$ man test
NAME
     test, [ -- condition evaluation utility

SYNOPSIS
     test expression
     [ expression ]

DESCRIPTION
     The test utility evaluates the expression and, if it evaluates to true, returns a zero (true) exit status; oth-
     erwise it returns 1 (false).  If there is no expression, test also returns 1 (false).

     All operators and flags are separate arguments to the test utility.

# 判斷檔名是否存在(Exist)
test -e REAME.md
# 判斷檔名是否存在且為檔案
test -f REAME.md
# 判斷檔名是否存在且為資料夾
test -d dir
#

# Newer than
test file1 -nt file2
# Older than
test file1 -ot file2
# True if file1 and file2 exist and refer to the same file.
test file1 -ef file2

# True if string is not the null string
test string
# True if the length of string is zero.
test -z string
# True if the length of string is nonzero.
test -n string

# True if the strings s1 and s2 are identical.
test s1 = s2
# True if the strings s1 and s2 are not identical.
test s1 != s2
# True if string s1 comes before s2 based on the binary value of their characters.
test s1 < s2
# True if string s1 comes after s2 based on the binary value of their characters.
test s1 > s2

# True if the integers n1 and n2 are algebraically equal.
test n1 -eq n2
# True if the integers n1 and n2 are not algebraically equal.
test n1 -ne n2
# True if the integer n1 is algebraically greater than the integer n2.
test n1 -gt n2
# True if the integer n1 is algebraically greater than or equal to the integer n2.
test n1 -ge n2
# True if the integer n1 is algebraically less than the integer n2.
test n1 -lt n2
# True if the integer n1 is algebraically less than or equal to the integer n2.
test n1 -le n2

## not
! expression
## and
test expression1 -a expression2
## or
expression1 -o expression2
## ()
( expression )