Shell Logic judgment

Format 1:if condition; Then statement; Fi

Format 2:if condition; Then statement; else statement; Fi

Format 3:if ...; Then ...; Elif ...; Then ...; else ...; Fi

Logical judgment expression: if [$a-gt $b]; If [$a-lt 5]; If [$b-eq 10] et-gt (>); -lt (<); -ge (>=); -le (<=);-eq (= =); -ne (! =) Note that there are spaces everywhere
can use && | | Combine multiple conditions
If [$a-gt 5] && [$a-lt 10]; Then
If [$b-gt 5] | | [$b-lt 3]; Then

If determine file, directory properties
[-F file] Determines if it is a normal file, and there is


[-D file] Determines if it is a directory and exists

[-E file] to determine whether files or directories exist

[-R File] to determine if the document is readable

[-W file] Determines whether the file is writable

[-X file] Determines whether the file is executable

Common operations, to determine if there is, if there is a delete, if not present, create


Some special uses of if judgment
If [-Z "$a"]?? This indicates what happens when the value of variable A is empty
If [-n ' $a '] means that the value of variable A is not empty
If Grep-q ' 123 ' 1.txt; Then?? What happens if the 1.txt contains a ' 123 ' row
if [!-e file]; Then what happens when the file doesn't exist?
if (($a <1)); Then ... Equivalent to if [$a-lt 1]; Then ...
Symbols such as <,>,==,!=,>=,<= cannot be used in []

Shell Logic judgment