Shell string comparison to determine whether a digital _linux shell

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

Integer Comparisons

-eq equals, such as: if ["$a"-eq "$b"]
-ne is not equal to, such as: if ["$a"-ne "$b"]
-GT is greater than, such as: if ["$a"-gt "$b"]
-ge is greater than or equal to, such as: if ["$a"-ge "$b"]
-lt is less than, such as: if ["$a"-lt "$b"]
-le is less than or equal, such as: if ["$a"-le "$b"]
< less than (requires double brackets), such as: (("$a" < "$b")
<= is less than or equal (requires double parenthesis), such as: (("$a" <= "$b")
> Greater than (requires double brackets), such as: (("$a" > "$b")
>= is greater than or equal (requires double parenthesis), such as: (("$a" >= "$b")

String Comparisons

= equals, such as: if ["$a" = "$b"]
= = equals, such as: if ["$a" = = "$b"], and = equivalent
Note: the function of = = = is different in [[]] and [], as follows:
1 [[$a = = z*]] # If $a starts with "Z" (pattern match) then it will be true
2 [[$a = = "z*"]] # if $a equals z* (character match), then the result is true
3
4 [$a = = z*] # File globbing and word splitting will occur
5 ["$a" = "z*"] # if $a equals z* (character match), then the result is true
A little explanation, about the file globbing is a shorthand for documents, such as "*.c" is, and so on.
But file globbing is not a strict regular expression, although in most cases the structure is more like.
!= is not equal to, such as: if ["$a"!= "$b"]
This operator will use pattern matching in the [[]] structure.
< less than, in ASCII alphabetical order. For example:
if [["$a" < "$b"]]
If ["$a" \< "$b"]
Note: In the [] structure, "<" needs to be escaped.
> Greater than, in ASCII alphabetical order. For example:
if [["$a" > "$b"]]
If ["$a" \> "$b"]
Note: In the [] structure, ">" needs to be escaped.
Refer to Example 26-11 to see examples of this operator application.
The-Z string is "null". That is, the length is 0.
-N string is not "null"
Attention:
Using-n tests in the [] structure must be caused by the variable "". Use a string that is not ""! -Z
Or the string itself, which is not quoted, is placed in the [] structure. Although in general you can
To work, but it's not safe. It is a good habit to use "" to test strings.

awk ' {print $} ' class.txt | grep ' ^[0-9.] ' > Res

–b returns True when file exists and is a block file
-C Returns True when file exists and is a character file
-D returns True when pathname exists and is a directory
-E Returns True when pathname specified file or directory exists
-F Returns True when file exists and is a regular file
-G returns True when the file or directory specified by pathname exists and the Sgid bit is set
-H Returns True when file exists and is a symbolic link file, which is not valid on some old systems
-K returns True when a file or directory specified by pathname exists and a "sticky" bit is set
-P Returns True when file exists and is a command pipe
-R when the file or directory specified by pathname is present and returned as true when it is readable
-S returns True when file size is greater than 0 o'clock
-U returns True when the file or directory specified by pathname exists and the SUID bit is set
-W Returns True when the file or directory specified by pathname exists and is executable. A directory that is accessed for its content is necessarily executable.
-O Returns True if the file or directory specified by pathname exists and the user specified by the active user ID of the current process is owned by the person.