Shell command--if

The IF in [] is actually called a quick method of test. Bash's numeric and string comparison operators:

Note = spaces on both sides

String	Numerical	is true, if
x = y	X-eq y
X! = y	X-ne y
X < y	X-lt y
X <= y	X-le y
X > Y	X-gt y
X >= y	X-ge y
-N X		X is not empty
-Z X		X is empty

File attribute judgment

-D File:file exists and is directory (DICT)

-e File:file exists(xist)

-F File:file exists and is normal file (File)

-R file: User has file Read permission (Read)

-S file:file exists and is not empty (?)

-W File: User has Write permission to file (write)

File1-nt file2:file1 file2 New (new tHan)

File1-ot file2:file1 older than file2 (old Than)

Shell command--if