists and is a directory, true is returned.
-E: returns true if the file or directory specified by pathname exists.
-F returns true if the file exists and is a regular file
-G returns true if the file or directory specified by pathname exists and the SGID bit is set.
-H returns true if the file exists and is a symbolic link file. This option is invalid in some old systems.
-K. If a file or directory specified by pathname exists and a "Sticky" bit is set, the system returns the true value.
-P: returns true if the file exists and is a command pipeline.
-R: returns true if the file or directory specified by pathname exists and is readable.
-S: returns true if the file size is greater than 0.
-U returns true if a file or directory specified by pathname exists and SUID is set.
-W returns true if the file or directory specified by pathname exists and is executable. A directory must be executable for its content access.
-O returns true if a file or directory specified by pathname exists and the user specified by the valid user ID of the current process is owned by it.